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Publication Number: FHWA-HRT-04-091
Date: August 2004

Signalized Intersections: Informational Guide

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CHAPTER 4 — TRAFFIC DESIGN AND ILLUMINATION

TABLE OF CONTENTS

4.0 TRAFFIC DESIGN AND ILLUMINATION

4.1 Traffic Signal Control Type

4.2 Traffic Signal Phasing

4.2.1   “Permissive-Only” Left-Turn phasing

4.2.2  “Protected-Only” Left-Turn phasing

4.2.3 Protected-Permissive Left-Turn phasing

4.2.4 Split Phasing

4.2.5 Prohibited Left-Turn phasing

4.2.6 Right-Turn phasing

4.3 Vehicle and Pedestrian Displays

4.3.1 Vehicle Displays

4.3.2  Pedestrian Displays

4.4  Traffic Signal Pole Layout

4.5 Traffic Signal Controller

4.6   Detection Devices

4.6.1 Vehicle Detection

4.6.2     Pedestrian Detection

4.7  Basic Signal Timing parameters

4.7.1 Pedestrian Timing

4.7.2 Vehicle timing—Green interval

4.7.3 Vehicle timing—Detector Timing

4.7.4 Vehicle timing—Vehicle Clearance

4.7.5  Vehicle timing—Cycle Length

4.8 Signing and Pavement Marking Design

4.9 Illumination Design

4.9.1 Illuminance

4.9.2  Veiling Illuminance

 

LIST OF FIGURES

23

Standard NEMA ring-and-barrier structure

24
Typical phasing diagram for “permissive-only” left-turn phasing
25
Possible signal head arrangements for “permissive-only” left-turn phasing
26
Typical phasing diagram for “protected-only” left-turn phasing
27
Possible signal head arrangements for “protected-only” left-turn phasing
28
Typical phasing diagram for protected-permissive left-turn phasing
29
Possible signal head and signing arrangement for protected-permissive left-turn phasing
30 
Illustration of the yellow trap
31
The protected-permissive left-turn display known as “Dallas display” uses louvers to  restrict visibility of the left-turn display to adjacent lanes
32
Typical phasing diagrams for split phasing
33
Common signal head arrangement for split phasing
34
Typical phasing diagram illustrating a right-turn overlap
35
Common signal head and signing arrangement for right-turn-overlap phasing
36
Examples showing five optional signal head locations
37
Pedestrian signal indicators
38
Example of advance street name sign for upcoming intersection
39 
Example of advance street name sign for two closely spaced intersections
40
Example of signing for a left-hand land trap
41
Example of advance overhead signs indicating lane use for various destinations
42
Example of pavement legends indicating destination route numbers  (“horizontal signage”) 

  

LIST OF TABLES

13
Advantages and disadvantages of various configurations for displaying vehicle signal  indications
14
Traffic signal controller advantages and disadvantages
15
Strengths and weaknesses of commercially available detector technologies
16
Location of advanced vehicle detectors
17
Recommended illuminance for the intersection of continuously lighted urban streets
18
RP-8-00 guidance for roadway and pedestrian/area classification for purposes of  determining intersection illumination levels

4.0    Traffic Design and Illumination

This chapter deals with the traffic signal hardware and software the infrastructure that controls the assignment of vehicular and pedestrian right-of-way at locations where conflicts or hazardous conditions exist.  The proper application and design of the traffic signal is a key component in improving the safety and efficiency of the intersection.

This chapter presents an overview of the fundamental principles of traffic design and illumination as they apply to signalized intersections. The topics discussed include:

  • Traffic signal control types.

  • Traffic signal phasing.

  • Vehicle and pedestrian detection.

  • Traffic signal pole layout.

  • Traffic signal controllers.

  • Basic signal timing parameters.

  • Signing and pavement marking.

  • Illumination.

4.1 Traffic Signal Control Type

Traffic signals operate in either pre-timed or actuated mode. Pre-timed signals operate with fixed cycle lengths and green splits.  Actuated signals vary the amount of green time allocated to each phase based on traffic demand.  Either type may be used in isolated (independent) or coordinated operation. Most pre-timed controls feature multiple timing plans, with different cycle, split, and offset values for different periods of the day. 

Actuated control does not rely on a fixed cycle length unless the intersection is in a coordinated system or under adaptive control. Actuated control provides variable lengths of green timing for phases that are equipped with detectors. The time for each movement depends on the characteristics of the intersection and timing parameters (which are based on demand at the intersection).

4.2 Traffic Signal Phasing

The MUTCD defines a signal phase as the right-of-way, yellow change, and red clearance intervals in a cycle that are assigned to an independent traffic movement or combination of traffic movements.(1) Signal phasing is the sequence of individual signal phases or combinations of signal phases within a cycle that define the order in which various pedestrian and vehicular movements are assigned the right-of-way.  The MUTCD provides rules for determining controller phasing, selecting allowable signal indication combinations for displays on an approach to a traffic control signal, and determining the order in which signal indications can be displayed.

Signal phasing at most intersections in the United States makes use of a standard National Electrical manufacturers association (NEMA) ring-and-barrier structure, shown in figure 23. This structure organizes phases to prohibit conflicting movements (e.g., eastbound and southbound through movements) from timing concurrently while allowing nonconflicting movements (e.g., northbound and southbound through movements) to time together. Most signal phasing patterns in use in the United States can be achieved through the selective assignment of phases to the standard NEMA ring-and-barrier structure.

View Alternative Text
Figure 23. Standard NEMA ring-and-barrier structure.

Depending on the complexity of the intersection, 2 to 8 phases are typically used, although some controllers can provide up to 40 phases to serve complex intersections or sets of intersections. Pedestrian movements are typically assigned to parallel vehicle movements.

Developing an appropriate phasing plan begins with determining the left-turn phase type at the intersection. The most basic form of control for a four-legged intersection is “permissive only” control, which allows drivers to make left turns after yielding to conflicting traffic or pedestrians and provides no special protected interval for left turns. As a general rule, the number of phases should be kept to a minimum because each additional phase in the signal cycle reduces the time available to other phases.

Provision of a separate left-turn lane may alleviate the problems somewhat by providing storage space where vehicles can await an adequate gap without blocking other traffic movements at the intersection. In most cases, the development of a signal phasing plan should involve an analytical analysis of the intersection. Several software packages are suitable for selecting an optimal phasing plan for a given set of geometric and traffic conditions for both individual intersections and for system optimization.

Pedestrian movements must be considered during the development of a phasing plan. For example, on wide roadways pedestrian timing may require timing longer than what is required for vehicular traffic, which may have an effect on the operation analysis.

4.2.1 “Permissive-Only” Left-Turn phasing

"Permissive-only" (also known as "permitted-only") phasing allows two opposing approaches to time concurrently, with left turns allowed after yielding to conflicting traffic and pedestrians. One possible implementation of this phasing pattern is illustrated in figure 24. Note that the two opposing movements could be run in concurrent phases using two rings; for example, the eastbound and westbound through movements shown in figure 24 could be assigned as phase 2 and phase 6, respectively.

“Permissive-only” phasing allows two opposing left-turn movements to occur concurrently upon yielding to conflicting vehicular and pedestrian movements. The base drawing is identical to figure 23. The phasing pattern shows all eight grids empty except for phases 2 and 4. Phase 2 allows pedestrians to cross the north and south legs of the intersection (east/west movements), protected through movements, and permissive left and right turn movements. Phase 4 shows a similar traffic configuration for the north and south approaches.
Figure 24. Typical phasing diagram for “permissive-only” left-turn phasing.

For most high-volume intersections, “permissive-only” left-turn phasing is generally not practical for major street movements given the high volume of the intersections. Minor side street movements, however, may function acceptably using “permissive-only” left-turn phasing, provided that traffic volumes are low enough to operate adequately and safely without additional left-turn protection.

"Permissive-only" displays are signified by a green ball indication. In this case, no regulatory sign is required, but the MUTCD (sections 2B.45 and 4D.06) allows the option of using the R10-12 regulatory sign ("LEFT TURN YIELD ON GREEN (symbolic green ball)").(1) As traffic volumes increase at the intersection, the number of adequate gaps to accommodate left-turning vehicles on the permissive indication may result in safety concerns at the intersection.  Common signal head arrangements that implement “permissive only” phasing are shown in figure 25; refer to the MUTCD for other configurations.

Signal head arrangement shows two vertical, three-section signal heads centered above two through lanes.
(a) Permissive left-turn phasing using three-section signal heads over the through lanes only.
Signal head arrangement shows two vertical three-section signal heads centered above two through lanes with a vertical three-section signal head centered above the left-turn lane and accompanied with a sign that reads “left-turn yield on green” in uppercase letters.

(b) Permissive left-turn phasing using three-section signal heads over the through lanes and a three-section signal head and accompanying sign over the left turn lane.

 

Figure 25. Possible signal head arrangements for “permissive-only” left-turn phasing.

 

4.2.2 “Protected-Only” Left-Turn phasing

"Protected-only" phasing consists of providing a separate phase for left-turning traffic and allowing left turns to be made only on a green left arrow signal indication, with no pedestrian movement or vehicular traffic conflicting with the left turn. As a result, left-turn movements with "protected-only" phasing have a higher capacity than those with "permissive-only" phasing due to fewer conflicts. This phasing pattern is illustrated in figure 26. Typical signal head and associated signing arrangements that implement "protected-only" phasing are shown in figure 27; refer to the MUTCD for other configurations. Chapter 12 of this document provides guidance on determining the need for protected left turns.

View Alternative Text
Figure 26. Typical phasing diagram for “protected-only” left-turn phasing.

 

Signal Head arrangement: “Protected only” phasing includes a three-section signal head (one for each lane) with arrows displayed for the red, yellow, and green indications.
(a) Protected left-turn phasing using a three-section signal head with red, yellow, and green arrows.
Signal Head arrangement: the red arrow is replaced by a red ball and a sign that reads “left-turn signal” in uppercase letters.

(b) Protected left-turn phasing using a three-section signal head with red ball, yellow arrow, and green arrow and an accompanying sign.

 

Figure 27. Possible signal head arrangements for “protected-only” left-turn phasing.

4.2.3 Protected-Permissive Left-Turn phasing

A combination of protected and permissive left-turn phasing is referred to as protected-permissive left-turn (PPLT) operation. This phasing pattern is illustrated in figure 28. A typical signal head and associated signing arrangement that implements protected-permissive phasing is shown in figure 29; refer to the MUTCD for other configurations.

View alternative text
Figure 28. Typical phasing diagram for protected-permissive left-turn phasing.
Signal head using a five-section head located directly above the lane line that separates the exclusive through and exclusive left-turn lane. A sign is located right of the five-section signal head that reads, “left turn yield on green” in uppercase letters.
(a) Protected-permissive left-turn phasing using a five-section head located directly above the lane line that separates the exclusive through and exclusive left-turn lane, along with an accompanying sign.
Signal head shows the five-section signal head located directly above the exclusive left-turn lane and three-section signal heads centered above the two through lanes.

(b) Protected-permissive left-turn phasing using a five-section signal head located directly above the exclusive left-turn lane.

 

Figure 29. Possible signal head and signing arrangement for protected-permissive left-turn phasing.

Observed improvements in signal progression and efficiency combined with driver acceptance have led to expanded usage of PPLT over the years. PPLT signals offer numerous advantages when compared to “protected-only” operation. These advantages are associated with both protected-permissive and lead-lag operation. They include the following (adapted with additions by the authors):(48)

  • Average delay per left-turn vehicle is reduced.
  • Protected green arrow time is reduced.
  • There is potential to omit a protected left-turn phase.
  • Arterial progression can be improved, particularly when special signal head treatments are used to allow lead-lag phasing.

Some disadvantages include the following:

  • The permissive phase increases the potential for vehicle-vehicle and vehicle-pedestrian conflicts.
  • There is a limited ability to use lead-lag phase sequences unless special signal head treatments are used (see below).

The controller phasing for protected-permissive mode is the most complicated phasing because of the safety implications created by the potential of what is known as the “yellow trap.” In a permissive-mode operation, the left-turning driver must obey the green display for the adjacent through movement, which also gives permission for the permissive left turn. When the yellow display for the adjacent through movement appears, the left-turning driver ordinarily expects the opposing through display to be yellow as well. The driver may now mistakenly believe that the left turn can be completed on the yellow display or immediately thereafter when the opposing through display will be red.

For ordinary lead-lead operation where both protected left-turn phases precede the permissive phases, this is not a concern, as both permissive phases end concurrently. However, this problem can occur when a permissive left turn is opposed by a lagging protected left turn. In this type of operation (known as lag-permissive), the yellow display seen by a left-turning driver is not indicative of the display seen by the opposing through driver. The opposing through display may be yellow or may remain green. A driver who turns left believing that the opposing driver has a yellow or red display when the opposing driver has a green display may be making an unsafe movement. This yellow trap is illustrated in figure 30.

Drivers who encounter this trap are those that attempt to make a permissive left-turn after a protected leading left-turn phase. Typically they have entered the intersection on a permissive green waiting to make a left turn when sufficient gaps occur in opposing through traffic. If the absence of gaps in opposing through traffic requires them to make their turn during the left-turn clearance interval, they may be “stranded" in the intersection because of the absence of gaps and because the opposing through movement remains green. More importantly, they may incorrectly presume that the opposing through traffic is being cleared at the same time that the adjacent through movement is being terminated. Therefore, they may complete their turn believing that opposing vehicles are slowing to a stop when in fact the opposing vehicles are proceeding into the intersection with a green ball signal indication.

There are two ways to eliminate the yellow trap. First, the phase sequence at the intersection can be restricted to simultaneous leading (lead-lead) or lagging (lag-lag) left-turn phasing. Second, the signal display can be altered to allow the left-turn signal head to display a permissive left turn independently of the adjacent through movements, which allows the through movements to terminate but allow a permissive left turn to continue during the opposite approach’s lagging protected left-turn phase. Some agencies have experimented with signal displays (e.g., “Dallas Display,” flashing circular red, flashing red arrow, flashing circular yellow, and flashing yellow arrow) that allow this type of operation. Of these, the “Dallas Display” optically restricts the visibility of the permissive movement using louvers; it is fully compliant with the MUTCD and is shown in figure 31.

View Alternative Text
Figure 30. Illustration of the yellow trap.(3)

The signal display for the PPLT movement is a five-section horizontal signal head with the following indications from left to right: red ball, louvered yellow ball, yellow arrow, green arrow, and louvered green ball. The signal display for the adjacent through lane is a three-section signal head with red, yellow, and green ball indications.
Figure 31. The protected-permissive left-turn display known as “Dallas display” uses louvers to restrict visibility of the left-turn display to adjacent lanes.(49)

A national NCHRP study, has examined the operational advantages and safety aspects of various PPLT control devices and signal arrangements. The study determined that a flashing yellow arrow PPLT display was consistently found to be equal or superior to existing PPLT displays both in a laboratory environment and in cities where the display was experimentally implemented in the field.(49)  The flashing yellow arrow display for PPLT is still considered experimental by the MUTCD and is undergoing further field testing.

4.2.4 Split Phasing

Split phasing consists of having two opposing approaches time consecutively rather than concurrently (i.e., all movements originating from the west followed by all movements from the east). Split phase can be implemented in a variety of ways depending on signal controller capabilities and how pedestrian movements are treated. Three basic variations, shown in figure 32, are described as follows:

  • Method A: Consecutive pedestrian phases using one ring. This method associates each pedestrian phase with its adjacent vehicle phase. This places pedestrians at potential conflict with right-turning traffic only. However, this may result in potentially consecutive pedestrian phases if pedestrian calls are present on both phases. For large intersections, the minimum time needed to serve these consecutive movements may result in excessively long cycle lengths. Implementation uses two consecutive phases in the same ring (e.g., phases 3 and 4), with pedestrian phases assigned to each.

  • Method b: Consecutive pedestrian phases using “exclusive” settings in controller. This method is functionally identical to method A. Implementation differs from method A in that a setting in the controller is needed to force the phases to time in an “exclusive” mode (e.g., the phase must not time with any other phases).

  • Method C: Concurrent pedestrian phases using two rings. This method, used by some agencies in certain situations, associates pedestrian movements with a single phase in one ring that, when actuated, operates concurrently with two consecutively timing vehicle phases in the second ring.  Details of implementation of this method can be found in Wainwright.(50)  This method can provide a considerably more efficient operation of the intersection, particularly where pedestrian crossing demands are large enough to warrant pedestrian signals but are relatively infrequent (not every cycle) during most or all of the day.  In most cycles, no pedestrian actuation occurs, so:

    • The split vehicular phases operate without any pedestrian timing considerations.
    • The sequential vehicular phase green times are directly related to their respective vehicular demands.
    • The green left arrow signal indication is displayed to each of the sequential vehicle phases to encourage efficient nonyielding movement.
    Method C is advantageous under some conditions, but should not be applied indiscriminately because it does have some potential liabilities as compared to the other two methods. Firstly, during the cycles when the pedestrian phase is actuated, left-turning vehicles can sometimes be placed in an awkward situation of not being able to clear the intersection when the vehicle phase terminates because conflicting pedestrians have not yet finished crossing. Secondly, pedestrians could face both left-turning and right-turning conflicting vehicles. Thirdly, if for some reason the timing parameters for the two crosswalks are different, then this method might be disadvantageous because placing both crosswalks on a single phase requires identical timing parameters for both crosswalks.

Split phasing is used infrequently at signalized intersections because a more efficient conventional phasing plan can usually be found. The following conditions could indicate that split phasing might be an appropriate design choice:

  • There is a need to accommodate multiple turn lanes on an approach, but sufficient width is not available to provide separate lanes. Therefore, a shared through/left lane is required. An operational analysis should be performed to ensure this option is superior compared to a single turn lane option under various phasing scenarios.
  • The left-turn lane volumes on two opposing approaches are approximately equal to the through traffic lane volumes and the total approach volumes are significantly different on the two approaches. Under these somewhat unusual conditions, split phasing may prove to be more efficient than conventional phasing.
  • a pair of opposing approaches is physically offset such that the opposing left turns could not proceed simultaneously or a permissive left turn could not be expected to yield to the opposing through movement.
  • The angle of the intersection is such that the paths of opposing left turns would not be forgiving of errant behavior by turning motorists.
  • The safety experience indicates an unusual number of crashes (usually sideswipes or head-on collisions) involving opposing left turns. This may be a result of unusual geometric conditions that impede visibility of opposing traffic.
  • a pair of opposing approaches each has only a single lane available to accommodate all movements and the left turns are heavy enough to require a protected phase.
  • One of the two opposing approaches has heavy demand and the other has minimal demand.  Under this condition, the signal phase for the minimal approach would be skipped frequently and the heavy approach would function essentially as the stem of a t intersection. 
View alternative text
(a) Method A: Consecutive pedestrian phases using one ring.
View alternative text

(b) Method b: Consecutive pedestrian phases using “exclusive” settings in controller.

Note: Separate “exclusive” setting must be used for phases 4 and 8; otherwise, operation results in simultaneous display of phases 4 and 8.

View alternative text

(c). Method C: Concurrent pedestrian phases using two rings.

 

Figure 32. Typical phasing diagrams for split phasing.

No standard method is provided in the MUTCD for indicating split phasing at an intersection, and the methods vary considerably depending on what type of phasing sequence has been used. A common way to implement method A or B described above involves using a four-section head displaying both a green ball and a green left-turn arrow simultaneously, as shown in figure 33. This method does not require the use of additional signs. Note that additional measures are needed with method C, as the protected left-turn arrow conflicts with the concurrent pedestrian phase, as follows:(50)

  • A special logic package can be used to suppress the green arrow display whenever the pedestrian phase is being served.
  • A static sign indicating “LEFT TURN YIELD TO PEDS ON GREEN (symbolic green ball)” can be located next to the leftmost signal head for emphasis.
  • A blankout sign indicating “LEFT TURN YIELD TO PEDS” can be activated when the conflicting vehicular and pedestrian phases are running concurrently.
This drawing shows a mast arm with two signal heads: a four-section signal head on the left (from top to bottom: red ball, yellow ball, green ball, green arrow) and a three-section signal head on the right (from top to bottom: red ball, yellow ball, green ball).
Figure 33. Common signal head arrangement for split phasing.

4.2.5 Prohibited Left-Turn phasing

An alternative to providing a left-turn phase is to prohibit left-turn movements at the subject intersection.  Under this scenario, left-turning drivers would be required to divert to another facility or turn in advance or beyond the intersection via a geometric treatment such as a jughandle or median U-turn.  Left-turns can be prohibited on a full- or part-time basis. The amount of traffic diverted, effects on transit routes, the adequacy of the routes likely to be used, and community impacts are all important issues to consider when investigating a turn prohibition. A variety of treatments that redirect left turns are discussed in chapter 10.

4.2.6 Right-Turn phasing

Right-turn phasing may be controlled in a permissive or protected manner with different configurations depending on the presence of pedestrians and lane configuration at the intersections.

Right turns have been operated on overlap phases to increase efficiency for the traffic signal. An overlap is a set of outputs associated with two or more phase combinations. As described earlier, various movements can be assigned to a particular phase. In some instances, right-turn movements operating in exclusive lanes can be assigned to more than one phase that is not conflicting. In this instance, a right turn is operated at the same time as the left turn, as shown in figure 34. The overlap forms a separate movement that derives its operation from its assigned phases (also called parent phases); for example, overlap A (OL A) is typically assigned to phase 2 (the adjacent through phase) and phase 3 (the nonconflicting left-turn phase from the cross street). During a transition between two parent phases, the overlap will remain green. To implement this type of true overlap, a three-section head with limited visibility must be used, as the right-turn display may be different from the adjacent through phase.

View alternative text
Figure 34. Typical phasing diagram illustrating a right-turn overlap.

More commonly, a five-section head with a combination of circular and arrow indications is used. Note that the MUTCD requires the display of a yellow change interval between the display of a green right-turn arrow and a following circular green display that applies to the continuing right-turn movement on a permissive basis. This yellow change interval is necessary to convey the change in right-of-way from fully protected during the green arrow to requiring a yield to pedestrians and other vehicles during the circular green. This can be implemented by assigning the right-turn arrows to the same phase as the nonconflicting left-turn phase on the cross street and the circular indications to the same phase as the adjacent through movement. A typical five-section signal head that implements protected-permissive right-turn phasing is shown in figure 35; refer to the MUTCD for other configurations.

Signal head arrangement uses a three-section signal display centered above the leftmost through lane and a five-section signal head centered above the lane line that separates the rightmost through lane and the exclusive right-turn lane.
(a) Right-turn overlap phasing using a five-section head located directly above the lane line that separates the exclusive through and exclusive right-turn lane.
Signal head arrangement shows three-section signal heads centered above both through lanes and the five-section signal head centered above the right-turn lane. The five-section head consists of a red ball centered over two vertical stacks: yellow and green balls on the left, and yellow and green right-turn arrows on the right.
(b) Right-turn overlap phasing using a five-section signal head centered above the right-turn lane.

Figure 35. Common signal head and signing arrangements for right-turn-overlap phasing.

This type of operation increases efficiency by providing more green time to this right-turn movement but may compromise the intersection’s usability for visually impaired pedestrians. The transition from the protected right-turn movement on the green arrow to the permissive right-turn movement on the green ball masks the sound of the adjacent through vehicles. This makes it difficult for visually impaired pedestrians to hear when the adjacent through vehicles begin to move, which is used as an audible cue for crossing the street.  Therefore, the use of accessible pedestrian signals to provide an audible indication of the start of the pedestrian phase may be needed to restore this cue.

4.3 Vehicle and Pedestrian Displays

Signal displays can be generally categorized into those for vehicles and for pedestrians. The following sections discuss each type.

4.3.1 Vehicle Displays

The location of signal heads should be evaluated based on visibility requirements and type of signal display.  While signal head placement is governed by MUTCD requirements for signal displays (discussed earlier in this chapter), the specific placement of signal heads is typically determined by local policies.  When designing the placement of signal heads, the following should be considered in addition to the minimum requirements described in the MUTCD:

  • Consistency with other intersections in the area.
  • A geometric design issue that could confuse a driver.
  • A large percentage of vehicles on one or more approaches that block lines of sight including trucks and vans.
  • The width of the intersection.
  • The turning paths of the vehicles.

At large signalized intersections, the safety and operation of the intersection may be enhanced through the use of additional signal heads, some of which are standard in some states. Figure 36 shows a typical intersection design with five types of optional heads:

Optional Head #1: This is a near-right-side side head that can be used to provide an advanced head at wide intersections as well as provide a supplemental head for vehicles that are unable to see the signal heads over the lanes due to their position behind large vehicles (trucks, etc.).

Optional Head #2: This is an extra through head that can be used to supplement the overhead signal heads. This head provides an indication for vehicles that might be behind large vehicles and may be more visible than the overhead signal head when the sun is near the horizon.

Optional Head #3: This is an extra left-turn head that can be used to guide left-turning vehicles across a wide intersection as they make their turn. It also helps visibility for vehicles behind large vehicles and for times of day when the sun is near the horizon.

Optional Head #4: This is a near-left-side head that can be used to provide an advance indication if visibility is hampered by a curve in the road upstream of the intersection. 

Optional Head #5: This is a head that can be used to provide a display in direct view of a right-turn lane and can also be used to provide a right-turn overlap phase in conjunction with the nonconflicting left-turn phase on the cross street.   The head should contain either three circular balls or be a five-section head with three balls and two right-turn arrows due to the concurrent pedestrian crossing.

(A) Optional Head #1 is on the near side for through vehicles, mounted on the pole nearest the approaching driver and facing the approaching driver. The drawing depicts a truck obscuring the visibility of the normal mast-arm-mounted signal heads for a passenger car behind the truck; the optional head can be seen to the right of the truck.
(a)     Optional Head #1: Near-side head for through vehicles.
(B) Optional Head #2: Far-side supplemental head for through vehicles, located on the same pole as the overhead mast arm signal heads on the far right side of the intersection. The drawing depicts a truck obscuring the visibility of the normal mast-arm-mounted signal heads for a passenger car behind the truck; the optional head can be seen to the right of the truck.
(b)     Optional Head #2: Far-side supplemental head for through vehicles.
(C) Optional Head #3: Far-side supplemental head for left-turning vehicles, located on the pole on the far left side of the intersection. The drawing depicts a truck obscuring the visibility of the normal mast-arm-mounted signal head for a passenger car behind the truck; the optional head can be seen to the left of the truck.

(c)     Optional Head #3: Far-side supplemental head for left-turning vehicles.

 

(D) Optional Head #4: Near-side head on a curving approach, located on the pole on the near left side of the intersection. The drawing depicts a car approaching on curve to the right, with the sight lines reaching only the optional head; the normal mast-arm-mounted signal head are out of view to the right.
(d) Optional Head #4: Near-side head on curving approach.
(E) Optional Head #5: Far-side head for right-turning vehicles, located on the mast arm opposite the subject approach. The drawing depicts a car waiting in a right-turn lane, with the optional head located within the lines of sight.
 

(e) Optional Head #5: Far-side head for right-turning vehicles.

 

Figure 36. Examples showing five optional signal head locations.

4.3.2 Pedestrian Displays

According to section 4E.03 of the 2003 MUTCD, pedestrian signal heads must be used in conjunction with vehicular traffic control signals under any of the following conditions:(1)

  • If a traffic control signal is justified by an engineering study and meets either Warrant 4, Pedestrian Volume, or Warrant 5, School Crossing (see MUTCD chapter 4C).
  • If an exclusive signal phase is provided or made available for pedestrian movements in one or more directions, with all conflicting vehicular movements being stopped.
  • At an established school crossing at any signalized location.
  • Where engineering judgment determines that multiphase signal indications (as with split-phase timing) would tend to confuse or cause conflicts with pedestrians using a crosswalk guided only by vehicular signal indications.

Pedestrian signals should be used under the following conditions:

  • If it is necessary to assist pedestrians in making a reasonably safe crossing or if engineering judgment determines that pedestrian signal heads are justified to minimize vehicle-pedestrian conflicts.
  • If pedestrians are permitted to cross a portion of a street, such as to or from a median of sufficient width for pedestrians to wait, during a particular interval but are not permitted to cross the remainder of the street during any part of the same interval.
  • If no vehicular signal indications are visible to pedestrians, or if the vehicular signal indications that are visible to pedestrians starting or continuing a crossing provide insufficient guidance for them to decide when it is reasonably safe to cross, such as on one-way streets, at t-intersections, or at multiphase signal operations.

The MUTCD provides specific guidance on the type and size of pedestrian signal indications (Section 4E.04). As noted in the MUTCD, all new pedestrian signals should use the UPRAISED HAND (symbolizing DON’T WALK) and WALKING PERSON (symbolizing WALK) indications, shown in figure 37. The pedestrian displays must be mounted so that the bottom of the pedestrian signal display housing (including mounting brackets) is no less than 2.1 m (7 ft) and no more than 3 m (10 ft) above sidewalk level.(1)

The diagram shows the pedestrian signal indications for the “Don’t Walk” and “Walk” phases.
Figure 37. Pedestrian signal indications.

Some signalized intersections have factors that may make them difficult for pedestrians who have visual disabilities to cross safely and effectively.  As noted in the MUTCD (section 4E.06), these factors include:(1)

  • Increasingly quiet cars.
  • Right turn on red (which masks the sound of the beginning of the through phase).
  • Continuous right-turn movements.
  • Complex signal operations (e.g., protected-permissive phasing, lead-lag phasing, or atypical phasing sequences).
  • Wide streets.

To address these challenges, accessible pedestrian signals have been developed to provide information to the pedestrian in a nonvisual format, such as audible tones, verbal messages, and/or vibrating surfaces. Detail on these treatments can be found in the MUTCD(1) and in several references sponsored by the U.S. Access board and the National Cooperative Highway Research Program (NCHRP).(51,52,53)

4.4 Traffic Signal Pole Layout

Three primary types of signal configurations display vehicle signal indications:

  • Pedestal or post-mounted signal displays.
  • Span-wire configurations.
  • Mast arms.

Table 13 identifies the advantages and disadvantages of each configuration.

Table 13. Advantages and disadvantages of various configurations for displaying vehicle signal indications.

Advantages

Disadvantages

Pedestal (post-mounted) vehicle signal

  •   Low cost
  •   Less impact on view corridors
  •   Lower maintenance costs
  •   Esthetics
  • Difficult to meet MUTCD visibility requirements, particularly at large signalized intersections

Span wire vehicle signal

  •   Can accommodate large intersections
  •   Flexibility in signal head placement
  •   Lower cost than mast arms
  • Higher maintenance costs
  • Wind and ice can cause problems
  • May be considered aesthetically unpleasing

Mast arm vehicle signal

  • Provides good signal head placement
  • Lower maintenance costs
  • Many pole esthetic design options
  • More costly than span wire
  • Mast arm lengths can limit use and be extremely costly for some large intersections

In addition to providing support for the optimal location of vehicle and pedestrian signal indications, signal poles need to be located carefully to address the following issues:

  • Pedestrian walkway and ramp locations.

  • Pedestrian pushbutton locations, unless separate pushbutton pedestals are provided.

  • Clearance from the travel way.

  • Available right-of-way and/or public easements.

  • Overhead utility conflicts, as most power utilities require at least 3.0 m (10 ft) clearance to power lines.

  • Underground utilities, as most underground utilities are costly to relocate and therefore will impact the location of signal pole foundations.

The MUTCD,(1) the ADAAG,(33) and the AASHTO Roadside Design Guide(54) all contain guidance regarding the lateral placement of signal supports and cabinets.  Generally, signal poles should be placed as far away from the curb as possible, not conflict with the pedestrian walking paths, and be located for easy access to the pushbuttons by disabled pedestrians. In some circumstances, it may be difficult or undesirable to locate a single pole that adequately serves both pedestrian ramps and provides adequate clearances.  In these cases, one or more pedestals with the pedestrian signal heads and/or pushbuttons should be considered to ensure visibility of the pedestrian signal heads and accessibility to the pushbuttons. 

4.5 Traffic Signal Controller

The traffic controller is the brain of the intersection.  There are two general categories of traffic signal controllers: pre-timed and actuated. In the past two decades, most electro-mechanical and early solid-state controllers have been replaced with NEMA, 170, and advanced traffic controllers (ATC), even in locations where the signal is operated in a pre-timed mode. Although most modern controllers can perform the functions needed at typical signalized intersections, some may not be able to handle: more complicated configurations (e.g., intersections with more than four legs or two closely spaced intersections); communications with other controllers of dissimilar brands; or accommodation of priority treatments (e.g., transit priority). Therefore, the choice of controller may play a significant role in the types of treatments that can be considered at a signalized intersection.

Traffic controllers can be generally classified into three types:

1.     NEMA.

2.     Type 170.

3.     ATC.

Some advantages and disadvantages of each type are described in table 14.

Table 14. Traffic signal controller advantages and disadvantages.

Advantages

Disadvantages

NEMA Controller

  • Specific vendor software
  • Reduced software/hardware problems
  • Cabinets are not standardized
  • Proprietary software
  • Proprietary features may not be interchangeable with other NEMA controllers
  • Typically require larger cabinets
  • May require extra spare parts if different models exist within one jurisdiction

Type 170 Controller

  • Standard layout and design
  • Many software choices
  • More easily adapted to special applications (i.e., ramp metering and Intelligent transportation Systems (ITS)).
  • Reduced spare parts inventory
  • Software and hardware compatibility problems
  • Software can be expensive
  • Liability can be greater with separate software/hardware vendors

Advanced traffic Controllers (ATC and 2070)

  • Compatible with the National Transportation Communications for ITS Protocol (NTCIP)
  • Much faster processing speeds
  • Additional phase inputs
  • Flexibility for ITs applications
  • Lack of proven software
  • Expensive
  • Current variations may not be interchangeable

In locating the controller cabinet, consider the following:

  • It should not interfere with sight lines for pedestrians or right-turning vehicles. 

  • It should be in a location that is less likely to be struck by an errant vehicle and where it does not impede pedestrian circulation, including wheelchairs and other devices that assist mobility.

  • A technician at the cabinet should be able to see the signal indications for two approaches while standing at the cabinet.

  • The cabinet should be located near the power source.

  • The cabinet location should afford ready access by operations and maintenance personnel, including consideration for where personnel would park their vehicle.

4.6 Detection Devices

The detectors (or sensors) at an intersection inform the signal controller that a vehicle, pedestrian, or bicycle is present at a defined location within the intersection or signal system.  The controller then uses this information to determine the amount of green time and the signal phases to serve. 

4.6.1 Vehicle Detection

Table 15, excerpted from the final draft of the Traffic Detector Handbook, 2003 edition, presents an overview of the strengths and weaknesses of commercially available detector technology.(55) The good performance of in-roadway detectors such as inductive loops, magnetic, and magnetometer detectors is based, in part, on their close location to the vehicle, which makes them insensitive to inclement weather due to a high signal-to-noise ratio. Their main disadvantage is their in-roadway installation, necessitating physical changes in the roadway as part of the installation process.  In addition, in-roadway detectors may be damaged or disrupted by utility cuts, pavement milling operations for resurfacing, and movement of pavement joints and cracks. Over-roadway detectors often provide data not available from in-roadway sensors, and some can monitor multiple lanes with one unit. The reader is encouraged to refer to the Traffic Detector Handbook for further discussion on detector technology.

Vehicle detectors provide advanced detection, left-turn lane presence detection, and stop-bar presence detection. Advanced detection extends a green signal to get an approaching vehicle through the signal. Left-turn lane presence detection detects left-turning vehicles that are waiting. Stop-bar presence detection will pick up any vehicles that may have entered to roadway from driveways and vehicles that might not have made it though the intersection on the previous green.

A fourth detector function is as a system detector.  On many large streets with coordinated signal systems, system detectors are used to collect midblock vehicle volume and occupancy data, which is analyzed by a master signal controller or central system to determine whether signal timing changes are needed.  The location of the system detectors varies based on the signal system and software being used, but typically they are located downstream of the intersection on the major roadway.

The location of the advanced detectors is often based on the dilemma zone boundary. The dilemma zone is that portion of the approach where a driver suddenly facing a yellow indication must make a decision whether to stop safely or to proceed through the intersection. As a result, the dilemma zone boundary is typically dictated by the minimum stopping distance. The actual distances vary by jurisdictional policies and should be reviewed before the traffic signal is designed. The typical location for advance detectors based on stopping sight distance is shown in table 16.

Table 15. Strengths and weaknesses of commercially available detector technologies.

Technology

Strengths

Weaknesses

Inductive Loop

  • Flexible design to satisfy large variety of applications
  • Mature, well understood technology
  • Large experience base
  • Provides basic traffic parameters (e.g., volume, presence, occupancy, speed, headway, and gap)
  • Insensitive to inclement weather such as rain, fog, and snow
  • Provides best accuracy for count data as compared with other commonly used techniques
  • Common standard for obtaining accurate occupancy measurements
  • High frequency excitation models provide classification data
  • Installation requires pavement cut
  • Improper installation decreases pavement life
  • Installation and maintenance require lane closure
  • Wire loops subject to stresses of traffic and temperature
  • Multiple detectors usually required to monitor a location
  • Detection accuracy may decrease when design requires detection of a large variety of vehicle classes
  • Destroyed by utility cuts or pavement milling operations

Magnetometer
(two-axis fluxgate magnetometer)

  • Less susceptible than loops to stresses of traffic
  • Insensitive to inclement weather such as snow, rain, and fog.
  • Some models transmit data over wireless radio frequency (RF) link
  • Installation requires pavement cut
  • Improper installation decreases pavement life
  • Installation and maintenance require lane closure
  • Models with small detection zones require multiple units for full lane detection

Magnetic
(induction or search coil magnetometer)

  • Can be used where loops are not feasible (e.g., bridge decks)
  • Some models are installed under roadway without need for pavement cuts, but boring under roadway is required
  • Insensitive to inclement weather such as snow, rain, and fog.
  • Less susceptible than loops to stresses of traffic
  • Installation requires pavement cut or tunneling under roadway
  • Cannot detect stopped vehicles unless special sensor layouts and signal processing software are used

Microwave Radar

  • Typically insensitive to inclement weather at the relatively short ranges encountered in traffic management applications
  • Direct measurement of speed
  • Multiple lane operation available
  • Continuous Wave (CW) doppler sensors cannot detect stopped vehicles

Active infrared (laser radar)

  • Transmits multiple beams for accurate measurement of vehicle position, speed, and class
  • Multiple-lane operation available
  • Operation may be affected by fog when visibility is less than ~6 m (20 ft) or blowing snow is present
  • Installation and maintenance, including periodic lens cleaning, require lane closure

Passive infrared

  • Multizone passive sensors measure speed
  • Passive sensor may have reduced vehicle sensitivity in heavy rain, snow, and dense fog
  • Some models not recommended for presence detection

Ultrasonic

  • Multiple-lane operation available
  • Capable of overheight vehicle detection
  • Large Japanese experience base
  • Environmental conditions such as temperature change and extreme air turbulence can affect performance; temperature compensation is built into some models
  • Large pulse repetition periods may degrade occupancy measurement on freeways with vehicles traveling at moderate to high speeds

Acoustic

  • Passive detection
  • Insensitive to precipitation
  • Multiple lane operation available in some models
  • Cold temperatures may affect vehicle count accuracy
  • Specific models are not recommended with slow moving vehicles in stop-and-go traffic

Video Image Processor

  • Monitors multiple lanes and multiple detection zones/lanes
  • Easy to add and modify detection zones
  • Rich array of data available
  • Provides wide-area detection when information gathered at one camera location can be linked to another
  • Installation and maintenance, including periodic lens cleaning, require lane closure when camera is mounted over roadway (lane closure may not be required when camera is mounted at side of roadway)
  • Performance affected by inclement weather such as fog, rain, and snow; vehicle shadows; vehicle projection into adjacent lanes; occlusion; day-to-night transition; vehicle/road contrast; and water, salt grime, icicles, and cobwebs on camera lens
  • Requires 15- to 21-m (50- to 70-ft) camera mounting height (in a side-mounting configuration) for optimum presence detection and speed measurement
  • Some models susceptible to camera motion caused by strong winds or vibration of camera mounting structure
  • Generally cost-effective when many detection zones within the camera field-of-view or specialized data are required

Source: Adapted from reference 55.

Table 16. Location of advanced vehicle detectors.

Speed

Calculated Stopping Distance

Single Detector Setback

Multiple Detector Setback

10% Probability of Stopping

90% Probability of Stopping

33 km/h (20 mph)

  22.0 m   (72.2 ft)

21 m   (70 ft)

-

-

40 km/h (25 mph)

  31.8 m (104.4 ft)

32 m (105 ft)

-

-

48 km/h (30 mph)

  42.9 m (140.8 ft)

43 m (140 ft)

-

-

56 km/h (35 mph)

  55.7 m (182.9 ft)

56 m (185 ft)

31 m (102 ft)

  77 m (254 ft)

64 km/h (40 mph)

  70.4 m (231.0 ft)

70 m (230 ft)

37 m (122 ft)

  87 m (284 ft)

72 km/h (45 mph)

  86.5 m (283.8 ft)

*

46 m (152 ft)

100 m (327 ft)

80 km/h (50 mph)

104.2 m (341.9 ft)

*

52 m (172 ft)

108 m (353 ft)

88 km/h (55 mph)

123.8 m (406.3 ft)

*

71 m (234 ft)

118 m (386 ft)

*  Use multiple detectors or volume-density modules.

Source:  (Reference 56 (table 7-1); reference 57 (table 4-3); metric values converted from U.S. customary provided in sources)

As shown in table 16, the stopping distance can be computed for both the average stopping condition as well as the probability ranges for stopping.  For most large intersections, a multiple-loop design should be used to account for the higher speeds and probabilities of stopping. More detailed information on detector placement, including the results of several calculation methods, can be found in the Manual of Traffic Detector Design.(58)

4.6.2 Pedestrian Detection

Pedestrian detection at actuated signals is typically accomplished through the use of pedestrian push buttons. Accessible pedestrian signal detectors, or devices to help pedestrians with visual or mobility impairments activate the pedestrian phase, may be pushbuttons or other passive detection devices. For pushbuttons to be accessible, they should be placed in accordance with the guidance in the MUTCD and located as follows (sections 4E.08 and 4E.09):(1)

  • Adjacent to a level all-weather surface to provide access from a wheelchair with a wheelchair-accessible route to the ramp.

  • Within 1.5 m (5 ft) of the crosswalk extended.

  • Within 3 m (10 ft) of the edge of the curb, shoulder, or pavement.

  • Parallel to the crosswalk to be used.

  • Separated from other pushbuttons by a distance of at least 3 m (10 ft).

  • Mounted at a height of approximately 1.1 m (3.5 ft) above the sidewalk.

Alternative methods of pedestrian detection, including infrared and microwave detectors, are emerging. Additional information on these devices can be found in FHWA’s Pedestrian Facilities User Guide—Providing Safety and mobility.(35)

4.7 Basic Signal Timing Parameters

Signal operation and timing have a significant impact on intersection performance. Controllers have a vast array of inputs that permit tailoring of controller operation to the specific intersection. This section provides guidance for the determination of basic timing parameters.

The development of a signal timing plan should address all user needs at a particular location including pedestrians, bicyclists, transit vehicles, emergency vehicles, automobiles, and trucks.  For the purposes of this section, signal timing is divided into two elements: pedestrian timing and vehicle timing. 

4.7.1 Pedestrian Timing

Pedestrian timing requirements include a WALK interval and a flashing DON’T WALK interval. The WALK interval varies based upon local agency policy. The MUTCD recommends a minimum WALK time of 7 s, although WALK times as low as 4 s may be used if pedestrian volumes and characteristics do not require an interval of 7 s (section 4E.10).(1) The WALK interval gives pedestrians adequate time to perceive the WALK indication and depart the curb before the clearance interval (flashing DON’T WALK) begins.

In downtown areas, longer WALK times are often appropriate to promote walking and serve pedestrian demand. School zones and areas with large numbers of elderly pedestrians also warrant consideration and the display of WALK time in excess of the minimum WALK time.

The MUTCD states that the pedestrian clearance time should allow a pedestrian crossing in the crosswalk to leave the curb and travel to at least the far side of the traveled way or to a median of sufficient width for pedestrians to wait before opposing vehicles receive a green indication. The MUTCD uses a walk speed of 1.2 m/s (4.0 ft/s) for determining crossing times.(1) However, the Pedestrian Facilities Users Guide recommends a lower speed of 1.1 m/s (3.5 ft/s); see chapter 2 for further discussion.(35) Pedestrian clearance time is calculated using equation 1:

 
Pedestrian clearance time equals crossing distance divided by walking speed.
(1)

 

where: Pedestrian Clearance time is in seconds
Crossing Distance is measured from the near curb to at least the far side of the traveled way or to a median; and
Walking Speed is typically 1.2 m/s (4 ft/s) or 1.1 m/s (3.5 ft/s) as indicated above.

Pedestrian clearance time is accommodated during either a combination of flashing DON’T WALK time and yellow clearance time or by flashing DON’T WALK time alone. The recommended practice is for the pedestrian clearance time to be accommodated completely within the flashing DON’T WALK time. However, at high-volume locations, it may be necessary as a tradeoff for vehicular capacity to use the yellow change interval as part of satisfying the calculated pedestrian clearance time.

4.7.2 Vehicle timing—Green interval

Ideally, the length of the green display should be sufficient to serve the demand present at the start of the green phase for each movement and should be able to move groups of vehicles, or platoons, in a coordinated system.  At an actuated intersection, the length of the green interval varies based on inputs received from the detectors.  Minimum and maximum green times for each phase are assigned to a controller to provide a range of allowable green times.  Detectors are used to measure the amount of traffic and determine the required time for each movement within the allowable range.

The minimum green time is the amount of time allocated to each phase so that vehicles in queue at the stop bar are able to start and clear the intersection. The minimum initial green time is established by determining the time needed to clear the vehicles located between the stop bar and the detector nearest the stop bar. Where presence detection is installed at the stop bar, a minimum interval may be set to a value that is less than 1.0 s.

Consider an intersection with the following properties: average vehicle spacing is 7.5 m (25 ft) per vehicle, initial start-up time is 2 s, and vehicle headway is 2 s per vehicle.  For an approach with a detector located 30 m (100 ft) from the stop bar, the minimum green time is 2 + (30 m/7.5 m x 2) = 2 + (100 ft/25 ft x 2) = 10 s. 

The maximum green time is the maximum limit to which the green time can be extended for a phase in the presence of a call from a conflicting phase. The maximum green time begins when a call is placed on a conflicting phase. The phase is allowed to "max-out" if the maximum green time is reached even if actuations have been received that would typically extend the phase.

4.7.3 Vehicle timing—Detector Timing

One advantage of actuated control is that it can adjust timing parameters based on vehicle or pedestrian demand. The detectors and the timing parameters allow the signal to respond to varied flow throughout the day. For pedestrians, detectors are located for convenient access; for vehicles, detector spacing is a function of travel speed and the characteristics of the street. The operation of the signal is highly dependent on detector timing. More information about detector timing, including settings for various detector configurations, is found in the FHWA Traffic Detector Handbook.(55)

One type of detector timing, known as volume-density timing, uses gap timers to reduce the allowable gap time the longer the signal is green. This type of timing makes the signal less likely to extend the green phase the longer the signal is green. A typical setting for a volume-density controller is to have the passage gap set to twice the calculated gap time to ensure the phase does not gap out too early. The minimum gap time might be set to less than the calculated gap time on multiple lane approaches, depending on the characteristics of the intersection.

Signal timing parameters may provide an opportunity to maximize the efficiency of the intersection. Signal timing parameters control how quickly the phase ends once traffic demand is no longer present. The one phase that is the exception is the coordinated phase, which receives the unused or additional time.

4.7.4 Vehicle timing—Vehicle Clearance

The vehicle clearance interval consists of the yellow change and red clearance intervals. The recommended practice for computing the vehicle clearance interval is the ITE formula (reference 56, equation 11-4), given in equation 2 (to use with metric inputs, use 1 m = 0.3048 ft):

 
Change period equals perception-reaction time of the motorist plus the quotient of the speed of the approaching vehicle in feet per second divided by the sum of 2 times the comfortable deceleration rate of the vehicle in feet per second squared plus 64.4 times the grade of the intersection approach (percent) (positive for upgrade, negative for downgrade), plus the quotient of the sum of the width of the intersection from curb to curb in feet and the length of the vehicle in feet, divided by the speed of the approaching vehicle in feet per second. (U.S. Customary)             
(2)

 

where:   CP = change period (s)
t = perception-reaction time of the motorist (s); typically 1
V = speed of the approaching vehicle (ft/s)
a = comfortable deceleration rate of the vehicle (ft/s2); typically 10 ft/s2
W = width of the intersection, curb to curb (ft)
L = length of vehicle (ft); typically 20 ft
g = grade of the intersection approach (%); positive for upgrade, negative for downgrade

For change periods longer than 5 s, a red clearance interval is typically used. Some agencies use the value of the third term as a red clearance interval. The MUTCD does not require specific yellow or red intervals but provides guidance that the yellow change interval should be approximately 3 s to 6 s and that the red clearance interval should not exceed 6 s (section 4D.10).(1) Note that because high-volume signalized intersections tend to be large and frequently on higher speed facilities, their clearance intervals are typically on the high end of the range. These longer clearance intervals increase loss time at the intersection and thus reduce capacity.

The topic of yellow and red clearance intervals has been much debated in the traffic engineering profession. At some locations, the yellow clearance interval is either too short or set improperly due to changes in posted speed limits or 85th-percentile speeds.  This is a common problem and frequently causes drivers to brake hard or to run through the intersection during the red phase. Because not all States follow the same law with regard to what is defined as "being in the intersection on the red phase," local practice for defining the yellow interval varies considerably. For this reason, red light photo enforcement should not be used during the period of red clearance required by the ITE formula.

Current thought is that longer clearance intervals will cause drivers to enter the intersection later and will breed disrespect for the traffic signal. Wortman and Fox conducted a study that showed that the time of entry of vehicles into the intersection increased due to a longer yellow interval.(59) Additional research is needed to examine the effect of lengthening the yellow interval on driver behavior. 

4.7.5 Vehicle timing—Cycle Length

For isolated, actuated intersections, cycle length varies from cycle to cycle based on traffic demand and signal timing parameters. For coordinated intersections, a background cycle length is used to achieve consistent operation between consecutive intersections. In general, shorter cycle lengths are preferable to longer ones because they result in less delay and shorter queues. However, the need to accommodate multiple pedestrian movements across wide roadways, coupled with complex signal phasing and minimum green requirements to accommodate signal progression in multiple directions, may sometimes require the use of even longer cycle lengths. Wherever possible, such use should be limited to peak traffic periods only.

In general, it is preferred that the cycle lengths for conventional, four-legged intersections not exceed 120 s, although larger intersections may require longer cycle lengths. Longer cycle lengths generally result in increased delay and queues to all users, particularly minor movements. There may also be a connection between longer cycle lengths and increased incidence of red-light running, although this has not been documented in research. Although longer cycle lengths result in fewer change periods per hour and thus fewer opportunities for red-light running, more drivers may be tempted to run the red light to avoid the extra delay caused by the longer cycle length.(60)

4.8 Signing and Pavement Marking Design

Signs and pavement markings are important elements of the design of an intersection. Because of the complexity of driver decisions, particularly at large signalized intersections, special attention to signing and pavement markings can maximize the safety and efficiency of the intersection. At signalized intersections, these traffic control devices serve several key functions, including:

  • Advance notice of the intersection.

  • Directional route guidance.

  • Lane use control, including indications of permissive or prohibited turning movements.

  • Regulatory control of channelized right turn movements (e.g., through the use of YIELD signs).

  • Delineation and warning of pedestrian crossing locations.

  • Delineation and warning of bicycle lane locations.

The FHWA’s MUTCD(1) is the primary reference for use in the design and placement of signs and pavement markings. Additional resources include state supplements to the MUTCD and reference materials such as ITE s Traffic Control Devices Handbook (TCDH)(61) and Traffic Signing Handbook.(62)

Designing effective signing and pavement marking at high-volume signalized intersections in particular often requires thinking beyond standard drawings of typical sign and pavement marking layouts at intersections. High-volume signalized intersections typically have more lanes than most intersections.  They may have redirected or restricted turning movements.  They often join two or more designated routes (e.g., State highways) that require directional guidance to the user. They are also frequently in urban areas where other intersections, driveways, and urban land use create visibility conflicts. The following questions, adapted from the ITETraffic Signing Handbook(62), represent a basic thought process that is recommended for engineers to follow when developing a sign layout at an intersection:

1. From a given lateral and longitudinal position on the roadway, what information does the user need, both in advance and at the intersection? At signalized intersections, is information on lane use at the intersection provided? Is advance street name information (“XX Street, Next Signal,” etc.) and (if appropriate) route number directional signage provided in advance of the intersection? Figure 38 gives an example of a simple advance street name sign on approach to an intersection, and figure 39 gives an example of an advance sign that provides street names for the next two signalized intersections.

The picture shows a simple advance street name sign that reads "Madison next signal" and warns that the red light is photo enforced.
Figure 38. Example of advance street name sign for upcoming intersection.

The picture shows a large advance sign that gives street names for the next two signalized intersections: the next signal is Gaither Road, and the second signal is Comprint Court.
Figure 39. Example of advance street name sign for two closely spaced intersections.

2. Are there any on- or off-road conditions that would violate driver expectancy? Lane drops, trap lanes, and right-hand exits for left turns are all examples where driver expectancy is violated and should be addressed by signing. Figure 40 shows an example of signage used to advise motorists of a trap lane.

The picture shows a car in the left lane and a sign in the median that reads, “left lane must turn left at Democracy Boulevard.”
Figure 40. Example of signing for a left-hand lane trap.

3. Is a specific action required by a road user? If the road user needs to be in an appropriate lane in advance of an intersection to make a movement at the intersection, signage is needed to convey this message to the user. Figure 41 provides an example of an overhead signs used to assist drivers in selecting the proper lane on approach to a signalized intersection.

The picture shows an overhead sign where the leftmost lane is a turn lane labeled Frontage Road, the next two left lanes are illinois West 56, I-88 West, and I-355 (a toll interstate). The three through lanes are Highland Avenue, and the right lane is illinois 56 East.
Figure 41. Example of advance overhead signs indicating lane use for various destinations.

4. Are signs located so that the road user will be able to see, comprehend, and attend to the intended message? Signs must be simple enough to be easily comprehended and attended to before the driver receives the next message. This requires adequate sign size, sign spacing, and attention to the number of elements on each sign. This may, for example, lend itself to the use of overhead signs in advance of large intersections, as well as large retroreflectorized or internally illuminated overhead signs (including street name signs) at intersections.

5. For what part of the driver population is the sign being designed? Have the needs of older drivers or nonlocal drivers been accommodated? This may require the use of larger lettering or sign illumination.

6. Does the sign “fit in” as part of the overall sign system? Signing at an intersection needs to be consistent with the overall sign layout of the connecting road system. For example, the consistent use of guide signs is helpful to freeway users in identifying the appropriate exit. Similar consistency is needed on arterial streets with signalized intersections.

Pavement markings also convey important guidance, warning, and regulatory lane-use information to users at signalized intersections. In addition to delineating lanes and lane use, pavement markings clearly identify pedestrian crossing areas, bike lanes, and other areas where driver attention is especially important.  Where in-pavement detection is installed for bicycles and motorcycles, appropriate markings should be painted to guide these vehicles over the portion of the loop that will best detect them.

Several supplemental pavement markings are particularly useful at large signalized intersections. For example, the use of lane line extensions into the intersection can be a helpful tool where the intersection is so large that the alignment of through or turning lanes between entering the intersection and exiting the intersection could be confused. This can occur, for example, where multiple turn lanes are provided, where the through lane alignments make a curve through the intersection, or where the receiving lanes at an intersection are offset laterally from the approach lanes. In addition, pavement legends indicating route numbers and/or destinations in advance of the intersection (i.e., “horizontal signage”) may be used to supplement signing for this purpose, as shown in figure 42.

The picture shows an approach with pavement markings that read “to (symbolic I-4) East” in the rightmost through lane, “to (symbolic I-4) West” in the leftmost through lane, and “yield to peds” in the left-turn lane.
Figure 42. Example of pavement legends indicating destination route numbers (“horizontal signage”)

4.9 Illumination Design

As noted in American National Standard Practice for Roadway Lighting (RP-8-00), “[t]he principal purpose of roadway lighting is to produce quick, accurate, and comfortable visibility at night. These qualities of visibility may safeguard, facilitate, and encourage vehicular and pedestrian traffic…[T]he proper use of roadway lighting as an operative tool provides economic and social benefits to the public including:

(a) Reduction in night accidents, attendant human misery, and economic loss.

(b) Aid to police protection and enhanced sense of personal security.

(c) Facilitation of traffic flow.

(d) Promotion of business and the use of public facilities during the night hours.”(63, p.1)

Specifically with respect to intersections, the document notes that “[s]everal studies have identified that the primary benefits produced by lighting of intersections along major streets is the reduction in night pedestrian, bicycle and fixed object accidents.” (section 3.6.2)(63) With respect to signalized intersections, roadway lighting can play an important role in enabling the intersection to operate at its best efficiency and safety. The highest traffic flows of the day (typically the evening peak period) may occur during dusk or night conditions where lighting is critically important, particularly in winter for North American cities in northern latitudes.

The document includes three different criteria for roadway lighting: illuminance, luminance, and small target visibility (STV). These are described as follows:

  • Illuminance is the amount of light incident on the pavement surface from the lighting source. 

  • Luminance is the amount of light reflected from the pavement toward the driver’s eyes. The luminance criterion requires more extensive evaluation.  Because the reflectivity of the pavement surfaces constantly changes over time, it is difficult to accurately estimate this criterion.

  • Small target visibility is the level of visibility of an array of targets on the roadway. The STV value is determined by the average of three components: the luminance of the targets and background, the adaptation level of adjacent surroundings, and the disability glare.

4.9.1 Illuminance

The two principal measures used in the illuminance method are light level and uniformity ratio. Light level represents the intensity of light output on the pavement surface and is reported in units of lux (metric) or footcandles (U.S. Customary).  Uniformity represents the ratio of either the average-to-minimum light level (Eavg/Emin) or the maximum-to-minimum light level (Emax/Emin) on the pavement surface.  The light level and uniformity requirements are dependent on the roadway classification and the level of pedestrian night activity.

The basic principle behind the lighting of intersections is that the amount of light on the intersection should be proportional to the classification of the intersecting streets and equal to the sum of the values used for each separate street. For example, if Street a is illuminated at a level of x and Street B is illuminated at a level of y, the intersection of the two streets should be illuminated at a level of x+y. RP-8-00 also specifies that if an intersecting roadway is illuminated above the recommended value, then the intersection illuminance value should be proportionately increased. If the intersection streets are not continuously lighted, a partial lighting system can be used. RP-8-00 and its annexes should be reviewed for more specific guidance on partial lighting, the specific calculation methods for determining illuminance, and guidance on the luminance and STV methods.(63)

Table 17 presents the recommended illuminance for the intersections within the scope of this document located on continuously illuminated streets. Separate values have been provided for portland cement concrete road surfaces (RP-8-00 Road Surface Classification R1) and typical asphalt concrete road surfaces (RP-8-00 Road Surface Classification R2/R3).

Table 18 presents the roadway and pedestrian area classifications used for determining the appropriate illuminance levels in table 17. RP-8-00 clarifies that although the definitions given in table 18 may be used and defined differently by other documents, zoning bylaws, and agencies, the area or roadway used for illumination calculations should best fit the descriptions contained in table 18 (section 2.0, p. 3).(63)

4.9.2 Veiling Luminance

Veiling luminance is produced by stray light from light sources within the field of view. This stray light is superimposed in the eye on top of the retinal image of the object of interest, which alters the apparent brightness of that object and the background in which it is viewed. This glare, known as disability glare, reduces a person’s visual performance and thus must be considered in the design of illumination on a roadway or intersection (annex C).(63) Table 17 shows the maximum veiling luminance required for good intersection lighting design.

Table 17. Recommended illuminance for the intersection of continuously lighted urban streets.

Pavement Classification2

Roadway Classification

Average Maintained Illuminance at Pavement1

Uniformity Ratio
(Eavg/Emin)3

Veiling Luminance Ratio (Lvmax/Lavg)4

Pedestrian/Area Classification

High
(lux (fc))

Medium
(lux (fc))

Low
(lux (fc))

R1

Major/Major

24.0 (2.4)

18.0 (1.8)

12.0 (1.2)

3.0

0.3

Major/Collector

20.0 (2.0)

15.0 (1.5)

10.0 (1.0)

3.0

0.3

Major/Local

18.0 (1.8)

14.0 (1.4)

9.0 (0.9)

3.0

0.3

Collector/Collector

16.0 (1.6)

12.0 (1.2)

8.0 (0.8)

4.0

0.4

Collector/Local

14.0 (1.4)

11.0 (1.1)

7.0 (0.7)

4.0

0.4

Local/Local

12.0 (1.2)

10.0 (1.0)

6.0 (0.6)

6.0

0.4

R2/R3

Major/Major

34.0 (3.4)

26.0 (2.6)

18.0 (1.8)

3.0

0.3

Major/Collector

29.0 (2.9)

22.0 (2.2)

15.0 (1.5)

3.0

0.3

Major/Local

26.0 (2.6)

20.0 (2.0)

13.0 (1.3)

3.0

0.3

Collector/Collector

24.0 (2.4)

18.0 (1.8)

12.0 (1.2)

4.0

0.4

Collector/Local

21.0 (2.1)

16.0 (1.6)

10.0 (1.0)

4.0

0.4

Local/Local

18.0 (1.8)

14.0 (1.4)

8.0 (0.8)

6.0

0.4

Notes: 1 fc = footcandles

           2 R1 is typical for portland cement concrete surface; R2/R3 is typical for asphalt surface.

           3 Eavg/Emin = Average illuminance divided by minimum illuminance

           4 Lvmax/Lavg = Maximum veiling luminance divided by average luminance.

Source: Reference 63, table 9 (for R2/R3 values); R1 values adapted from table 2.

Table 18.  RP-8-00 guidance for roadway and pedestrian/area classification
for purposes of determining intersection illumination levels

Roadway Classification

Description

Average Daily Vehicular Traffic Volumes (ADT)1

Major

That part of the roadway system that serves as the principal network for through-traffic flow. The routes connect areas of principal traffic generation and important rural roadways leaving the city. Also often known as “arterials,” thoroughfares,” or “preferentials.”

More than
3,500

Collector

Roadways servicing traffic between major and local streets. These are streets used mainly for traffic movements within residential, commercial, and industrial areas. They do not handle long, through trips.

1,500 to 3,500

Local

Local streets are used primarily for direct access to residential, commercial, industrial, or other abutting property.

100 to 1,500

Pedestrian Conflict area Classification

Description

Possible Guidance on Pedestrian Traffic Volumes2

High

Areas with significant numbers of pedestrians expected to be on the sidewalks or crossing the streets during darkness. Examples are downtown retail areas, near theaters, concert halls, stadiums, and transit terminals.

More than 100 pedestrians/hour

Medium

Areas where lesser numbers of pedestrians use the streets at night. Typical are downtown office areas, blocks with libraries, apartments, neighborhood shopping, industrial, older city areas, and streets with transit lines.

11 to 100 pedestrians/hour

Low

Areas with very low volumes of night pedestrian usage. These can occur in any of the cited roadway classifications but may be typified by suburban single family streets, very low density residential developments, and rural or semirural areas.

10 or fewer pedestrians/hour

Notes: 1 For purposes of intersection lighting levels only.

           2 Pedestrian volumes during the average annual first hour of darkness (typically 18:00-19:00), representing the total number of pedestrians walking on both sides of the street plus those crossing the street at non-intersection locations in a typical block or 200 m (656 ft) section. RP-8-00 clearly specifies that the pedestrian volume thresholds presented here are a local option and should not be construed as a fixed warrant.

Source: Reference 63, sections 2.1, 2.2, and 3.6


Part II

Project Process and Analysis Methods

Part II describes the key elements of a typical project process (chapter 5) from project initiation to implementation and monitoring.  Part II also includes a description of safety analysis methods (chapter 6) and operational analysis methods (chapter 7) that can be used in the evaluation of a signalized intersection.  The chapters in part II provide the reader with the tools needed to determine deficiencies of a signalized intersection and areas for improvement and mitigation.  The findings from part II should be used to identify applicable treatments in part III.

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