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

Signalized Intersections: Informational Guide

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CHAPTER 9 — INTERSECTION-WIDE TREATMENTS

TABLE OF CONTENTS

9.0 INTERSECTION-WIDE TREATMENTS

9.1 Pedestrian Treatments

9.1.1 Reduce Curb Radius

9.1.2 Provide Curb Extensions

9.1.3 Modify Stop Bar Location

9.1.4 Improve Pedestrian Signal Displays

9.1.5 Modify Pedestrian Signal Phasing

9.1.6 Grade-Separate Pedestrian Movements

9.2 Bicycle treatments

9.2.1 Provide Bicycle Box

9.2.2 Provide Bike Lanes

9.3 Transit treatments

9.3.1 Relocate transit Stop

9.4 Traffic Control Treatments

9.4.1 Change signal Control from Pre-Timed to Actuated

9.4.2 Modify Yellow Change interval and/or Red Clearance interval

9.4.3 Modify Cycle Length

9.4.4 Late Night/Early Morning Flash Removal

9.5 Street Lighting and Illumination

9.5.1 Provide or Upgrade illumination

 

LIST OF FIGURES

67
A curb radius from 4.6 m (15 ft) to 15.2 m (50 ft) increases the pedestrian crossing distance from 18.9 m (62 ft) to 30.5 m (100 ft), all else being equal
68
Intersection with curb extension
69
Examples of countdown and animated eyes pedestrian signal displays
70
A pedestrian grade separation treatment
71
Typical lighting layouts

 

LIST OF TABLES

44
Summary of issues for curb radius reduction
45
Summary of issues for curb extensions
46
Summary of issues for stop bar alterations
47
Safety benefits associated with addition of pedestrian signals: Selected findings
48
Summary of issues for pedestrian signal display improvements
49
Summary of issues for pedestrian signal phasing modifications
50
Summary of issues for pedestrian grade separation
51
Summary of issues for providing a bicycle box
52
Summary of issues for providing bicycle lanes
53
Summary of issues for near-side/far-side transit stops
54
Safety benefits associated with upgrading an intersection from pre-timed to actuated operation: Selected findings
55
Summary of issues for providing signal actuation
56
Safety benefits associated with modifying clearance intervals: Selected findings
57
Summary of issues for extending yellow/red clearance intervals
58
Summary of issues for cycle length modifications
59
Safety benefits associated with removal of signal from late night/early morning flash mode: Selected findings
60
Summary of issues for flash mode removal
61
Safety benefits associated with providing illumination: Selected findings
62
Summary of issues for providing illumination

9. Intersection-wide Treatments

This chapter discusses five groups of intersection-wide treatments:

  • Pedestrian treatments.
  • Bicycle treatments.
  • Transit treatments.
  • Traffic control treatments.
  • Illumination.

9.1 Pedestrian Treatments

Accommodation of pedestrians has a significant effect on both the design and operations of a signalized intersection and should therefore be an integral part of the design process. Key factors to consider are:

  • Crossing locations with a high number of pedestrians should be protected (where possible) from conflicting through traffic.
  • Crossing distances should be minimized as much as possible.
  • Adequate crossing time needs to be provided.
  • Pedestrian ramps need to be located within the crosswalk; diagonal ramps are discouraged.
  • Pedestrian ramp location and design must meet ADA requirements.

One common way to better accommodate pedestrians and improve their safety is to reduce their crossing distance. Reducing crossing distance decreases a pedestrian's exposure to traffic, which may be particularly helpful to pedestrians who are disabled or elderly. It also reduces the amount of time needed for the pedestrian phase, which reduces the delay for all other vehicular and pedestrian movements at the intersection. Three common methods of reducing pedestrian crossing distance are:

  • Curb radius reduction.
  • Curb extensions.
  • Provision of median crossing islands.

Traffic engineers have also made modifications to the location of the stop bar and crosswalk to try to control where motorists stop on the intersection approach and where pedestrians cross.

Traffic control improvements directly applicable to pedestrians include:

  • Improving the signal display to the pedestrian through the use of redundancy, including the use of pedestrian signals, accessible pedestrian signals, and enhancements to the pedestrian signal display.
  • Modifying the pedestrian signal phasing.

Each of these treatments is discussed in the following sections; median crossing islands were addressed in chapter 8.

9.1.1 Reduce Curb Radius

Description

Curb radius reduction has been suggested in research because one common pedestrian crash is when right-turning vehicles at intersections strike pedestrians. A wide curb radius typically results in high-speed turning movements by motorists. Existing guidelines recommend reconstructing the turning radius to a tighter turn to reduce turning speeds, shorten the crossing distance for pedestrians, and improve sight distance between pedestrians and motorists. Figure 67 demonstrates that increasing the curb radius increases pedestrian crossing distance. Tighter turning radii are most important where street intersections are not at right angles.(101)

Click for full description

Click for full description

Click for full description

Figure 67. A curb radius increase from 4.6 m (15 ft) to 15.2 m (50 ft) increases the pedestrian crossing distance from 18.9 m (62 ft) to 30.5 m (100 ft), all else being equal.

Applicability

Reducing the curb radii is an appropriate consideration wherever there are pedestrians. Small curb radii facilitate the use of two perpendicular curb ramps rather than a single diagonal ramp (see chapter 3 for further discussion). Note that the need to accommodate the design vehicle may limit how much the curb radius can be reduced. Depending on State vehicle code, it may be acceptable to allow large vehicles to turn right into the second lane (the lane next to the curb lane).

Safety Performance

Reducing the curb radius lowers the speed of right-turning vehicles. It is expected that the frequency of pedestrian-vehicle collisions will be reduced as a result, and any remaining collisions will be of a lesser severity due to the lower speeds involved. However, vehicles turning right will be forced to decelerate more rapidly in attempting the right turn. This may lead to rear-end conflicts with through vehicles, particularly if a separate right-turn lane is not provided and the through movements have high speeds.

Operational Performance

Reducing pedestrian crossing distance with smaller curb radii reduces the amount of time needed to serve the pedestrian clearance time. This may result in shorter cycle lengths and less delay for all users. However, a curb radius reduction may reduce the capacity of the affected right-turn movement.

Multimodal Impacts

Pedestrians benefit from a shorter crossing distance and the reduced speed of right-turning vehicles.

Larger vehicles and transit may have difficulty negotiating the tighter corner, either swinging out too far into the intersection or having their rear wheels ride up over the curb onto the sidewalk. Caution should be exercised in reducing curb radius if right-turning large trucks or buses are frequent users. It may be necessary to move the stopbar locations on the roadway the trucks are turning into to allow them to briefly swing wide into the opposing lanes.

Physical Impacts

Reducing the curb radius reduces the size of the intersection and allows for additional space for landscaping or pedestrian treatments. Traffic signal equipment may need to be relocated.

Socioeconomic Impacts

Depending on the degree of improvement, low to moderate construction costs will be associated with the reconstruction of the curb radius.

Enforcement, Education, and maintenance

The effectiveness of this treatment may be enhanced by police enforcement of drivers failing to come to a complete stop on a red signal when making a right turn and/or not yielding to pedestrians in the crosswalk.

Summary

Table 44 summarizes issues associated with curb radius reduction.

Table 44. Summary of issues for curb radius reduction.

Characteristic

Potential benefits

Potential Liabilities

Safety

Reduction in right-turning vehicle/pedestrian collisions.

Fewer right-turn-on-red violations.

May increase right-turning/through vehicle rear-end collisions due to increased speed differential.

Large vehicle off-tracking.

Operations

Less overall delay due to reduced time needed to serve pedestrian movement.

Reduction in capacity for affected right-turn movement.

Multimodal

Shorter crossing distance.

Facilitates the use of two perpendicular ramps rather than a single diagonal ramp.

May be more difficult for large trucks and buses to turn right.

Physical

Reduces the size of the intersection.

None identified.

Socioeconomic

Low to moderate costs.

None identified.

Enforcement, Education, and maintenance

None identified.

Enforcement of yielding to pedestrians may be necessary.

 

9.1.2 Provide Curb Extensions

Description

Curb extensions, also known as "bulbouts" or "neckdowns," involve extending the sidewalk or curb line into the street, reducing the effective street width. These are often used for traffic calming on neighborhood streets, but the technique is applicable for higher volume signalized intersections. Curb extensions improve the visibility of the pedestrian crosswalk. They reduce the amount of roadway available for illegal or aggressive motorist activities such as failing to yield to pedestrians, making high-speed turns, and passing in the parking lane. It has also been observed that motorists are more inclined to stop behind the crosswalk at a curb extension, and that pedestrians are more inclined to wait on the curb extension than in the street. An example of a curb extension is shown in figure 68.

Application

This treatment would be applicable to urban intersections with heavy pedestrian traffic and a high number of pedestrian collisions. It would not be appropriate at high-speed rural intersections, and caution should be used at intersections with a high proportion of right-turning movements. Curb extensions can be used to terminate parking lanes; care should be exercised if they are used to terminate travel lanes.

The earlier observations on reduced curb radius also apply here.

Figure 68. Intersection with curb extension. Photo. The picture shows a curb extension that narrows the street width.
Photograph Credit: Synectics Transportation Consultants, Inc.

Figure 68. Intersection with curb extension.

Safety Performance

By reducing the pedestrian crossing distance and subsequent exposure of pedestrians to traffic, this treatment should reduce the frequency of pedestrian collisions. A New York City study suggested that curb extensions appear to be associated with lower frequencies and severities of pedestrian collisions.(102) Curb extensions should also reduce speeds on approaches where they are applied.

Operational Performance

The operational performance effects of curb extensions are similar to those for reduced curb radii. The reduction in pedestrian crossing distance reduces the amount of time needed to serve the pedestrian clearance time. This may result in shorter cycle lengths and less delay for all movements. However, the reduced curb radius resulting from the curb extension may reduce the capacity of the affected right-turn movement. If a right-turn lane is present, the curb radius reduction should not impede through movements.

Because curb extensions are essentially a traffic-calming treatment, they will likely reduce speeds and may possibly divert traffic to other roads; right-turn movements would be particularly affected by this treatment. Emergency services (fire, ambulance, and police) should be consulted if this treatment is being considered.

Multimodal Impacts

Pedestrians benefit greatly from the provision of curb extensions. The curb extension can greatly improve the visibility between pedestrians and drivers. In addition, the reduction in pedestrian crossing distance reduces pedestrian exposure and crossing time.

Bicycle movements and interactions with motor vehicles need to be considered in the design of any curb extensions.

Caution should be used if this treatment is being considered along heavy truck routes. All types of trucks and transit, in particular those needing to turn right at the intersection, would be negatively affected by this treatment.

Physical Impacts

Drainage should be evaluated whenever curb extensions are being considered, as the curb extension may interrupt the existing flow line.

Socioeconomic Impacts

Costs associated with this improvement would be low to moderate.

Enforcement, Education, and maintenance

No specific effects have been identified.

Summary

Table 45 provides a summary of the issues associated with curb extensions.

Table 45. Summary of issues for curb extensions.

Characteristic

Potential benefits

Potential Liabilities

Safety

Reduction in right-turning vehicle/pedestrian collisions.

Fewer right-turn-on-red violations.

May increase right-turning/through vehicle rear- end collisions due to increased speed differential.

Large vehicle offtracking.

Operations

Less overall delay due to reduction in time needed to serve pedestrian movement.

May adversely affect operation if curb extension replaces a travel lane.

Right-turn movements delayed.

Emergency vehicles may be significantly delayed.

Multimodal

Shorter crossing distance.

Facilitates the use of two perpendicular ramps rather than a single diagonal ramp.

Better visibility between pedestrians and drivers.

May be more difficult for large trucks and buses to turn right.

Physical

None identified.

Drainage may be adversely affected.

Socioeconomic

Low to moderate costs.

None identified.

Enforcement, Education, and maintenance

None identified.

None identified.

9.1.3 Modify Stop Bar Location

Description

Visibility is a key consideration for determining the location of stop bars. The FHWA Pedestrian Facilities Users Guide-Providing Safety and mobility suggests the use of advance stop lines as a possible countermeasure.(35) At signalized pedestrian crossing locations, the vehicle stop line can be moved 5 to 10 m (15 to 30 ft) further back from the pedestrian crossing than the standard 1.2 m (4 ft) distance to improve visibility of through cyclists and crossing pedestrians for motorists (and particularly truck drivers) who are turning right. Advanced stop lines benefit pedestrians, as the pedestrians and drivers have a clearer view and more time to assess each other's intentions when the signal phase changes.

Applicability

Relocating the stop bar may be applicable to intersections with a high number of right-turn-on-red vehicle/pedestrian collisions.

One reference has recommended that marked crosswalks alone are insufficient in situations:

  • Where the speed limits exceeds 64.4 km/h (40 mph).
  • On roadways with four or more lanes without a raised median or crossing island that has (or will soon have) an ADT of 12,000 or greater.

  • On roadways with four or more lanes with a raised median or crossing island that has (or will soon have) an ADT of 15,000 or greater.(35)

Safety Performance

One evaluation study found that advance stop lines resulted in reducing right-turn-on-red conflicts with cross traffic; more right-turn-on-red vehicles also make a complete stop behind the stop line. Another study determined that stop line relocation resulted in better driver compliance with the new location and increased elapsed time for lead vehicles entering the intersection. This may decrease the risk of pedestrian collisions involving left-turning vehicles.(101,103,104) However, placing the crosswalk at least 3 m (10 ft) or more from the cross-street flow line or curb also provides more time to drivers to react for the presence of pedestrian crossing on the street they are about to enter.(105)

Operational Performance

Advance stop lines increase the clearance time for vehicles passing through the intersection. As a result, there may be an increase in loss time.

Multimodal Impacts

Advance stop lines keep the opposing lanes at intersections free, allowing trucks to turn wide and thereby allowing smaller curb radii that are more pedestrian friendly.

Physical Impacts

No physical needs have been identified.

Socioeconomic Impacts

Minimal costs are associated with stop bar alterations.

Enforcement, Education, and maintenance

Enforcement of the relocated stop bars may be necessary.

Summary

Table 46 summarizes the issues associated with stop bar alterations.

Table 46. Summary of issues for stop bar alterations.

Characteristic

Potential benefits

Potential Liabilities

Safety

Decreased risk of pedestrian collisions involving left-turning vehicles.

None identified.

Operations

None identified.

Increase in vehicular clearance time and loss time.

Multimodal

Relocation of stop bars facilitates turning movements of heavy trucks.

None identified.

Physical

No physical needs identified.

None identified.

Socioeconomic

None identified.

None identified.

Enforcement, Education, and maintenance

None identified.

Enforcement of the stop bars may be necessary.

9.1.4 Improve Pedestrian Signal Displays

Traffic signals should allow adequate crossing time for pedestrians and an adequate clearance interval based on walking speed. Pedestrian signal enhancements include:

  • Separate pedestrian signals (WALK/DON'T WALK).
  • Accessible pedestrian signal.
  • Countdown displays.
  • Animated eyes display.

Application

Chapter 4 provided guidance on the use of pedestrian signals and accessible pedestrian signals. Current thinking suggests that redundancy in information to pedestrians benefits all pedestrians. For example, sighted pedestrians may react more quickly to the WALK indication when provided an audible cue in addition to the pedestrian signal display. Therefore, accessible pedestrian signals may enhance the usability of the intersection for all pedestrians, not just those with visual impairments.

Countdown signals, shown in figure 69a, display the number of seconds remaining before the end of the DON'T WALK interval. The WALKING person symbol and flashing and steady UPRAISED HAND symbol still appear at the appropriate intervals. The countdown signals do not change the way a signal operates; they only provide additional information to the pedestrian. Countdown pedestrian signals have been included in the 2003 MUTCD for optional use.(1)

Another innovative pedestrian signal treatment is an animated eyes display, shown in figure 69b. The animated, light-emitting diode (LED) signal head, is used to prompt pedestrians to look for turning vehicles at the start of the WALK indication. The signal head includes two eyes that scan from left to right. Animated eyes have been included in the 2003 MUTCD for optional use with the pedestrian signal WALK indication.(1)

Figure 69. Examples of countdown and animated eyes pedestrian signal displays. Photos. The photo to the left shows a countdown display with a Flashing don't Walk symbol next to a digital number (eight). The digital number counts down second by second and reflects the amount of time remaining during the "flashing don't walk" interval. The photo to the right shows an "animated eyes" display in a fluorescent blue color. The "eyes" are shown directly above the walk symbol and indicate that pedestrians should look out for conflicting movements.
(a) Countdown display.
Figure 69. Examples of countdown and animated eyes pedestrian signal displays. Photos. The photo to the left shows a countdown display with a Flashing don't Walk symbol next to a digital number (eight). The digital number counts down second by second and reflects the amount of time remaining during the "flashing don't walk" interval. The photo to the right shows an "animated eyes" display in a fluorescent blue color. The "eyes" are shown directly above the walk symbol and indicate that pedestrians should look out for conflicting movements.
(b) Animated eyes display.

Figure 69. Examples of countdown and animated eyes pedestrian signal displays.

 

Safety Performance

Collision modification factors listed in table 47 all suggest that pedestrian signals improve safety. However, a number of older studies found that pedestrian signalization does not improve safety.(106,107) Larger pedestrian signal heads were described in the literature as a treatment to enhance conspicuity; however, no research on the effect on pedestrian safety was found.

Accessible pedestrian signals assist visually impaired pedestrians. Different devices generating audible messages (audible at pedestrian head or audible at push button), vibration at push button, and transmitted messages are in use.(108) A recent study found a 75-percent reduction in the percentage of pedestrians not looking for threats and a similar reduction in conflicts at an intersection equipped with speakers providing messages prompting pedestrians to look for turning vehicles during the walk interval.(109)

Countdown displays may reduce vehicle-pedestrian conflicts resulting from pedestrians attempting to cross the intersection at inappropriate times. Several studies of these pedestrian countdown signals found no statistically significant reductions in pedestrian crash rates. The countdowns did result in a higher percentage of successful crossings by pedestrians (completed their crossing before conflicting traffic received the right of way). (See references 105, 110, 111, and 112.)

Preliminary results from studies of the use of animated-eye displays show increased pedestrian observation of traffic behavior, even after 6 months. Pedestrian/vehicle conflicts appear to decrease at a variety of intersection configurations. Overuse of the device may decrease its effectiveness.(105,113)

Table 47 presents the results of selected references involving the addition of pedestrian signals.

Table 47. Safety benefits associated with addition of pedestrian signals: Selected findings.

Treatment

Implication

Install WALK/DON'T WALK signals (68)

15 to 17% estimated reduction in pedestrian collisions

Add pre-timed, protected pedestrian phase (98)

10% estimated reduction in pedestrian collisions

 

Operational Performance

These treatments should have a negligible effect on vehicle operations. Use of redundant visual and audible displays may reduce the delay pedestrians experience in initiating their crossing, which may reduce the delay for right-turning vehicles.

Multimodal Impacts

Some treatments described above are of specific benefit to people with visual disabilities, although all pedestrians are likely to benefit from redundancy. They should be considered when modifying intersections.

Apart from pedestrians, there are no specific impacts to other transportation modes.

Physical Impacts

No particular specific physical needs have been identified.

Socioeconomic Impacts

Pedestrian signals and the pedestrian signal enhancements described above have moderate costs.

Enforcement, Education, and maintenance

As some of the treatments described above have not seen widespread use (e.g., the animated eyes display), some education of the meaning of the devices should be considered upon their introduction to the public.

Summary

Table 48 summarizes the issues associated with pedestrian signal display improvements.

Table 48. Summary of issues for pedestrian signal display improvements.

Characteristic

Potential benefits

Potential Liabilities

Safety

Give pedestrians improved awareness of traffic.

None identified.

Operations

None identified.

None identified.

Multimodal

All pedestrians, but especially visually impaired pedestrians, are likely to benefit.

None identified.

Physical

None identified.

None identified.

Socioeconomic

None identified.

Some enhancements are expensive.

Enforcement, Education, and maintenance

None identified.

Education may be necessary.

9.1.5 Modify Pedestrian Signal Phasing

Description

In general, shorter cycle lengths and longer WALK intervals provide better service to pedestrians and encourage greater signal compliance. The MUTCD uses a walk speed of 1.2 m/s (4.0 ft/s) for determining crossing times.(1) However, the Pedestrian Facilities User Guide recommends a lower speed of 1.1 m/s (3.5 ft/s), as discussed in chapter 2.(35) Other researchers suggest that this speed is still inadequate to meet the needs of older pedestrians. A 15th-percentile walking speed of older pedestrians has been recommended as the criterion to be used to assess adequacy of crossing time.(114)

Three options beyond standard pedestrian signal phasing are:

  • The leading pedestrian interval.
  • The lagging pedestrian interval.
  • The exclusive pedestrian phase.

A leading pedestrian interval entails retiming the signal splits so that the pedestrian WALK signal begins a few seconds before the vehicular green. As the vehicle signal is still red, this allows pedestrians to establish their presence in the crosswalk before the turning vehicles, thereby enhancing the pedestrian right-of-way.

A lagging pedestrian interval entails retiming the signal splits so that the pedestrian WALK signal begins a few seconds after the vehicular green for turning movement. The 2001 ITE guide, Alternative treatments for At-Grade Pedestrian Crossings, indicates that this treatment is applicable at locations where there is a high one-way to one-way turning movement and works best where there is a dedicated right-turn lane.(105) This benefits right-turning vehicles over pedestrians by giving the right turners a head start before the parallel crosswalk becomes blocked by a heavy and continuous flow of pedestrians.

An exclusive pedestrian signal phase allows pedestrians to cross in all directions at an intersection at the same time, including diagonally. It is sometimes called a "barn dance" or "pedestrian scramble." Vehicle signals are red on all approaches of the intersection during the exclusive pedestrian signal phase. The objective of this treatment is to reduce vehicle turning conflicts, decrease walking distance, and make intersections more pedestrian-friendly. The 2001 ITE guide refers to research that indicates that leading intervals were more effective treatments than this scramble pattern.(105)

Application

Leading pedestrian phasing may be considered where:

  • There is moderate to heavy pedestrian traffic.
  • A high number of conflicts/collisions occur between turning vehicles and crossing pedestrians.

Lagging pedestrian phasing may be considered where:

  • There is moderate to heavy pedestrian traffic.
  • There is right-turn channelization that is heavily used by vehicles.
  • A high number of conflicts/collisions occur between right-turning vehicles and crossing pedestrians.

Exclusive pedestrian phasing (scramble) may be considered where:

  • There is heavy pedestrian traffic.
  • Delay for vehicular turning traffic is excessive due to the heavy pedestrian traffic.
  • There are a large number of vehicle-pedestrian conflicts involving all movements.

Note that for any of the three treatments, the use of accessible pedestrian signals is recommended to give people with visual disabilities information of the walk phase in the absence of predictable surging traffic.

Safety Performance

Several studies have demonstrated that imposing of leading pedestrian intervals significantly reduces conflicts for pedestrians.(102,105,115) Crash analysis conducted at 26 locations with leading pedestrian intervals in New York City (based on up to 10 years of data) showed that leading pedestrian intervals have a positive effect on pedestrian safety, especially where there is a heavy concentration of turning vehicles. This evidently occurs regardless of pedestrian volume.

None of the studies of lagging pedestrian intervals considered the safety effect of this treatment.

Using exclusive pedestrian intervals that stop traffic in all directions has been shown to reduce pedestrian crashes by 50 percent in some locations (i.e., downtown locations with heavy pedestrian volumes and low vehicle speeds and volumes).(101,116)

Operational Performance

The leading pedestrian phase will increase delay at the intersection due to a loss in green time. A solution for the issue of loss of green time for vehicles when using a leading pedestrian interval is based on trading the leading pedestrian interval seconds at the beginning of the cycle for seconds at the end of the cycle. The effect would be that all movements get less green time, but that time is optimized. However, this timing was not investigated empirically.(102)

A main operational disadvantage of lagging pedestrian intervals is that they cause additional delays to pedestrians.

With concurrent signals, as described above, pedestrians usually have more crossing opportunities and shorter waits. Unless a system more heavily penalizes motorists, pedestrians will often have to wait a long time for an exclusive pedestrian phase. As a result, many pedestrians will simply choose to ignore the signal and cross if and when a gap in traffic occurs.(101,116) In addition, an exclusive pedestrian phase may increase the overall cycle length of the intersection, thus increasing delay for all users. On the other hand, an exclusive pedestrian phase removes pedestrians from the vehicular phases, thus increasing vehicular capacity during those phases.

Multimodal Impacts

Pedestrians may become impatient or ignore a lagging pedestrian interval or exclusive pedestrian phase and begin crossing the road during the DON'T WALK phase.

Physical Impacts

No specific physical needs were identified.

Socioeconomic Impacts

Minimal costs are associated with the retiming of the pedestrian signals. The exclusive pedestrian phase, if implemented, may require additional signing and pavement markings to indicate that diagonal crossings may be made (see 2003 MUTCD, section 3B.17(1)).

Enforcement, Education, and maintenance

Where leading or lagging pedestrian phases are being considered, they should be accompanied by police enforcement to ensure that vehicles and pedestrians obey traffic signals.

Summary

Table 49 summarizes the issues associated with pedestrian signal phasing modifications.

Table 49. Summary of issues for pedestrian signal phasing modifications.

Characteristic

Potential benefits

Potential Disbenefits

Safety

Reduce pedestrian/vehicle collisions.

None identified.

Operations

Exclusive phase: Increased capacity for vehicular turning movements.

Lead phase: Increased vehicular delay.

Exclusive phase: Increased vehicular delay due to potentially longer cycle length.

Multimodal

Lead phase: Reduced pedestrian delay.

Lag phase: Increased pedestrian delay.

Exclusive phase: Increased pedestrian delay due to potentially longer cycle length.

Physical

None identified.

None identified.

Socioeconomic

Lead or lag phases: Little or no cost.

Exclusive phase: Moderate costs.

Enforcement, Education, and maintenance

None identified.

Enforcement may be necessary.

9.1.6 Grade-Separate Pedestrian Movements

Description

In some situations, it may be feasible to consider separating pedestrian movements from an intersection. Pedestrian overpasses and underpasses allow for the uninterrupted flow of pedestrian movement separate from the vehicle traffic. However, it increases out-of-direction travel, both horizontally and vertically, for the pedestrian in the process.

Applicability

Pedestrian grade separation, an example of which is shown in figure 70, may be appropriate in situations where:

  • An extremely high number of pedestrian/vehicle conflicts or collisions are occurring at the existing crossing location.
  • School crossings exist or high volumes of children cross.
  • A crossing has been evaluated as a high-risk location for pedestrians.
  • Turning vehicles operate with high speeds.
  • Sight distance is inadequate.

Usually, a warrant for a grade pedestrian separation is based on pedestrian and vehicle volume, vehicle speed, and area type. Warrants usually differ for new construction projects and existing highways. In the first case, greater opportunities for grade separation are available. In some cases, safety can be a major factor; e.g., New Jersey Department of Transportation guidelines consider pedestrian overpasses and/or underpasses warranted if a safety evaluation indicates that erection of a fence to prohibit pedestrian crossing.(117)

 

Figure 70. a pedestrian grade separation treatment. Photo. The picture shows a wide road with a median. A raised footbridge set on concrete piers provides a grade-separated crossing for pedestrians to eliminate conflicts with vehicles.

Photograph Credit: Synectics transportation Consultants, Inc.

Figure 70. A pedestrian grade separation treatment.

Safety Performance

Pedestrian grade separations ideally should completely remove any pedestrian/ vehicle conflicts at the location in question. However, studies have shown that many pedestrians will not use overpasses or underpasses if they can cross at street level in about the same amount of time, or if the crossing takes them out of their way. Some pedestrians may avoid a pedestrian tunnel or overpass due to personal security concerns.

Operational Performance

Completely eliminating a pedestrian crossing area should improve traffic flow. However, a pedestrian overpass is not likely to be used if it is too inconvenient. Use of a median pedestrian barrier should be considered to reduce midblock crossings and encourage pedestrians to use the grade-separated crossing.

Multimodal Impacts

Pedestrian access and convenience may be negatively affected by grade separation. Pedestrians with disabilities or low stamina may have difficulty with the out-of-direction travel and elevation changes associated with grade separation.

Physical Impacts

Construction of a bridge overpass or tunnel is required. Note that any new or modified pedestrian grade separation treatment must comply with ADA requirements. This may involve adding long ramps with landings at regular intervals or installing elevators.

Socioeconomic Impacts

Grade separation can be very expensive and difficult to implement. As a result, grade separation is usually only feasible where pedestrians must cross high-speed, high-volume arterials.(101) In most cases, other treatments are likely to be more cost effective.

Enforcement, Education, and maintenance

Maintenance issues associated with litter and graffiti are significant with pedestrian overpasses and underpasses. Additional police enforcement may be needed because of the fear of crime in these facilities.

Summary

Table 50 summarizes the issues associated with pedestrian grade separation.

Table 50. Summary of issues for pedestrian grade separation.

Characteristic

Potential benefits

Potential Liabilities

Safety

Reduced pedestrian-vehicle collisions.

Pedestrians may cross in unexpected locations due to inconvenience of grade separation.

Operations

Improved vehicular capacity.

None identified.

Multimodal

Fewer conflicts between pedestrians and vehicles.

Increased walking distance, delay, and difficulty for pedestrians.

Physical

None identified.

Grade separation structure required, as well as ramps or elevators to meet ADA requirements.

Socioeconomic

None identified.

Significant costs (grade separation).

Enforcement, Education, and maintenance

None identified.

Graffiti removal and enforcement for personal security may be necessary.

9.2 Bicycle treatments

9.2.1 Provide Bicycle Box

Description

A bicycle box uses advance stop bars that are placed on the approach to a signalized intersection, typically in the rightmost lane, at a location upstream from the standard stop bar location. These create a dedicated space for bicyclists a bicycle box-to occupy while waiting for a green indication. Advance stop bars are used in conjunction with bicycle lanes or other similar bicycle provisions.

Applicability

This treatment may be applicable in situations where vehicle/bicycle collisions have been observed in the past, or vehicle/bicycle conflicts are observed in field observations. The treatment may be considered if a bike lane exists on the approach.

In locations with a high volume of right-turning traffic, use of this treatment may be problematic.

Safety Performance

Such a treatment was found to be effective in Europe, resulting in a 35 percent reduction in through-bicycle/right-turning-vehicle collisions.(118)

Operational Performance

This treatment is not expected to have a significant effect on traffic operations unless a high volume of right-turning traffic is present.

Multimodal Impacts

Bicycle boxes permit bicyclists to pass other queued traffic on the intersection approach leg, giving them preferential treatment in proceeding through the intersection.

Enforcement, Education, and maintenance

Concerns with providing a bicycle box include motorist violation of existing stop bar, a lack of uniformity with other intersections, and right-turn-on-red movements.

Summary

Table 51 summarizes the issues associated with providing a bicycle box.

Table 51. Summary of issues for providing a bicycle box.

Characteristic

Potential benefits

Potential Liabilities

Safety

Potential reduction in collisions between through bicycles and right-turning vehicles.

None identified.

Operations

None identified.

This treatment may not be compatible with a high volume of right-turning traffic.

Multimodal

Bicyclists can bypass queued traffic, thus reducing delay.

None identified.

Physical

None identified.

None identified.

Socioeconomic

None identified.

None identified.

Enforcement, Education, and maintenance

None identified.

Enforcement of the box may be necessary.

9.2.2 Provide Bike Lanes

Description

While bicycle lanes are frequently used on street segments, AASHTO cautions against the use of bicycle lane markings through intersections.(21) Special lanes for bicyclists can cause problems to the extent that they encourage bicyclists and motorists to violate the rules of the road for drivers of vehicles. Specifically, a bike lane continued to an intersection encourages right-turning motorists to stay in the left lane, not the right (bike) lane, in violation of the rule requiring that right turns be made from the lane closest to the curb. Similarly, straight-through, or even left-turning, cyclists are encouraged to stay right.

Some advocate placing the bike lane between the through lane and the right-turn only lane. A right-turn-only lane encourages motorists to make right turns by moving close to the curb (as the traffic law requires). A cyclist going straight can easily avoid a conflict with a right-turning car by staying outside of the right-turn lane. A bike lane to the left of the turn lane encourages bicyclists to stay out of the right-turn lane when going straight.

Applicability

This treatment may be applicable in situations where there are a high number of bicyclists using the road or where bicycle use is being promoted or encouraged.

Safety Performance

Some European literature suggests that bicycle lane markings can increase motorist expectation of bicyclists; one Danish study found a 36-percent reduction in bicycle collisions when these were marked.(119) Other research concludes that bicycle paths along arterials typically increase cyclists' vulnerability to a collision at signalized intersections; however, raised and brightly colored crossings reduce the number of bicycle/vehicle conflicts and should improve safety.(120)

Multimodal Impacts

Bicycle lanes delineate roadway space between motor vehicles and bicycles and provide for more predictable movements by each.(21)

Physical Impacts

Bicycle lanes may require additional right-of-way unless width is taken from the existing travel and/or parking lanes, either by lane narrowing or the removal of a lane.

Summary

Table 52 summarizes of the issues associated with providing bicycle lanes.

Table 52. Summary of issues for providing bicycle lanes.

Characteristic

Potential benefits

Potential Liabilities

Safety

Potential reduction in vehicle/bicycle collisions.

None identified.

Operations

None identified.

None identified.

Multimodal

Bicycle lanes delineate roadway space between motor vehicles and bicycles and provide for more predictable movements by each.

None identified.

Physical

None identified.

Bicycle lanes may require additional right-of-way unless width is taken from existing lanes.

Socioeconomic

None identified.

None identified.

Enforcement, Education, and maintenance

None identified.

None identified.

 

9.3 Transit treatments

9.3.1 Relocate Transit Stop

Placement of bus stops in the vicinity of intersections can have a significant influence on the safety and operational performance. Approximately 2 percent of pedestrian accidents in urban areas and 3 percent in rural areas are related to bus stops.(121) Proper placement and provisions at bus stops can reduce several safety and mobility problems. Traffic engineers often have two choices with regard to bus stop placement in the vicinity of an intersection: on the near side (upstream) or far side (downstream). The 1996 Transit Cooperative Research Program (TCRP) Report 19: Guidelines for the Location and Design of Bus stops provides a comprehensive comparative analysis of far-side, near-side, and midblock placement of bus stops.(121)

Application

Relocation of a transit stop to a location upstream of the intersection (near side) should be considered in situations where there is congestion on the far side of the intersection during peak periods.

Relocation of a transit stop to a location downstream of the intersection (far side) should be considered in situations where:

  • There is a heavy right-turn movement.
  • There have been a number of conflicts between vehicles trying to turn right, through vehicles, and stationary near-side buses, resulting in rear-end and sideswipe collisions.
  • There have been a number of pedestrian collisions because pedestrians cross in front of a stationary bus and are struck by a vehicle.

Safety Performance

One advantage of near-side placements is that the bus driver has the entire width of the intersection available to pull away from the curb. Near-side bus placements increases conflicts between right-turning vehicles, through traffic, and the bus itself. When the bus is stopped at the bus stop, traffic control devices, signage, and crossing pedestrians are blocked from view. Vehicles on the adjacent approach to the right may have difficulty seeing past a stopped bus while attempting a right turn on red.

Far-side bus stop placements minimize conflicts between right-turning vehicles and buses. Relocating the bus stop to the far side of the intersection can also improve safety because it eliminates the sight distance restriction caused by the bus and encourages pedestrians to cross the street from behind the bus instead in front of it.(122) The 1996 TCRP report recommends a minimum clearance distance of 1.5 m (5 ft) between a pedestrian crosswalk and the front or rear of a bus stop.(121) Finally, the bus driver can take advantage of gaps in the traffic flow that are created at signalized intersections. However, far-side bus stops may cause rear-end collisions, as drivers often do not expect buses to stop immediately after the traffic signal.

In conclusion, as a whole, it would appear that far-side bus stops offer greater overall safety.

Operational Performance

Near-side bus stop placements minimize interference with through traffic in situations where the far side of the intersection is congested. This type of placement also allows the bus driver to look for oncoming traffic, including other buses with potential passengers for the stopped bus. However, if the bus stop is being used for more than one bus, the right and through lanes may be temporarily blocked.

Far-side bus stop placements improve the right-turn capacity of the intersection. Yet they may block the intersection during peak periods by stopping buses or by a traffic queue extending back into the intersection. Also, if the light is red, it forces the bus to stop twice, decreasing the efficiency of bus operations.

Multimodal Impacts

Near-side bus stop placements allow pedestrians to access buses closest to the crosswalk, and allows pedestrians to board, pay the fare, and find a seat while the bus is at a red light. However, placing the bus stops on the near side of intersections or crosswalks may block pedestrians' view of approaching traffic and the approaching drivers view of pedestrians.(101)

Physical Impacts

Near-side bus stops/bus shelter placements may interfere with the placement of a red-light camera.

Socioeconomic Impacts

Relocation of a bus stop is a relatively low-cost improvement, unless it involves the relocation of a bus bay and shelter.

Enforcement, Education, and maintenance

Some jurisdictions have implemented or are considering a yield-to-bus law. If implemented, this would require all motorists to yield to buses pulling away from a bus stop and reduce transit/vehicle conflicts.

Far-side bus bays provide a safe haven for police officers carrying out red light running or speed enforcement and can also facilitate U-turns.

From a driver education point of view, the traffic engineer and transit agency may consider consistently placing the bus stop either on the near side or the far side, so that motorists have an expectation of where the bus is going to stop at all signalized intersections in their jurisdiction.

Summary

Table 53 summarizes of the issues associated with providing near-side or far-side transit stops.

Table 53. Summary of issues for near-side/far-side transit stops.

Characteristic

Potential benefits

Potential Liabilities

Safety

Right-turning vehicle conflicts (far side).

Sight distance issues for crossing pedestrians/vehicles on adjacent approach (far side).

Rear-end conflicts (near side).

Right-turning vehicle conflicts (near side).

Sight distance issues for crossing pedestrians/vehicles on adjacent approach (near side).

Rear-end conflicts (far side).

Operations

Eliminates double stopping (near side).

Right-turn/through lanes may be blocked (near side).

Intersection may be blocked (far side).

Multimodal

Passenger can board while light is red (near side).

Less walking distance to crosswalk (near side).

None identified.

Physical

None identified.

May interfere with red-light camera placement (near side).

Socioeconomic

None identified.

Relocation (far or near) may be costly if it involves relocation of bus bay/bus shelter.

Enforcement, Education, and maintenance

Far-side bus bays provide space for enforcement vehicles.

Enforcement of yielding to buses may be necessary.

9.4 Traffic Control Treatments

Intersection-wide traffic control treatments have either operational or safety benefits on all approaches and all movements. Signal coordination improves traffic flow for through traffic and provides gaps for left-turn movements. Signal preemption and priority identifies and accommodates critical movements and users. Signal controller upgrades (from pre-timed to actuated) accommodate intersections where traffic flow is highly variable, reducing delays and driver frustration. Clearance interval adjustments can address a red light running problem. Cycle length can also be adjusted based on the nature of the traffic flow through the intersection. Finally, the advisability of removal of a signalized intersection from late night/early morning flash mode should be evaluated.

9.4.1 Change signal Control from Pre-Timed to Actuated

Description

Traffic signal control at an intersection may be pre-timed or actuated. This mode of control could be a function of the capabilities of the controller (older controllers may not have actuated capabilities), or it could be a byproduct of the lack of detection at the intersection (for example, a modern controller with full actuated capabilities may be required to run pre-timed if no detection is in place). The mode of control used can have a profound effect on the operational efficiency and safety of the signalized intersection.

a pre-timed controller operates within a fixed cycle length using preset intervals and no detection. Pre-timed traffic control signals direct traffic to stop and permit it to proceed in accordance with a single predetermined time schedule or series of schedules.

The traffic engineer may want to consider upgrading an intersection from pre-timed to actuated control. These signals service movement based on demand. Actuated signals use detection to respond to vehicle calls and are categorized as either semiactuated or fully actuated. Semiactuated traffic signals have detectors located on the minor approaches and in the left-turn lanes of the major approaches. Fully actuated traffic signals have detection on all approaches.

Selecting the best type of control for a location requires full knowledge of local conditions, but, in general, can be based on:

  • Variations in peak and average hourly traffic volumes on the major approaches.
  • Variations in morning and afternoon hourly volumes.
  • Percentage of volumes on the minor approaches.
  • Usage by large vehicles, pedestrians, and bicycles.

Applicability

Converting a signal from pre-timed to actuated may be considered in situations:

  • Where fluctuations in traffic cannot be anticipated and thus cannot be programmed with pre-timed control.
  • At complex intersections where one or more movements are sporadic or subject to variations in volume.
  • At intersections that are poorly placed within a traffic corridor of intersections with pre-timed traffic signals.
  • To minimize delay in periods of light traffic.

Safety Performance

Actuated traffic signals provide better service to all movements at an intersection, reducing driver frustration and the likelihood of red light running. However, they also make it more difficult for pedestrians with visual impairments to predict what will happen in the intersection.

There is little research on the effect of signal actuation on collisions, apart from some references from Michigan and New York State (table 54). These references suggest that actuated signalized intersections have fewer collisions than intersections with fixed timing.

Table 54. Safety benefits associated with upgrading an intersection from pre-timed to actuated operation: Selected findings.

Treatment

Finding

Upgrade signal controller (98)

20 to 22% estimated reduction in all collisions.

Install signal actuation (98)

20% estimated reduction in all collisions.

Change in signal operations (pre-timed to traffic actuated)(123)

28% estimated reduction in all collisions.

32% estimated reduction in right-angle collisions.

26% estimated reduction in rear-end/overtaking collisions.

60% estimated reduction in head-on/sideswipe collisions.

30% estimated reduction in left-turn collisions.

 

Operational Performance

Actuated intersections used in appropriate situations, can reduce delays to vehicles, particularly in light traffic situations and for movements from minor approaches.

Actuated traffic control is not necessary in situations where traffic patterns and volumes are predictable and do not vary significantly. They may not be the best choice where there is a need for a consistent starting time and ending time for each phase to facilitate signal coordination with traffic signals along a traffic corridor. Actuated signals are dependent on the proper operation of detectors; therefore, they are affected by a stalled vehicle, vehicles involved in a collision, or construction work. This may disrupt operations at a signalized intersection.

Multimodal Impacts

Pre-timed traffic signals may be more acceptable than traffic-actuated signals in areas where there is large and fairly consistent pedestrian traffic crossing the road. Actuated traffic signals may cause confusion with the operation of pedestrian push buttons. Actuated pedestrian push buttons must be located in appropriate locations and be accessible to be ADA compliant.

Physical Impacts

Detectors are required on the approaches where actuation is needed. Depending on the type of detector, this may create physical impacts (see chapter 4 for further discussion of detector types).

Socioeconomic Impacts

Generally speaking, actuated traffic controllers cost more to purchase and install than pre-timed traffic controllers, although almost all traffic controllers purchased today are capable of actuated operation. Detection can be a significant percentage of the cost of a signalized intersection.

Enforcement, Education, and maintenance

Pre-timed traffic signals may lead to driver frustration in low-volume situations, as in the late evening/early morning hours, as the driver may be waiting for the signal to change green while no other vehicles are present on the other approaches. This may lead to red light running.

Traffic-actuated signals are more complicated and less easily maintained than pre-timed traffic signals, especially because of detector maintenance needs.

Summary

Table 55 summarizes the issues associated with providing signal actuation.

Table 55. Summary of issues for providing signal actuation.

Characteristic

Potential benefits

Potential Liabilities

Safety

Improves safety.

Reduces driver frustration, red light running.

None identified.

Operations

Provides better service to minor approaches.

Accommodates widely fluctuating volumes.

Can sometimes reduce smooth platooning in coordinated systems.

Requires proper operation of detectors.

Multimodal

None identified.

May be problematic in high pedestrian areas.

Physical

None identified.

Detectors required.

Socioeconomic

None identified.

Can be costly.

Enforcement, Education, and maintenance

Enforcement needs may decrease.

Maintenance costs will likely increase to maintain detection.

9.4.2 Modify Yellow Change interval and/or Red Clearance interval

Description

The yellow change interval warns approaching traffic of the change in assignment of right-of-way. Yellow change intervals, a primary safety measure used at traffic signals, are the subject of much debate. The yellow change interval is normally between 3 and 6 s. Since long yellow change intervals may encourage drivers to use it as a part of the green interval, a maximum of 5 s is commonly employed. Local practice dictates the length of the change interval.

Current thought is that longer intervals will cause drivers to enter the intersection later and breed disrespect for the traffic signal. One before-and-after study showed that the time vehicles entered the intersection increased with a longer yellow change interval.(124) Additional research is needed to further understand the effect of lengthening the yellow change interval on driver behavior.

The red clearance interval is an optional interval that follows the yellow change interval and precedes the next conflicting green interval. The red clearance interval provides additional time following the yellow change interval before conflicting traffic is released. The decision to use a red clearance interval is determined based on engineering judgment and assessment of any of the following criteria:

  • Intersection geometrics.
  • Collision experience.
  • Pedestrian activity.
  • Approach speeds.
  • Local practices.

The red clearance interval is either set by local policy or calculated using an equation that determines the time needed for a vehicle to pass through the intersection. The equation most commonly used is described in various documents(125) (and chapter 4). As intersections are widened to accommodate additional capacity, the length of the calculated clearance interval increases. This increase may contribute to additional lost time at the intersection, which negates some of the expected gain in capacity due to widening.

Applicability

Modifying the yellow or red clearance interval may be considered where:

  • A high number of angle/left-turn collisions occur due to through/left-turning drivers failing to clear the intersection or stop before entering the intersection at onset of the red.
  • A high number of rear-end collisions occur because drivers brake sharply to avoid entering the intersection at the onset of the red.
  • A high incidence of red-light violations is recorded.

Safety Performance

A 1985 study examined the relationship between the timing of clearance intervals and crash rates.(126) The study focused on 91 intersections across the United States that represent a variety of intersection characteristics including average approach speed, cross-street width, yellow phase, and all-red phase. Results from a cluster analysis showed that intersections with inadequate clearance intervals (meaning the implied deceleration rate exceeded 1982 Transportation and traffic Engineering Handbook recommendations of 3 m/s2 (10 ft/s2) resulted in either: (1) drivers entering the intersection without adequate cross-street protection, thus increasing the risk of right-angle collisions; or (2) drivers braking suddenly to avoid entering the intersection, thus increasing the risk for rear-end collisions. The study concludes that accepted standards for timing clearance intervals are commonly ignored and that improved procedures need to be adopted throughout the United States.

A 1997 paper studied the effects of clearance interval timing on red light running and late exits (vehicles that fail to exit the intersection before the onset of green for the cross-street movement, resulting in a right-of-way conflict).(127) The clearance interval timing for the intersections that were studied was compared to the recommended timing calculated when using the procedures identified in "Determining Vehicle Change intervals: a proposed Recommended Practice."(128) Results from the study showed that the number of red light running and late exit instances were highest at intersections where clearance intervals were too short. Based on the results of the study, drivers did not appear to become habituated to longer yellow signals or increased all-red periods. The paper concludes by indicating that although the positive influence of a longer clearance interval may partially erode over time, the findings of the research suggest that longer change intervals can provide a sustainable safety benefit.

A 2000 paper(129) evaluated the potential crash effects associated with modifying clearance intervals to conform with ITE's "Determining Vehicle Change intervals: a proposed Recommended Practice."(128) The study focused on 122 intersections that were randomly assigned to experimental and control groups. During the 3-year period following the implementation of the signal timing changes, the intersections with the modified time experienced an 8 percent reduction in reported crashes, a 12 percent reduction in injury crashes, and 37 percent reduction in pedestrian and bicycle crashes. Given the positive results from the study, the authors conclude that modifying the clearance intervals to conform to the ITE proposed recommended practice should be strongly considered by agencies to reduce the number of crashes.

A Texas study of 12 intersection approaches suggested that red light running frequency is minimized with yellow change intervals between 4.0 and 4.5 seconds.(130)

A Michigan study concerning the adjustment of yellow change intervals and all-red intervals at three intersections showed a significant reduction in red light running violations. All red intervals were lengthened from 0.1 second to 1.6 to 4.0 seconds, according to ITE guidelines. Reductions in red light running violations were significant and some preliminary data appears to suggest that collisions have been reduced as well.(131)

Table 56 presents selected findings associated with signal clearance modifications.

Table 56. Safety benefits associated with modifying clearance intervals: Selected findings.

Treatment

Finding

Add all-red clearance interval (98)

15 to 30% estimated reduction in all collisions.

Increase yellow change interval (98)

15 to 30% estimated reduction in all collisions.

Retime traffic signal (98)

10% estimated reduction in all collisions.

Add all-red clearance interval and increase yellow change interval(132)

15% estimated reduction in all collisions.

30% estimated reduction in right-angle collisions.

Operational Performance

Extending the yellow and red interval will increase the amount of lost time, decreasing the overall efficiency of the intersection.

Multimodal Impacts

Either extending the yellow and/or red clearance interval or providing a red clearance interval will benefit pedestrians, going them additional time to clear the intersection. The elderly or people with mobility disabilities may benefit substantially.

Physical Impacts

No physical impacts are associated with this treatment.

Socioeconomic Impacts

The treatment has been shown to reduce red light running at a wide variety of signalized intersections. It is a low-cost alternative to the use of police or automated enforcement.

Enforcement, Education, and maintenance

Local practice varies as to legal movements during the yellow phase. Police, traffic engineering staff, and the public need to be clear and in agreement about what is permissible in their jurisdiction.

Summary

Table 57 summarizes the issues associated with modifying yellow and/or red clearance intervals at signalized intersections.

Table 57. Summary of issues for modifying yellow/red clearance intervals.

Characteristic

Potential benefits

Potential Liabilities

Safety

Angle collisions are reduced.

Left-turn collisions are reduced.

Rear-end collisions are reduced.

None identified.

Operations

None identified.

Increased lost time.

Multimodal

The elderly and people with mobility disabilities have more time to cross.

None identified.

Physical

No physical requirements.

None identified.

Socioeconomic

Low-cost alternative to police and automated enforcement.

None identified.

Enforcement, Education, and maintenance

Red-light enforcement may become less necessary.

None identified.

9.4.3 Modify Cycle Length

Description

The calculation and selection of cycle length requires good judgment on the part of the traffic engineer/analyst. General practice is to have a cycle length between 50 and 120 s. For low-speed urban roads, a shorter cycle length is preferable (50 to 70 s). For wider roadways (over 15 m (50 ft)) with longer pedestrian crossing times (greater than 20 s), or in situations where heavier traffic is present and left-turning vehicles are not being effectively accommodated, a cycle length of 60 to 90 s may be preferable. At high-volume intersections where heavy turning movements are accommodated by multiple phases, a cycle length of 90 to 120 s may be most appropriate.(133) However, cycle lengths longer than 120 s may be needed at large intersections to accommodate multiple long pedestrian crossings in combination with heavy turning movements, especially during peak periods.

Safety Performance

Longer cycle lengths may lead to driver frustration and red light running, as it may take several cycles for a motorist to get through the intersection, particularly when attempting a left turn against opposing traffic.

No known research or specific collision modification factors exist for modifying cycle length.

Operational Performance

A cycle length of 90 s is often considered optimum, since lost time is approaching a maximum, capacity is approaching a minimum, and delay is not too great.(133) Longer cycle lengths may lead to excessive queuing on the approach and will interfere with turning movements (left- and right-turn channelization) if through traffic is severely backed up.

Conversely, intersection capacity drops substantially when cycle lengths fall below 60 s, as a greater percentage of available time is used up in the yellow and red clearance intervals.

Multimodal Impacts

A shorter cycle length may not provide pedestrians with sufficient time to safely cross the intersection, particularly if it has turning lanes. Conversely, a longer cycle length may encourage impatient pedestrians to cross illegally during the red phase.

Physical Impacts

No physical impacts are associated with the modification of cycle length.

Socioeconomic Impacts

No significant costs are associated with this treatment, apart from labor.

Summary

Table 58 summarizes the issues associated with cycle length modification.

Table 58. Summary of issues for cycle length modifications.

Characteristics

Potential benefits

Potential Liabilities

Safety

Less red light running (with shorter cycle lengths).

More red light running (with longer cycle lengths).

Operations

Reduction in delay optimized at 90 s.

Excessive queuing (with longer cycle lengths).

Inadequate capacity (with cycle lengths that are too short).

Multimodal

None identified.

Inadequate crossing time for pedestrians (with cycle lengths that are too short).

Physical

None identified.

None identified.

Socioeconomic

None identified.

None identified.

Enforcement, Education, and maintenance

None identified.

More red light running (with longer cycle lengths).

9.4.4 Late Night/Early Morning Flash Removal

Description

Some jurisdictions operate traffic signals in flashing mode during various periods of the night, the week, or for special events. Flashing operation may be of some advantage to traffic flow, particularly with pre-timed signals, when traffic is very light (late evening/early morning hours, or on a Sunday or holiday in an industrial area).

Two modes of flashing operation are typically used: red-red and red-yellow. Red-red (all approaches receive a flashing red indication) is used where traffic on all approaches is roughly the same. In this instance, the intersection operates the same as an all-way stop. Red-yellow (the minor street receives a flashing red indication and the major street receives a flashing yellow indication) is used in situations where traffic is very light on the minor street. In this instance, the intersection operates similar to a two-way stop.

Safety Performance

A 1987 Michigan study involved the conversion of 59 signalized intersections previously operating in late-night/early morning flashing mode.(134) Late night and early morning collisions before and after the conversion were compared and tested using a paired t-test. Right-angle collisions during when signalized intersections were in flash mode dropped by 91 percent; right-angle injury collisions dropped by 95 percent. Rear-end collisions increased slightly, but the change was not significant.

In a study in Winston-Salem, NC, signals from 20 intersections were taken out of late night/early morning flashing operation.(135) There was a 78-percent reduction in right-angle collisions and a 32-percent reduction in all collisions during times the traffic signal had been in flashing mode.

Some studies have indicated a safety benefit of removing traffic signals from flashing mode under some circumstances, as positive control is provided rather than leaving the driver to decide when it is safe to proceed into the intersection.

Selected study findings associated with the removal of a traffic signal from a flashing mode operation (such as during the late-night/early morning time period) are shown in table 59.

Table 59. Safety benefits associated with removal of signal from late night/early morning flash mode: Selected findings.

Treatment

Finding

Remove signal from late night/early morning flash mode (135)

78% estimated reduction in right-angle collisions during time of previous flashing operation.

32% estimated reduction in all collisions during time of previous flashing operation.

Operational Performance

If the signalized intersection removed from flashing operation is not fully actuated and responsive to traffic demand, there will be a tendency for red-light violations and/or complaints about unnecessary long waits on red signals.

Multimodal Impacts

Removing a traffic signal from a flash mode will require vehicles to come to a complete stop during the red phase. This treatment should give vehicles more time to see, respond, and yield to any pedestrians.

Physical Impacts

No physical impacts are associated with this treatment.

Socioeconomic Impacts

No costs are associated with this treatment.

Enforcement, Education, and maintenance

When a traffic signal is taken out of flash mode, police enforcement could be undertaken at the location to ensure that habituated drivers are not continuing to proceed through the intersection as if the signal were still operating in flashing mode. The traffic engineer may consider temporary signage/publicity to inform motorists of the change in operations and to explain the safety benefits.

Summary

Table 60 summarizes the issues associated with flash mode removal.

Table 60. Summary of issues for flash mode removal.

Characteristic

Potential benefits

Potential Liabilities

Safety

Angle collisions are reduced.

None identified.

Operations

None identified.

Increased delay for through traffic.

Multimodal

Motorists forced to yield to pedestrians.

None identified.

Physical

None identified.

None identified.

Socioeconomic

None identified.

None identified.

Enforcement, Education, and maintenance

None identified.

Enforcement and temporary signage may be needed for a period after conversion.

9.5 Street Lighting and Illumination

9.5.1 Provide or Upgrade illumination

Description

The purpose of roadway lighting is to enhance visibility for drivers, bicyclists, and pedestrians, thereby improving their ability to see each other and the physical infrastructure of the intersection. This allows them to react more quickly and accurately to each other when natural light goes below a certain level ¾ either at night or during bad weather.

Applicability

Intersection lighting should be considered at all signalized intersections. Upgrades may be justified if more collisions than expected are occurring at night, and particularly if the nighttime collisions involve pedestrians, bicyclists, and/or fixed objects.

Design Features

The illumination design at an intersection should meet lighting criteria established by the illuminating Engineering Society of North America (IESNA) in IESNA RP-8-00, American National Standard Practice for Roadway Lighting.(63) The basic principles and design values for intersections have been presented previously (chapter 4) and include overall light level and uniformity of lighting.

Some of the factors that affect the light level and uniformity results include:

  • Luminaire wattage, type, and distribution.
  • Luminaire mounting height.
  • Pole placement and spacing.

These factors are interrelated. For example, higher mounting heights improve uniformity by spreading the light over a larger area; however, the overall light level decreases unless larger wattages are used or poles are placed closer together. A good illumination design balances these various factors against an overall desire to minimize the number of poles and fixtures (both for cost savings and for minimizing the number of fixed objects in the right-of-way).

Pole Placement and Spacing

Besides the types of poles and fixtures, the placement is also an important aspect of a good roadway design. Several factors need to be considered in pole placement. First is safety. Most important is to place the pole at an offset distance that can assist in preventing crashes (vehicles and pedestrians). Second is to determine the pole spacing that is most efficacious for the initial and the long-term maintenance costs and yet still meets the lighting requirement. At intersections, shared use of poles for signal equipment and illumination is recommended. Figure 71 shows examples from RP-8-00 of illumination pole layouts typical at signalized intersections with and without channelized right-turn lanes. However, recent research to improve lighting at midblock pedestrian crosswalks suggests that it may be desirable to locate poles approximately one third to one half the luminaire mounting height back from the crosswalk to improve lighting for pedestrians, which may involve having separate poles for signal equipment and luminaires.(136) For intersections where separate pedestrian pedestals are provided at the crosswalk, the mast arm poles for vehicle signal heads could be located to be optimal for illumination as well.

Click for full description

(a) Typical lighting layout for intersection without right-turn bypass lane.

Click for full description

(b) Typical lighting layout for intersection with right-turn bypass lane.

Figure 71. Typical lighting layouts.(63, figure D3)

Safety Performance

When illumination and visibility are optimal, the chance of nighttime accidents declines, and traffic flow is enhanced. Roadway lighting also increases sight distance, security, and the use of surrounding facilities.

Selected findings from the literature suggest safety improvements associated with adding illumination to an intersection (see table 61).

Table 61. Safety benefits associated with providing illumination: Selected findings.

Treatment

Finding

Add lighting (132)

30% reduction in all collisions.

50% reduction in nighttime collisions.

Add lighting (137)

43% reduction in fatal crashes.

17% reduction in injury crashes.

Operational Performance

There is no documented relationship between illumination and the operational performance of an intersection. The authors believe that illumination likely has little effect on traffic flow, delay, and queuing.

Multimodal Impacts

As noted above, illumination has been demonstrated to reduce pedestrian crashes and provide a more secure environment for all intersection users at night.

Physical Impacts

The provision of illumination typically has little effect on the overall footprint of an intersection. Commonly, combination poles are used to support both signal heads and luminaires, so additional poles are rarely needed in the immediate vicinity of the intersection. However, the recent research cited previously suggests the possibility of improved pedestrian visibility using additional poles upstream from the crosswalk.

Socioeconomic Impacts

Illumination has a positive benefit in reducing the fear of crime at night and in promoting business and the use of public streets at night.(63)

In addition to the initial capital cost and maintenance of illumination fixtures, illumination requires energy consumption. The Roadway Lighting Committee of IESNA believes that lighting of streets and highways is generally economically practical and that such preventive measures can cost a community less than the crashes caused by inadequate visibility.(63) Judicious design of luminaire types, wattages, mounting height, and pole spacing may increase visibility at the intersection without significantly increasing energy costs.

Summary

Table 62 summarizes the issues associated with providing illumination.

Table 62. Summary of issues for providing illumination.

Characteristic

Potential benefits

Potential Disbenefits

Safety

Reported reductions in nighttime collisions.

None identified.

Operations

None identified.

None identified.

Multimodal

May reduce pedestrian crashes.

None identified.

Physical

Little impact.

None identified.

Socioeconomic

May reduce fear of nighttime crime.

Additional energy consumption.

Enforcement, Education, and maintenance

None identified.

Maintenance of illumination will be necessary.

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