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

Signalized Intersections: Informational Guide

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508 FIGURE CAPTIONS

Figures

Figure 1. Traffic controls such as official signs need to be close to the road, distinctive from other information presentations, brief, and explicit. This photo provides an example of signs that are close to the road but may be confused with background information. Photo. This photo shows that the traffic light and the one-way sign on the post are obscured by the Marathon Taverna and "park in rear" signs.

Figure 2. In terms of both official signs and advertising displays, too many displays may have the effect of causing drivers to "tune out," and recall will be poor. This photo shows an example of sign clutter where the regulatory sign is difficult to isolate from the background advertising signs. Photo. This photo shows an example of sign clutter where the regulatory sign with center lane only arrows is difficult to isolate from the busy background of "affordable," "entrance," and "finance" banners and the "we buy cars" sign.

Figure 3. Enforcement cameras, as shown in the photo above, are used at signalized intersections to identify red light runners. Photo. The photo shows a post-mounted enforcement camera that captures red light runners.

Figure 4. Typical dimensions of a bicyclist. Drawing. The drawing shows the rear view of a cyclist with the required dimensions for operating space. The bicyclist needs 0.75 meters (30 inches) width with 0.125 meters (five inches) clearance on both sides for a total of 1 meter (40 inches). The height clearance is 2.5 meters, or 100 inches.

Figure 5. Bicyclist conflicts at signalized intersections. Diagram. The diagram depicts bicyclist conflicts at a four-way intersection. Of the 24 potential conflicts at signalized intersections shown in this figure, six are unique to bicyclists. These include conflicts between right-turning vehicles and through bicyclists and conflicts between through vehicles and bicyclists positioning themselves to turn left with vehicular traffic. An additional 18 conflicts are shown that are in common with motor vehicles, such as left-turn conflicts with opposing through movements.

Figure 6. Examples of pedestrians of various abilities preparing to cross an intersection. Photo. The picture shows a city intersection with a crosswalk delineated with pavers. Seven pedestrians are preparing to cross the street. Six are standing and one is in a wheelchair.

Figure 7. Typical dimensions for a wheelchair. Diagram. The diagram shows the typical dimensions needed for a wheelchair to turn in all directions. The Americans with Disabilities act accessibility Guidelines specify a 1.525-meter (60-inch) square area for 180-degree turns.

Figure 8. Crosswalks are used by a variety of users with different speed characteristics. Pedestrian walking speeds generally range between 0.8 to 1.8 meters per second (2.5 to 6.0 feet per second). Photo. The picture shows a city street with a crosswalk and pedestrians with different speed characteristics. a person pushing a baby stroller and three pedestrians are crossing in this photo.

Figure 9. Pedestrian conflicts at signalized intersections. Diagram. This graphic shows crossing conflicts for vehicle/pedestrian and vehicle/vehicle movements at a four-leg intersection. There are 16 vehicle/pedestrian and 16 vehicle/vehicle crossing conflicts. There are four vehicle/pedestrian conflicts on each leg of the intersection, one on the approach and three on the departure. The approach conflict consists of pedestrians conflicting with all approaching vehicles. The departure conflicts consist of pedestrians conflicting with vehicles completing left-turn, through, and right-turn movements.

Figure 10. The photograph shows a raised median that restricts left-turn egress movements from a driveway located between two signalized intersections. Photo. The picture shows a raised median on a six-lane divided arterial. A driveway into a large shopping center is located approximately 80 to 100 meters (240 to 300 feet) from signalized intersections on each side. The driveway access is restricted by the median to right-in/right-out/left-in.

Figure 11. Pavement markings can be used to delineate travel lanes within wide intersections as shown in the photograph. Photo. The picture shows a wide four-way signalized intersection with double left-turn lanes on all approaches. Dotted pavement lines are used to guide left-turning vehicles into the appropriate lanes.

Figure 12. Various right-turn treatments may be used, depending on the speed environment. Diagram. The diagram illustrates (A) how channelization islands and larger curb radii accommodate higher speed right-turn movements and (B) how smaller curb radii can accommodate lower speed right-turn movements.  

Figure 13. Providing a dedicated left-turn lane reduces potential collisions between left-turning and through vehicles, increasing the capacity of the approach for both left and through traffic. Diagram. The diagram shows a four-lane, four-way intersection. In the top example (A), the major street has two lanes in each direction, with left-turning vehicles sharing the lane with through vehicles. Through vehicles stack up behind a vehicle waiting to turn left. In the bottom example (B), a dedicated left-turn lane allows a left-turning vehicle to wait without impeding through traffic on the adjacent through lanes.

Figure 14. The photo shows how double left- and right-turn lanes can be used to accommodate high-priority movements. Photo. The picture shows a T-intersection with crosswalks and double left- and right-hand turn lanes that accommodate high-volume turning movements.

Figure 15. Intersection skews increase both the intersection width and pedestrian crossing distance. Diagram. The diagram depicts three roadway intersections. The top diagram shows an intersection with a 90-degree angle between approaches, resulting in a crosswalk length of 18.6 meters (61 feet) and distance across the intersection of 31.1 meters (102 feet) from the leading edge of a crosswalk on the near side of the intersection to the trailing edge of a crosswalk on the far side of the intersection. The lower two diagrams show intersections with skewed angles of 75 and 60 degrees. This results in increased crosswalk lengths of 19.2 meters (63 feet) and 23.2 meters (76 feet), respectively, and increased distance across the intersection of 32.9 meters (108 feet) and 36.6 meters (120 feet), respectively.

Figure 16. The photograph illustrates a multileg intersection. Photo. The aerial picture shows a busy multileg intersection with five roads converging and seven crosswalks of varying lengths.

Figure 17. Potential conflicts at intersections with three and four legs. Diagram. The three-leg intersection shows nine potential conflicts: three diverging, three merging, and three crossing the intersection. The four-leg intersection shows 32 total potential conflicts: 8 diverging, 8 merging, and 16 crossing the intersection.

Figure 18. Curb ramp components. Diagram. The diagram shows a flared ramp in a sidewalk with labeled components. The ramp consists of a central ramp area, detectable warnings on the ramp next to the gutter, and angled flares on each side. The sidewalk consists of a landing area behind the ramp and approaches on each side.

Figure 19. Examples of preferred designs. Diagram. The diagram shows six preferred designs that promote accessibility. (A) Perpendicular curb ramps with flares and a level landing. (B) Perpendicular curb ramps with returned curbs and a level landing. (C) Two parallel curb ramps with a lowered curb between the ramps. (D) Two parallel curb ramps with a lowered curb between the ramps. (E) Two combination curb ramps on a corner with a wide turning radius. (F) A curb extension with two perpendicular curb ramps with returned curbs and level landings.

Figure 20. Examples of acceptable curb ramp designs. Diagram. The diagram shows six acceptable curb ramp designs that still allow accessibility. (A) Perpendicular curb ramps design perpendicular to the curb on a corner with a wide turning radius. (B) Diagonal curb ramp with flares and a level landing, in addition to at least 1.22 meters (48 inches) of clear space. (C) Diagonal curb ramp with returned curbs, a level landing, and sufficient clear space in the crosswalk. (D) Single parallel curb ramp with at least 1.22 meters (48 inches) clear space. (E) Two built-up curb ramps. (F) Partially built-up curb ramps.

Figure 21. Examples of inaccessible designs. Diagram. This figure shows four examples of inaccessible curb designs. (A) Perpendicular curb ramps without a landing. (B) On a corner with a wide turning radius, curb ramps are aligned parallel with the crosswalk. (C) Diagonal curb ramp with no clear space or no level area at the bottom of the curb ramp. (D) Diagonal curb ramps without a level landing.

Figure 22. This crosswalk design incorporates the use of detectable warning surfaces into the curb ramps to facilitate navigation by a visually impaired pedestrian. Photo. The picture shows a visually impaired person using a cane stepping onto the detectable warning surface of a ramp, about to enter the street.

Figure 23. Standard NEMA ring-and-barrier structure. Diagram. The diagram shows a structure that organizes phases to prohibit conflicting movements from moving concurrently (I.E., eastbound through and northbound through) and allows nonconflicting movements to occur together (I.E., northbound through and southbound through). The diagram shows eight phases in two rows (representing rings 1 and 2) of four cells each, numbered 1 to 4 and 5 to 8, respectively. A thick vertical line, representing a barrier, is located between phases 2 and 3 and phases 6 and 7; a second barrier is located after phases 4 and 8. Arrows wrap around from phase 4 back to phase 1 and from phase 8 back to phase 5. The four phases depicted in ring 1 are (1) protected westbound left-turn movement; (2) east/west pedestrian movements on the south leg, eastbound through movement, and permissive eastbound left-turn and right-turn movements; (3) protected northbound left-turn movement; and (4) north/south pedestrian movements on the west leg, southbound through movement, and permissive southbound left-turn and right-turn movements. The four phases in ring 2 are (5) protected eastbound left-turn movement; (6) east/west pedestrian movements on the north leg, westbound through movement, and permissive westbound left-turn and right-turn movements; (7) protected southbound left-turn movement; and (8) north/south pedestrian movements on the east leg, northbound through movement, and permissive northbound left-turn and right-turn movements.

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Figure 24. Typical phasing diagram for "permissive-only" left-turn phasing. Diagram. "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 25. Possible signal head arrangements for "permissive-only" left-turn phasing. Diagram. The diagram shows two signal head arrangements that indicate "permissive-only" left-turn phasing. The first arrangement shows two vertical, three-section signal heads centered above two through lanes. The second 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.

Figure 26. Typical phasing diagram for "protected-only" left-turn phasing. Diagram. "Protected-only" phasing provides a separate phase for left-turning traffic that can only be made with a green arrow signal indication and without conflicting pedestrian or vehicular movements. This diagram shows the phasing for "protected only" left turns. The base drawing is identical to figure 23. The phases include: (1) a protected westbound left-turn movement; (2) east/west pedestrian movements on the south leg, eastbound through movement, and permissive eastbound right-turn movement; (3) a protected northbound left-turn movement; (4) north/south pedestrian movements on the west leg, a southbound through movement, and a permissive southbound right-turn movement; (5) a protected eastbound left-turn movement; (6) east/west pedestrian movements on the north leg, a westbound through movement, and a permissive westbound right-turn movement; (7) a protected southbound right-turn movement; and (8) north/south pedestrian movements on the east leg, a northbound through movement, and a permissive northbound right-turn movement.

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Figure 27. Possible signal head arrangements for "protected-only" left-turn phasing. Drawing. The diagram shows two ways to arrange signal heads to indicate "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. Signal Head arrangement: the red arrow is replaced by a red ball and a sign that reads "left-turn signal" in uppercase letters.

Figure 28. Typical phasing diagram for protected-permissive left-turn phasing. Diagram. This diagram shows a combination of eight protected and permissive phases. The base drawing is identical to figure 23. The phases include: (1) protected westbound left-turn movement; (2) east/west pedestrian movements on the south leg, eastbound through movement, and permissive eastbound left-turn and right-turn movements; (3) protected northbound left-turn movement; (4) north/south pedestrian movements on the west leg, southbound through movement, and permissive southbound left-turn and right-turn movements; (5) protected eastbound left-turn movement; (6) east/west pedestrian movements on the north leg, westbound through movement, and permissive westbound left-turn and right-turn movements; (7) protected southbound left-turn movement; and (8) north/south pedestrian movements on the east leg, northbound through movement, and permissive left-turn and right-turn movements.

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Figure 29. Possible signal head and signing arrangement for protected-permissive left-turn phasing. Drawing. The diagram shows two possible ways to arrange signal heads to indicate 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. 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.


Figure 30. Illustration of the yellow trap. Drawing. This diagram shows sequential phasing for a protected-permissive left-turn (PPLT) movement that experiences a yellow trap. The diagram shows a five-section signal head display for the PPLT movement, a three-section signal head display for an adjacent through lane, and a three-section signal head display for the opposing through movement. The six sequential indications shown for the PPLT display are: (1) red ball indications for the three depicted signal heads (label reads "All red"); (2) a green-arrow indication for the PPLT head and red ball indications for all three signal heads (label reads "Protected left turn"); (3) a yellow-arrow indication for the PPLT head and red ball indications for all three signal heads (label reads "Clearance interval (end protected left turn)"); (4) green-ball indications for the three signal heads (label reads "Permissive phase"); (5) yellow ball indication for the PPLT and adjacent through signal heads and a green ball indication for the opposing through signal head (label reads "Change interval (Yellow trap)"); and (6) red ball indication for the PPLT and adjacent through signal heads and a green ball indication for the opposing through signal head (label reads "Opposing through phase indication still green").

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Figure 31. The protected-permissive left-turn display known as "Dallas display" uses louvers to restrict visibility of the left-turn display adjacent lanes. Drawing. 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 32. Typical phasing diagrams for split phasing. Diagram. Three methods are shown for implementing split phasing. The base drawings are identical to figure 23. Method A: Consecutive pedestrian phases using one ring. The six phases are: (1) westbound protected left turn movement; (2) east/west pedestrian movements on the south leg, eastbound through movement, and permissive eastbound left-turn and right turn movements; (3) north/south pedestrian movements on the east leg, northbound through movement, and permissive northbound left-turn and right-turn movements; (4) north/south pedestrian movements on the west leg, southbound through movement, and permissive southbound left-turn and right-turn movements; (5) protected eastbound left-turn movement; and (6) east/west pedestrian movements on the north leg, westbound through movement, and permissive westbound left-turn and right-turn movements. Phases (7) and (8) are empty.

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Method B: Consecutive pedestrian phases using "exclusive" settings in controller. This method is functionally identical to Method A, except the setting in the controller requires the phases to time in an "exclusive" mode. The six phases are: (1) westbound protected left-turn movement; (2) east/west pedestrian movements on the south leg, eastbound through movement, and permissive eastbound left-turn and right-turn movements; (4, located where phase 3 would normally be) north/south pedestrian movements on the west leg, southbound through movement, and permissive southbound left-turn and right-turn movements; (8, located where phase 4 would normally be) north/south pedestrian movements on the east leg, northbound through movement, and permissive northbound left-turn and right-turn movements; (5) protected eastbound left-turn movement; and (6) east/west pedestrian movements on the north leg, westbound through movement, and permissive westbound left-turn and right-turn movements. Phase 7 and where phase 8 would normally be are empty.

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Method C: Concurrent pedestrian phases using two rings. Pedestrian movements are in a single phase in one ring that operates concurrently with two consecutively timing vehicle phases in the second ring. The seven phases are: (1) protected westbound left-turn movement; (2) east/west pedestrian movements on the south leg, eastbound through movement, and permissive eastbound left-turn and right-turn movements; (3) northbound through movement and permissive northbound left-turn and right-turn movements; (4) southbound through movement and permissive southbound left-turn and right-turn movements; (5) protected eastbound left-turn movement; (6) east/west pedestrian movements on the north leg, westbound through movement, and permissive westbound left-turn and right-turn movements; and (8) exclusive north/south pedestrian movements on the east and west legs. Phase 7 is empty.

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Figure 33. Common signal head arrangement for split phasing. Drawing. 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 34. Typical phasing diagram illustrating a right-turn overlap. Diagram. Right turns are operated on overlap phases to increase efficiency for right-turn movements. Overlap outputs are associated with two or more phase combinations. The base drawing is identical to figure 23. The eight phases are: (1) protected westbound left-turn movement; (2) east/west pedestrian movements on the south leg, eastbound through movement, and permissive eastbound right-turn movement; (3) protected northbound left-turn movement and protected eastbound right-turn movement (Overlap A); (4) north/south pedestrian movements on the west leg, southbound through movement, and permissive southbound right-turn movement; (5) protected eastbound left-turn movement; (6) east/west pedestrian movements on the north leg, westbound through movement, and permissive westbound right-turn movement; (7) protected southbound left-turn movement; and (8) north/south pedestrian movements on the east leg, northbound through movement, and permissive northbound right-turn movement.

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Figure 35. Common signal head and signing arrangement for right-turn-overlap phasing. Drawing. The diagram shows two ways to arrange signal heads to indicate right-turn-overlap phasing. 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. 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.

Figure 36. Examples showing five optional signal head locations. Diagram. The drawings show the same intersection configuration with vehicles approaching from different directions and signal heads at different locations. (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. (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. (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. (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. (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.

Figure 37. Pedestrian signal indicators. Diagram. The diagram shows the pedestrian signal indications for the "don't Walk" and "Walk" phases.

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

Figure 39. Example of advance street name sign for two closely spaced intersections. Photo. 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 40. Example of signing for a left-hand lane trap. Photo.  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 41. Example of advance overhead sign indicating lane use for various destinations. Photo. 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 42. Examples of pavement legends indicating destination route numbers ("horizontal signage"). Photo. 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 43. Exclusion of property-damage-only collisions (such as this one from an analysis) may mask valuable information. Photo. The picture shows a car that recently has been involved in a collision. Its hood is open, and a fireman is attending to the rear of the vehicle.

Figure 44. The potential for error in coding the location of a collision should be understood. Photo. The picture shows a car that has struck a plate-glass window of a building. a police officer is talking with two witnesses in the background on the corner of a signalized intersection.

Figure 45. Selecting a candidate intersection using a combined collision frequency/collision rate method, where each diamond represents an intersection. Graph. The horizontal axis is the collision rate per million vehicles entering the intersection (ranging from 0.0 to 2.0), and the vertical axis is the 5-year average frequency collision rate (ranging from 0 to 40). The graph depicts 10 scattered data points, with 1 point greater than 30 collisions per year and a collision rate of 1.50 per million vehicles entering the intersection.

Figure 46. Example of SPF curve. Graph.  The horizontal axis displays average daily traffic (ADT) ranging from 0 to 80,000 vehicles. The vertical axis shows collisions per year ranging from 0 to 50. The graph depicts eight data points. A best-fit curve is plotted that ranges from 7 collisions at 5,000 ADT to 40 collisions at 80,000 ADT. The large diamond above the curve that plots at 45 collisions at 50,000 ADT is an intersection performing worse than predicted.

Figure 47. Identification of potential problems. Flowchart. This flowchart identifies the steps to perform a safety diagnosis to determine causal factors for collisions in intersections. The first oval reads "Safety Diagnosis" and the steps include: assemble the collision data, perform a collision diagnostic analysis, determine overrepresentation (sites that experience more collisions than is expected based on their characteristics), and conduct a site visit. If the information collected is satisfactory, then the define problem statement box follows. If the information collected is not satisfactory, then the traffic engineer must conduct further studies, such as positive guidance and traffic conflict analysis.

Figure 48. The original police collision report may contain valuable information regarding collisions that have occurred at the intersection. Photo. The picture shows a police collision report that contains a handwritten description and a sketch of the intersection and collision; dimensions, vehicle positions, and trajectories are noted.

Figure 49. Possible taxonomy for collision type classification. Diagram. This diagram graphically depicts 22 types of collisions. They are: rear end; head on; sideswipe, same direction; sideswipe, opposite direction; overtaking; right turn, rear end; right turn, oncoming; left turn, oncoming; left turn, rear end; left turn, opposing through; right angle; right turn sideswipe; through with right; left turn sideswipe; through with left; left and right turn sideswipe; single vehicle with parked car; single vehicle with other than parked car; vehicle with pedestrian; vehicle with bicycle; bicycle with pedestrian; and a question mark for other.

Figure 50. Conducting a site visit. Photo. The photo shows a person taking notes and observing traffic conditions at an intersection. The observer is wearing a reflective orange vest.

Figure 51. Examples of problem statements. List. Problem Statement #1: Rear-end collisions and collisions occurring between 3 and 6 P.M. are overrepresented. The collision diagram shows that almost all of these occur on the westbound approach. Based on the site visit, the initial problem statement is that these are occurring due to lack of traffic signal visibility for westbound drivers, movement into and out of a commercial driveway on the near side of the intersection, a polished pavement surface on this approach, and glare from the afternoon sun.

Problem Statement #2: Fatal and injury collisions were overrepresented, and four fatal or injury collisions involved pedestrians. The collision diagram indicates that all occurred on the southwest corner of the intersection and are related to the right-turn lane channelization. Based on the site visit and subsequent further analysis, the initial problem statement is that these are occurring due to the design of the right-turn channelization operating under yield control, which contributes to excessive driver speed, drivers failing to yield to pedestrians, and the presence of a bus shelter that partially blocks the view of the crosswalk.

Figure 52. Identification of possible treatments. Flowchart. The steps to follow to identify possible treatments are: list possible treatments, screen treatments, apply study finding or collision modification factor (CMF), quantify the safety benefit, and select treatments.

Figure 53. Still reproduction of graphic from an animated traffic operations model. Photo. This figure shows a screen capture of the animation output from a microsimulation model. This particular example shows heavy vehicle queues on all approaches to a signalized intersection.

Figure 54. Overview of intersection traffic analysis models. Flowchart. This flowchart shows four steps of an operational analysis: (1) Quick Estimation Method; (2) HCM Operational Procedure; (3) Arterial and Network Timing Models; and (4) Microscopic Simulation Models. The Quick Estimation Method (QEM) requires volumes, number of lanes, and a maximum cycle length for input and produces a V/C ratio and intersection status as performance measures for the intersection as a whole. The QEM is based on critical movement analysis, is a good place to begin any intersection analysis, generates an initial signal timing plan, and is not generally suitable for detailed analysis. The HCM Operational Procedure requires a complete description of intersection geometrics and operational parameters, as well as initial timing from the QEM. It produces V/C ratio, control delay, maximum queue length, and level of service as performance measures for each lane group. The HCM Operational Procedure is a more detailed analysis with additional input data, produces performance measures by lane group, and has been adopted as a standard by many public agencies. Arterial and network timing models require similar inputs as is required for an HCM analysis, along with link characteristics such as length and speed; the method also takes saturation flow from an HCM analysis as an input. These models can produce an optimized signal timing plan along with generally the same performance measures as the HCM, plus number of stops and fuel consumption, and can provide implementable timing for an HCM analysis or microscopic simulation models. Arterial and network timing models recognize the time relationships between individual signals, propagate traffic macroscopically through the system, and produce optimal signal timing plans. Microscopic simulation models require input similar to the arterial and network timing models; however, more calibration of input data is needed for credible results. These models produce generally the same performance measures as the arterial and network timing models, plus air quality measures. Microscopic simulation models propagate each vehicle through the system as a separate entity, with updates typically once per second; they recognize queue blocking and overflow effects; each movement is treated individually, as opposed to lane group aggregation; and animated graphics are produced for improved visualization.

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Figure 55. Pedestrian LOs based on cycle length and minimum effective pedestrian green time. Graph. The horizontal axis shows cycle length ranging from 60 to 210 seconds. The vertical axis is effective green time for pedestrians (the minimum walk time plus four seconds flashing "don't walk" in uppercase letters) ranging from 10 to 170 seconds. Five lines have been drawn to mark the boundaries between consecutive levels of service (E.G., between a and b). All levels of service from A to F show that the amount of green time needed for pedestrian crossings increases with longer cycle lengths. For instance, for cycle lengths more than 150 seconds, a minimum pedestrian effective green time of 40 seconds is required to maintain level of service D.

Figure 56. Graphical summary of the Quick Estimation Method. Flowchart. This flowchart outlines seven steps for applying the Quick Estimation Method. The steps are: (1) Identify lane configurations; (2) Develop a signal phasing plan; (3) Determine the highest lane volume served in each phase; (4) Sum all phase volumes; (5) Determine the maximum critical volume that the intersection can accommodate; (6) Determine the critical volume-to-capacity ratio; and (7) Determine the intersection status from the critical volume-to-capacity ratio.

Figure 57. Issues associated with intersections with a narrow median. Diagram. The diagram shows one leg of a six-lane road with curbs and bike paths on both sides. There is an additional left-turn lane with a narrow median. The figure notes that there is insufficient room in the median for use as a pedestrian refuge and that pedestrians are required to cross in one stage from curb to curb.

Figure 58. Issues associated with intersections with a wide median. Diagram. The diagram shows an intersection with four legs. Each has six through lanes of traffic, bike paths on each side, a left-turn lane, and wide pedestrian refuges. The figure notes that pedestrians may have to cross in two stages, bicycles and motor vehicles have longer clearance time, and supplemental signal heads may be necessary.

Figure 59. Median pedestrian treatments. Drawing. The drawing shows two types of median treatments with detectable warning surfaces: the cut-through median and the ramped median. The dimensions include the width of the detectable warning surface at 915 millimeters (36 inches) minimum, 1525 millimeters (60 inches) preferred; the width of the median at 1.22 millimeters (48 inches) minimum, 1.83 millimeters (72 inches) preferred for a one-stage crossing and minimum for a two-stage crossing; and the width of the ramp at 1.22 millimeters (48 inches) minimum, 1.525 millimeters (60 inches) preferred.

Figure 60. Median pedestrian signal treatments. Diagram. The diagram shows the options for locating pedestrian signals. Option (A) shows a one-stage pedestrian crossing with signal heads and accessible pedestrian pushbuttons on both ends of the crosswalk. Option (B) shows a two-stage crossing with separate pedestrian displays and accessible pedestrian pushbuttons for each half of the crosswalk. The figure shows optional positions within the median on either side of the crosswalk for locating the pedestrian display and pushbutton. Option (C) shows a one-stage crossing with an optional two-stage crossing. Pedestrian displays are provided at the ends of the crosswalk, and accessible pedestrian pushbuttons are provided at the ends and on both sides of the crosswalk in the median to accommodate pedestrians crossing in two stages.

Figure 61. This refuge island enables two-stage pedestrian crossings. Photo. The photo shows a six-lane road with a turn lane and a wide median, with pedestrians in the crosswalks on either side of the median. Pedestrian displays are provided in the median. Fences are provided within the median to discourage pedestrians from crossing in one stage.

Figure 62. Comparison of physical and functional areas of an intersection. Diagram. The diagram shows the physical area of the intersection as a light gray shaded area bounded by curb returns of the intersection. The functional area is shown in darker gray beyond the physical area of the intersection and covers upstream and downstream segments to include deceleration distance, perception/reaction time distance, queue storage, stopping distance, and clearing the intersection.

Figure 63. Diagram of the upstream functional area of an intersection. Diagram. The diagram shows a series of vehicles in a right-turn lane before the intersection, with dimensions marked lowercase D1, D2, and D3. Moving from right to left, the vehicle begins perception-reaction at the start of D1. Where D1 ends and D2 begins, the vehicle begins deceleration and lateral movement. Within D2, the vehicle clears the traffic lane with a speed differential equal to or less than 15 kilometers pe r hour (10 miles pe r hour). Where the lateral movement is completed (at the end of D2/beginning of D3), full deceleration begins. Within D3, the vehicles are in queue. The minimum functional length is equal to D1 plus D2 plus D3.

Figure 64. Access points near signalized intersections. Diagram. Three examples are shown that illustrate the functional area available for access between two signalized intersections. (A) The intersection influence areas of the two signalized intersections do not overlap, and a region for potential access is available between the functional areas of the two signals. (B) The intersection influence areas of the two intersections partially overlap, creating a region for partial access between the two intersections. (C) The intersection influence areas of the two intersections fully overlap, creating a substantial amount of potential adverse effects related to access between the two intersections.

Figure 65. Access management requiring U-turns at a downstream signalized intersection. Diagram. The diagram shows a right-in/right-out/left-in (RIROLI) intersection located upstream of a full-access intersection that allows U-turn maneuvers. drivers that desire to turn left out of the RIROLI intersection instead turn right and make a downstream U-turn maneuver at a signalized intersection.

Figure 66. Access management requiring U-turns at an unsignalized, directional median opening. Diagram. The diagram shows a right-in/right-out/left-in (RIROLI) intersection and a median U-turn opening located before a signalized intersection. drivers that desire to turn left out of the RIROLI intersection instead turn right and make a U-turn at the midblock U-turn intersection.

Figure 67. A curb radius from 4.6 meters (15 feet) to 15.2 meters (50 feet) increases the pedestrian crossing distance from 18.9 meters (62 feet) to 03.5 meters (100 feet), all else being equal. Diagram. The diagram demonstrates the effect that curb radii have on the pedestrian crossing distance. Three examples are shown with an 18.2-meter (60-foot)-wide road. The top example has curb radii equal to 4.6 meters (15 feet), the middle example has curb radii equal to 7.5 meters (25 feet), and the bottom example has curb radii equal to 15.2 meters (50 feet). The pedestrian crossing distances increase from 18.9 meters (62 feet) in the top example, to 21.3 meters (70 feet) in the middle example, and 30.5 meters (100 feet) in the bottom example.

Return to Figure 67

Figure 68. Intersection with curb extension. Photo. The picture shows a curb extension that narrows the street width.

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.

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.

Figure 71. Typical lighting layouts. Drawing. Two typical lighting layouts are depicted. The first drawing shows a major five-lane road intersected by a crossroad. Partial lighting is required at the four corners of the intersection. Additional units for continuous lighting are required on the approaches to the intersection and are spaced in a staggered fashion on both sides of the road. The second drawing is a major four-lane road with a secondary road and a right-hand bypass lane. Partial lighting is required at the intersection corners and at the entrance and exit of the slip lane. Additional units for continuous lighting are required on the approaches to the intersection and are spaced in a staggered fashion on both sides of the road.

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Figure 72. Illustration of conflict points for a four-leg signalized intersection. Diagram. The four-leg signalized intersection shows 32 total potential conflicts: 8 diverging, 8 merging, and 16 crossing in the intersection.

Figure 73. Diagrams of different types of intersection realignment. Diagram. (A) This example depicts two roadways intersecting at an acute angle, a major roadway as left-to-right and the minor roadway as lower-left to upper-right. The minor roadway is shifted to one side and curving so that it intersects the major roadway at an angle closer to 90 degrees. All realignment occurs to one side of the minor roadway. (B) This example is similar to (A) except that the realignment occurs on both sides of the minor roadway, resulting in a new intersection at the approximate location of the original intersection. (C) This example depicts the same two roadways as (A) but breaks the intersection into two T-intersections. With the realignment, left turns destined for each of the minor roadways are stored between the two intersections. (D) This configuration is a mirror image of (C), with the minor roadway depicted upper-left to lower-right. With the realignment, left turns destined for each of the minor roadways are stored outside the two intersections. (E) The diagram depicts a major roadway on a curve, with a minor roadway intersecting it at the start of the curve at an acute angle. The diagram realigns the minor roadway using a curve to make its intersection with the major roadway a 90-degree angle.

Return to Figure 73

Figure 74. Example of deflection in travel paths for through vehicles. Photo. This photo shows a pickup truck proceeding through an intersection; opposite the vehicle are two lanes of cars waiting to turn left. The pickup driver is required to shift to the right to continue through the intersection, as the alignment of the through lane partially intersects the oncoming traffic.

Figure 75. Conflict point diagram for two closely spaced t-intersections. Diagram. Each T-intersection shows nine total potential conflicts: three diverging, three merging, and three crossing.

Figure 76. Diagram of a jughandle intersection. Diagram. The diagram shows a four-leg intersection with a jughandle ramp in one quadrant of the intersection.

Figure 77. Vehicular movements at a jughandle intersection. Diagram. (A) The major street movements that desire to turn left at the intersection instead turn right onto the jughandle in advance of the intersection, turn left at the cross street, then proceed through the major intersection. All right-turning traffic from the major street uses the jughandle as well.(B) The minor street turn movements occur at the major intersection and are not changed by the jughandle.

Figure 78. Example of a jughandle intersection. Photo. This picture shows a five-lane road with a jughandle ramp to the right in advance of the intersection.

Figure 79. Another example of a jughandle intersection. Photo. This jughandle veers slightly to the right and ties back into the major intersection, which is a T-intersection to the left. There is no intersecting cross street to the right.

Figure 80. Design layout of near-side jughandle. Drawing. The drawing shows a four-lane divided major street with a median running left and right. A jughandle connects the major street to the minor street in the lower left quadrant. Labeled dimensions include deceleration length along a ramp on the major street; an exit radius of 75 to 90 meters (250 to 300 feet) desirable, 45 meters (150 feet) minimum; a tangent section of jughandle ramp as needed for superelevation transition (no access permitted); a curb radius of 10 meters (35 feet) minimum at the intersection of the jughandle with the minor street; and a length on the minor street between the major street and jughandle sufficient for queue storage (30 meters (100 feet) minimum).

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Figure 81. Design layout of far-side jughandle. Drawing. The drawing shows a four-lane divided major street with a median. A loop ramp and bypass lane occupy one quadrant of the intersection. Traffic intending to turn left from the major street onto the minor street pass through the intersection, turn right onto the loop ramp, join the minor street, and pass through the intersection traveling on the minor street. Labeled dimensions include a ramp radius of 22.5 meters (75 feet) minimum, 27 meters (90 feet) desirable; a distance on the minor street of 30 meters (100 feet) minimum between where the loop ramp joins the minor street and the intersection with the major street; and tangent sections on the bypass lane as needed for superelevation transition (no access permitted).

Return to Figure 81

Figure 82. Example of jughandle and associated signing. Photo. The picture shows a jughandle exiting to the right in advance of a signalized intersection. White-hatched pavement markings are used in the gore area of the jughandle ramp to delineate through movements from right-turn movements. A white-on-green sign in the gore area of the ramp indicates "Carmen ave" in uppercase letters with an arrow pointing to the upper right. Below it, a black-on-white sign indicates "all turns" in uppercase letters with an arrow pointing to the upper right.

Figure 83. Signal phasing of a jughandle intersection. Diagram. Two phasing options are shown for a jughandle intersection. The first shows a two-phase signal with major street through movements (without left-turn movements) followed by minor street movements (left turns are permissive). The second option shows a three-phase signal where the minor street left-turn movements are protected and occur in advance of the minor street through movements.

Figure 84. Conflict point diagram for a four-leg intersection with two jughandles. Diagram. The diagram shows that this road configuration has 18 conflict points. Five occur at each of the two jughandles (one crossing, two merging, and two diverging), and eight occur in the intersection (four crossing, two merging, and two diverging).

Figure 85. Diagram of a median U-turn crossover from the main line. Diagram. The intersection diagram shows a four-lane divided east/west major street with a wide median and a four-lane north/south minor street with right-turn lanes and a narrow median. Two median U-turn crossovers on the major street eliminate left turns at intersections and move them to median crossovers beyond the intersection.

Figure 86. Vehicular movements at a median U-turn intersection. Diagram. Major street traffic that desires to turn left at the intersection instead travels through the intersection, makes a median U-turn beyond the intersection, and turns right onto the minor street. Minor street traffic that desires to turn left at the intersection instead turns right, makes a median U-turn, and travels through the intersection on the major street.

Figure 87. Example of median U-turn signing in Michigan. Photo. The picture shows a direction sign in advance of a U-turn location. The through arrow points north to Business 96 (Larch Street). The U-turn arrow points south to Cedar Street.

Figure 88. Diagram of general placement of median U-turn crossover. Diagram. The diagram shows a median U-turn on one side of a major intersection. The note reads that the nose of the crossover must align with the center lane of the side street. Another arrow points to the intersection and mentions that the optimum directional crossover spacing for signal progression is 200 meters (660 feet) (plus or minus 30 meters (100 feet)) from a major intersection. Another note reads, "The number of crossovers per mile is determined by need." Generally, 200 meters (660 feet) spacing is used in urban areas, and 400 meters (1320 feet) spacing is used in rural areas.

Figure 89. Diagram of a median U-turn crossover from the main line with a narrow median. Diagram. The intersection shows four legs with four lanes of traffic and a turn lane. The medians are narrow, and the pavement is widened using jughandles to accommodate the turning radius of large vehicles making the U-turn.

Figure 90. Conflict diagram for a four-leg signalized intersection with median U-turns. Diagram. A median U-turn configured intersection has four crossing angle conflicts and 12 merging/diverging conflicts.

Figure 91. Diagram of a continuous flow intersection. Diagram. The figure depicts a continuous flow intersection. Left-turning vehicles cross over opposing traffic upstream of the intersection, travel on the left side of opposing traffic up to the intersection, and turn left without conflict from the opposing traffic. The figure provides several example dimensions: 107 meters (350 feet) of left-turn storage upstream of the crossover, a crossover length of 38 meters (125 feet), a distance between the crossover and the main intersection of 99 meters (325 feet), a right-turn storage length of 76 meters (250 feet), and a right-turn acceleration lane of 92 meters (300 feet).

Figure 92. Vehicular movements at a continuous flow intersection. Diagram. This diagram shows that vehicles desiring to turn left at the major intersection instead turn left in advance at a signalized crossing, continue through on a parallel road, and turn left at the major intersection concurrently with through traffic (because the left-turning vehicles are positioned to the left of through traffic).

Figure 93. Continuous flow intersection. Photo. The picture shows an example of a continuous flow intersection.

Figure 94. Displaced left turn at a continuous flow intersection. Photo. The picture shows an example of the displaced left turn at a continuous flow intersection in an urban setting.

Figure 95. Signal phasing of a continuous flow intersection. Diagram. The first stage shows street a movements at the major intersection, left turns at the advance intersections on Street a, and through movements at the advance intersections on Street B. The second stage shows street a movements at the major intersection and through movements at all four advance intersections. The third stage shows street a movements at the major intersection, through movements at the advance intersection on Street a, and left-turns at the advance intersections on Street B. The fourth, fifth, and sixth stages are a repeat of the first three stages but are shown for the street B approaches.

Figure 96. Conflict diagram for a continuous flow intersection with displaced left turns on the major street only. Diagram. The continuous flow intersection has 30 potential conflict points: 14 merging/diverging, 6 crossing (left turn), and 10 crossing (angle).

Figure 97. Diagram of a quadrant roadway intersection. Diagram. The diagram shows a quadrant roadway located in the lower left quadrant of the intersection. The quadrant roadway serves all traffic that desires to turn left at the major intersection.

Figure 98. Vehicular movements at a quadrant roadway intersection. Diagram. A diagram is shown with a major intersection and a quadrant roadway located in the lower left quadrant. All left-turn movements are eliminated at the major intersection and are redirected to the quadrant roadway. Some right-turning vehicles also use the quadrant roadway.

Figure 99. Signal phasing of a quadrant roadway intersection. Diagram. A quadrant roadway can be operated with three stages. Right-turn movements at the quadrant intersections are overlapped with the protected left-turn movement on the adjacent approach. The first stage serves east-west traffic at the major intersection, east-west traffic at the west intersection, and eastbound traffic at the south intersection. The second stage serves east-west traffic at the major intersection, westbound traffic at the west intersection, and northbound traffic at the south intersection. The third stage serves north-south traffic at the major intersection, northbound traffic at the west intersection, and southbound traffic at the south intersection.

Figure 100. Conflict point diagram for four-leg signalized intersection with quadrant roadway. Diagram. The quadrant roadway intersection has 28 potential conflict points: 20 merge diverge, 4 crossing (left turn), and 4 crossing (angle).

Figure 101. Illustration of super-street median crossover. Diagram. This type of intersection is similar to the median U-turn, but major street left-turn movements are permitted and all minor-street traffic turns right. The super-median crossover in the diagram shows 12 lanes of traffic traveling east/west and 6 lanes traveling north/south with medians in all directions. The main intersection shows a channelization island with four others in the main intersection and two on the east/west road after the narrow median. Traffic travels in loops around the intersection directed by the channelization islands. Pedestrians are allowed to cross the major street from the lower left quadrant to the center channelization island and then to the upper right quadrant.

Figure 102. Vehicular movements at a super-street median crossover. Diagram. All minor street movements turn right at the main intersection. Traffic that desires to continue through on the minor street makes a right turn at the main intersection, a U-turn downstream of the main intersection at the crossover, then turns right onto the minor street. Traffic that desires to turn left at the main intersection turns right at the main intersection, makes a U-turn at the crossover, then continues through on the major street. Traffic on the major street can make all turn movements at the main intersection (left, through, and right).

Figure 103. Signal phasing of a super-street median crossover. Diagram. A super-street median crossover can operate with two independent two-phase signals, one for each direction of the major street. For each intersection, the first phase allows protected right turns from the minor street, left turns from the major street, and north/south pedestrian crossings. The second phase allows east/west pedestrian crossings, major street through traffic, and permissive right turns.

Figure 104. Conflict diagram for a super-street median crossover. Diagram. The super-street median crossove r has 20 potential conflict points: 5 at each minor street intersection with the major roadway and 5 at each U-turn crossover. Of the total, 18 are merge/diverge conflicts and 2 are left-turn crossing conflicts.

Figure 105. Illustration of a split intersection. Diagram. A split intersection divides the major road into one-way streets, each having a separate intersection with the minor street. The shape of the intersection is similar to that of a diamond interchange but without the grade separation.

Figure 106. Conflict point diagram for a split intersection. Diagram. The split intersection has 22 potential conflict points; 11 at each intersection. Within each intersection there are 6 merge/diverge conflicts, 3 left turn crossing conflicts, and 2 angle crossing conflicts.

Figure 107. Diagram of a single-point interchange. Diagram. The single-point diamond interchange (or single-point urban interchange) operates as a single signalized intersection. The main road is grade-separated from the minor road. Left turns to and from the ramps on the major road are angled at approximately 45 degrees and align opposite each another.

Figure 108. Diagram of a compressed diamond interchange. Diagram. The major road is grade-separated from the minor road in this compressed diamond configuration. The ramps form two intersections with the minor road. The minor road left turn lanes are configured side-by-side on the bridge structure.

Figure 109. Typical signal phasing of a single-point interchange. Diagram. In this three-phase signalization, the first phase serve protected left turns on the arterial street, the second phase serves east/west pedestrian traffic in both lanes and protected arterial street through traffic, and the third phase serves left-hand turns from the ramps.

Figure 110. Typical signal phasing of a compressed diamond interchange. Diagram. The compressed diamond interchange operates with six stages. Each stage calls out movements for both ramp intersections so that both intersections operate as one. The drawings are oriented assuming that the nongrade-separated arterial street runs east-west and the ramps run north-south. The first stage serves east-west through movements at the southbound ramp intersection and eastbound-only movements at the northbound ramp intersection. The second stage serves the southbound off-ramp movements and traffic traveling in the eastbound direction at the northbound ramp. The third stage serves the southbound off-ramp movements and east-west through movements at the northbound ramp. The fourth stage serves westbound only movements at the southbound ramp intersection and east-west through movements at the northbound ramp. The fifth stage serves the northbound off-ramp movements and westbound-only movements at the southbound ramp. The sixth stage serves the northbound off-ramp movements and east-west through movements at the southbound ramp. Pedestrian crossings are allowed on at least two approaches in each stage.

Return to Figure 110

Figure 111. Single-point diamond interchange conflict point diagram. Diagram. The single-point diamond intersection has 24 potential conflict points: 12 in the main intersection (8 left-turn crossing and 4 merge/diverge conflicts), and 16 conflicts at the ramp merge and diverge points.

Figure 112. Compressed diamond interchange conflict point diagram. Diagram. The compressed diamond intersection has 30 potential conflict points. Of these, 13 occur at each interchange ramp and 4 occur at the upstream/downstream ramp merge and diverge points with the mainline. Of the 13 ramp intersection conflicts, 8 are merge/diverge, 3 are left-turn crossing, and 2 are angle crossing.

Figure 113. Signal head with a double red signal indication. Photo. The photo shows a signalized intersection with a signal head that has a double red signal indication on the near side of the intersection. The double red indications are positioned side-by-side and are centered above the vertical yellow and green indications.

Figure 114. Lane-aligned signal heads. Photo. The picture shows one signal head and lane use signs for each lane of traffic (two left turns, two straight through traffic lanes, and one right turn) placed on a mast arm.

Figure 115. Illustration of sight distance triangles. Diagram. Two diagrams are shown that illustrate the sight distance triangle for a minor road vehicle that approaches a major road. The first shows the sight triangle looking to the driver's left and represents the field of vision that a minor road driver would see when looking for an oncoming vehicle. The area within the shaded triangle represents the clear sight triangle. The second illustration shows the sight triangle for a minor road driver looking to his o r her right.

Figure 116. Diagram of a single left-turn lane. Diagram. The diagram shows an illustration of a left-turn lane. The key design elements of the left-turn lane include: the length of storage (L), the radius of reversing curve (R), the stopping sight distance (S), the tangent distance required to accommodate a reversing curve (T), the width of the intersection (W), and the taper for widening on the approach.

Figure 117. Narrow (2.4-meter (8-foot)) left-turn lanes may be used effectively in retrofit situations. Photo.

The example shows a narrow left-turn lane at a retrofitted signalized intersection. Vehicles in the left turn lane shy away from the median curb toward the stripe that separates the left-turn lane from the adjacent through lane.

Figure 118. Example of positive offset. Diagram. This diagram shows an example of a positive offset where the left-turn lanes are positioned further to the left, which removes them from the sight lines of opposing left-turn drivers. The diagram on the left shows a normal left turn lane with no offsets. The right diagram shows shaded portions or buffers on both lanes on the right side turn lane parallel to the through travel route. This marking causes left-turning drivers to position closer to the median and improves sight distance.

Figure 119. Intersection with turn paths delineated for dual left-turn lanes in Tucson, Arizona (Kolb Road/22nd Street), June 1998. Photo. This aerial view of a large intersection in Tucson, Arizona shows delineated paths for dual left-turn lanes on all approaches. The dual left turn lanes are offset to the left to improve sight distance for protected-permissive operation.

Figure 120. Diagram of an auxiliary through lane. Diagram. Auxiliary lanes of limited length usually occur on the mainline of a high-volume signalized intersection before the intersection, and taper back downstream of the intersection to give added capacity for through movements. The diagram shows the main road with a lane addition taper, storage in advance of the intersection, an auxiliary lane extending away from the intersection, and a lane drop taper.

Figure 121. Examples of delineated paths. Diagram. This diagram shows dotted through lane lines in an intersection to guide through movements that are not aligned through the intersection.

Figure 122. Diagram of a typical right-turn lane. Diagram. This diagram shows a right-turn lane with a taper (not steeper than 4 to 1 with a minimum length of 15 meters (50 feet)), a deceleration lane length, and a right-turn lane width of 3.6 meters (12 feet).

Figure 123. Narrow (2.4-meter (8-foot)) right-turn lanes may be used effectively in retrofit situations. Photo. This photo of a signalized intersection in an urban setting shows a narrow right-turn lane with a sign that reads "right lane must turn right" in uppercase letters. Four cars wait in the queue to turn right.

Figure 124. Example illustration of a channelized right-turn lane. Diagram. This diagram shows a channelized right turn located outside of a bicycle lane. A channelization island is located at the corner of the intersection and has ramps for pedestrians. The diagram shows a right-turn radius of 20 meters (70 feet) for the right-turn lane and a 50-60 degree angle for the junction of the channelized right-turn lane with the cross street.

Figure 125. Example use of variable lane use sign to add a third left-turn lane during certain times of day. Photos. (A) The top picture shows a fiber optic variable lane use sign that allows through and right-turn movements in the outside lane. (B) At the same intersection during the evening peak period, the sign changes to allow left-turn, through, and right-turn movements.

Figure 126. Example of variable lane use sign to add a second right-turn lane along a corridor during certain times of day. Photo. The picture shows fiber optic variable lane use signs on a mast arm in advance of the intersection and on the signal mast arm at the intersection. The variable sign shows a shared through-right movement and is located to the left of a fixed sign indicating right turn only.

Equations

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

Equation 2. 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.

Equation 3. The chi-square test value equals the quotient of the square of (X, which is the frequency of the collision type being investigated, minus the product of P, which is the average ratio for the collision type being investigated, times N, which is the total number of collisions at the site), all divided by P times N, plus the quotient of the square of (N minus X, minus N times (1 minus P)), all divided by (N times 1 minus P).

Equation 4. This is equation 3, with actual numbers inserted for the variables. The chi-square test value equals the quotient of 18 minus the product of 0.254 times 50, all squared, divided by the product of 0.254 times 50, plus the quotient of the sum of 50 minus 18, minus 50 times the sum of 1 minus 0.254, all squared, all divided by 50 times the sum of 1 minus 0.254, which equals 2.96.

Equation 5. Safety benefit in dollars equals the expected reduction in property-damage-only collisions times the societal costs of property-damage-only collisions plus the expected reduction in injury collisions times the societal costs of injury collisions plus the expected reduction in fatal collisions times the societal cost of fatal collisions.

Equation 6. The capacity of a bicycle lane, in bicycles per hour, equals the saturation flow rate of the bicycle lane, in bicycles per hour, times the ratio of the effective green time for bicycle lane, in seconds, divided by the signal cycle length, in seconds, which, in this case, equals 2000 times the ratio of the effective green time for bicycle lane, in seconds, divided by the signal cycle length, in seconds.

Equation 7. Control delay, in seconds per bicycle, equals the quotient of a numerator divided by a denominator. The numerator equals 0.5 times the signal cycle length, in seconds, times the square of the difference 1 minus the ratio of the effective green time for bicycle lane divided by the signal cycle length, both in seconds. The denominator equals 1 minus the effective green time for bicycle lane divided by the signal cycle length, both in seconds, times minimum of either: the flow rate of bicycles in the bicycle lane (one direction), in bicycles pe r hour, divided by the capacity of the bicycle lane in bicycles per hour, or the value of 1.0.

Equation 8. The average pedestrian delay, in seconds, equals 0.5 times the square of the difference of cycle length minus effective green time for pedestrians, both in seconds, all divided by the cycle length.

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United States Department of Transportation - Federal Highway Administration