Signalized Intersections: Informational Guide

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CHAPTER 4 — TRAFFIC DESIGN AND ILLUMINATION

TABLE OF CONTENTS

4.0 TRAFFIC DESIGN AND ILLUMINATION

4.1 Traffic Signal Control Type

4.2 Traffic Signal Phasing

4.2.1   “Permissive-Only” Left-Turn phasing

4.2.2  “Protected-Only” Left-Turn phasing

4.2.3 Protected-Permissive Left-Turn phasing

4.2.4 Split Phasing

4.2.5 Prohibited Left-Turn phasing

4.2.6 Right-Turn phasing

4.3 Vehicle and Pedestrian Displays

4.3.1 Vehicle Displays

4.3.2  Pedestrian Displays

4.4  Traffic Signal Pole Layout

4.5 Traffic Signal Controller

4.6   Detection Devices

4.6.1 Vehicle Detection

4.6.2     Pedestrian Detection

4.7  Basic Signal Timing parameters

4.7.1 Pedestrian Timing

4.7.2 Vehicle timing—Green interval

4.7.3 Vehicle timing—Detector Timing

4.7.4 Vehicle timing—Vehicle Clearance

4.7.5  Vehicle timing—Cycle Length

4.8 Signing and Pavement Marking Design

4.9 Illumination Design

4.9.1 Illuminance

4.9.2  Veiling Illuminance

LIST OF FIGURES

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23

Standard NEMA ring-and-barrier structure

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

LIST OF TABLES

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

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

Some disadvantages include the following:

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

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

• A special logic package can be used to suppress the green arrow display whenever the pedestrian phase is being served.
• A static sign indicating “LEFT TURN YIELD TO PEDS ON GREEN (symbolic green ball)” can be located next to the leftmost signal head for emphasis.
• A blankout sign indicating “LEFT TURN YIELD TO PEDS” can be activated when the conflicting vehicular and pedestrian phases are running concurrently.

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

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

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

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

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

• Pedestrian walkway and ramp locations.

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

• Clearance from the travel way.

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

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

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

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

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

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

• The cabinet should be located near the power source.

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

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

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

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

• Parallel to the crosswalk to be used.

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

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

• Advance notice of the intersection.

• Directional route guidance.

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

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

• Delineation and warning of pedestrian crossing locations.

• Delineation and warning of bicycle lane locations.

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

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

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

Part II

Project Process and Analysis Methods

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

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