Traffic Signals

Traffic Signals
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Terminology

Movement:
- A pair of directed street segments (e.g., straight, right turn, left turn)
Phase:
- The timing unit associated with one or more movements with the same initial directed street segment (e.g., northbound straight and northbound right)

Terminology (cont.)

Interval:
- A duration of time during which the signal indications do not change (e.g., a pedestrian phase contains three intervals: Walk, Flashing Don't Walk, Solid Don't Walk)
Cycle:
- The sequence of signalizations for all movements at an intersection

Users and Movements

Users:
- Vehicles
- Pedestrians
- Bicyclists
Types of Movements:
- Protected - no conflict is possible
- Permissive - the movement can be made only after yielding

An Example 4-Leg Intersection

Vehicular Movements are Shown with Purple Arrows
Phases are Numbered in Gold

Ring-and-Barrier Diagrams

Purpose:
- Organizing phases
Components:
- Rings illustrates sequential movements
- Barriers avoid conflicting movements (e.g., separate N-S movements from E-W movements)

Ring-and-Barrier Diagrams (cont.)

An Example for a 4-Leg Intersection

Left-Turn Options

Permissive:
- Reduces overall delay but also reduces safety
Protected:
- Reduces delay for turning vehicles but increases overall delay
Prohibited:
- Reduces conflicts but increases delay (overall and for turning vehicles)

Left-Turn Options (cont.)

Ring-and-Barrier Diagram for Protected Left-Turn Phasing

Nerd Humor

(Courtesy of xkcd)

Vehicular Green Interval

Minimum Green:
- The least amount of time that a green signal indication will be displayed for a movement (which can be based on driver expectancy, pedestrian crossing times, and/or queue clearance)
Maximum Green:
- The maximum amount of time that a green signal indication can be displayed in the presence of conflicting demand

Vehicular Change and Clearance Intervals

Yellow Change:
- Based on driver perception+reaction time and the distance needed to safely stop or to travel safely through the intersection
- Permissive - A driver can enter the intersection during the entire yellow interval and be in the intersection during the red indication as long as the vehicle entered the intersection during the yellow interval
- Restrictive A - A vehicle may not enter an intersection when the indication is yellow unless the vehicle can clear the intersection by the end of yellow
- Restrictive B - A vehicle may not enter an intersection unless it is impossible or unsafe to stop
Red Clearance:
- An interval at the end of the yellow change interval during which the phase has a red-signal display before the display of green for the following phase

Systems of Traffic Signals

Isolated:
- Each traffic signal operates in isolation
Coordinated:
- Signals at multiple intersections are synchronized to enhance the operation of one or more directional movements in a system

Systems of Traffic Signals (cont.)

An Example Time-Space Diagram for Coordinated Signals

Systems of Traffic Signals (cont.)

Some Existing Optimization Packages:
- Synchro
- PASSER^TM II
- TRANSYT-7F
Complicating Factors:
- Heavy side street volumes
- Turn bay interactions
- Oversaturated conditions