Smart Traffic Signals

Smart Traffic Signals
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

The Idea:
- Use "demand for service" in addition to signal timings to improve the performance of traffic signals
Measures of Performance:
- Delay (e.g., shorten phases late at night)
- Throughput
- Safety (e.g., provide a safe phase termination for high-speed movements by reducing the chance of being in the indecision zone)

Variants:

Semi-Actuated Control:
- Uses detection only for the minor movements
Fully-Actuated Control:
- All movements are detected and all phases are actuated

Detecting the "Demand for Service"

Pedestrians:
- Principally user-activated push buttons
Vehicles:
- Inductive loops
- Digital cameras equipped with image analysis algorithms
- Acoustic detectors

Operating Modes

Detectors:
- Pulse Mode - point detection that results in a short "on" pulse
- Presence Mode - time-interval detection that starts with the arrival of the vehicle and ends with its departure
Controller Memory:
- Locking - the first actuation on a channel during the red interval triggers a call for service and is retained until the assigned phase is serviced (regardless of whether there are demanders waiting to be served)
- Non-Locking - the actuation is recognized only during the time the demander is present (hence can't be used with pulse detection)

Detection for High-Speed (> 35mph) Approaches

Visualization of the Indecision Zone:
Defining the Indecision Zone:
- Distance from stop line
- Travel time to stop line (e.g., 85% of drivers stop if they are more than 3 secs from the stop line)
- Stopping sight distance (SSD)

Detection for High-Speed Approaches (cont.)

Position of Upstream Detector:
- Typically located at the start of the indecision zone
Number of Detectors:
- Typically 3 or 4

Common Designs - Basic

Characteristics:
- Detection zone at the stop line
- Detectors in presence mode
- Non-locking memory
Guidelines:
- Detection zone should not be shorter than 20 feet
- A queued vehicle should not be able to stop between the trailing edge of the detection zone and the crossroad

Common Designs - Volume-Density

Characteristics:
- 6-foot detector upstream of the stop line (determined by the desired maximum allowable headway)
- Detectors in presence mode
- Locking memory
Guidelines:
- Not suitable for large turning movements

Common Designs - Multiple Detector

Characteristics:
- One 6-foot detector at 60 feet (this is the last detector)
- One 6-foot detector at the upstream safe stopping distance (this is the first detector)
- Detectors in presence mode
- Non-locking memory
Guidelines:
- Suitable for high percentage of turning movements because these vehicles "gap out" as they slow to make the turn