Module 7 : Traffic Signal Design

Lecture 35 : Signalized Intersection Delay Models

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	Types of delay The most common measure of operational quality is delay, although queue length is often used as a secondary measure. While it is possible to measure delay in the field, it is a difficult process, and different observers may make judgments that could yield different results. For many purposes, it is, therefore, convenient to have a predictive model for the estimate of delay. Delay, however, can be quantified in many different ways. The most frequently used forms of delay are defined as follows: Stopped time delay Approach delay Travel time delay Time-in-queue delay Control delay These delay measures can be quite different, depending on conditions at the signalized intersection. Fig 1 shows the differences among stopped time, approach and travel time delay for single vehicle traversing a signalized intersection. The desired path of the vehicle is shown, as well as the actual progress of the vehicle, which includes a stop at a red signal. Figure 1: Illustration of delay measures Note that the desired path is the path when vehicles travel with their preferred speed and the actual path is the path accounting for decreased speed, stops and acceleration and deceleration. Stopped Time Delay Stopped-time delay is defined as the time a vehicle is stopped in queue while waiting to pass through the intersection. It begins when the vehicle is fully stopped and ends when the vehicle begins to accelerate. Average stopped-time delay is the average for all vehicles during a specified time period. Approach Delay Approach delay includes stopped-time delay but adds the time loss due to deceleration from the approach speed to a stop and the time loss due to re-acceleration back to the desired speed. It is found by extending the velocity slope of the approaching vehicle as if no signal existed. Approach delay is the horizontal (time) difference between the hypothetical extension of the approaching velocity slope and the departure slope after full acceleration is achieved. Average approach delay is the average for all vehicles during a specified time period. Travel Time Delay It is the difference between the driver's expected travel time through the intersection (or any roadway segment) and the actual time taken. To find the desired travel time to traverse an intersection is very difficult. So this delay concept is is rarely used except in some planning studies. Time-in-queue Delay Time-in-queue delay is the total time from a vehicle joining an intersection queue to its discharge across the STOP line on departure. Average time-in-queue delay is the average for all vehicles during a specified time period. Time-in-queue delay cannot be effectively shown using one vehicle, as it involves joining and departing a queue of several vehicles. Control Delay Control delay is the delay caused by a control device, either a traffic signal or a STOP-sign. It is approximately equal to time-in-queue delay plus the acceleration-deceleration delay component. Delay measures can be stated for a single vehicle, as an average for all vehicles over a specified time period, or as an aggregate total value for all vehicles over a specified time period. Aggregate delay is measured in total vehicle-seconds, vehicle-minutes, or vehicle-hours for all vehicles in the specified time interval. Average individual delay is generally stated in terms of seconds per vehicle for a specified time interval.