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Section 8: Intersection Delay Study

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Introduction

To meet the requirements for Warrant 10, “Peak Hour Delay,” a delay study must be performed at the subject intersection. Typically, an intersection delay study is conducted at intersections or major driveways where congestion problems exist. This study is considered as a detailed investigation of the stopped-time delay conditions at an intersection being evaluated for signalization.

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Time of Study

The intersection delay study should be performed during periods of congestion. Typically, the peak delay occurs during the peak hour, which can be identified from the traffic counts. The peak delay may occur during the major street’s peak hour or during the minor street’s peak hour, so care should be taken when determining the study time period. In some cases, both time periods need to be studied to determine the peak delay hour. It may be desirable to start the delay study 30 minutes before the beginning of the peak hour and end it 30 minutes afterwards to ensure that the peak delay is recorded.

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Equipment

Unless the district has a delay meter, the intersection delay data is usually collected manually. In most cases, one observer is required for each intersection approach being evaluated. In some cases, traffic volumes are too heavy for one person to handle alone, and a second observer is used.

Each observer needs

  • a stop watch or wristwatch with a second hand
  • a clipboard and paper to record the delay data.
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Form

The Intersection Delay Study Field Sheet can be used to record the data. A sample of the form is provided in Appendix A of this manual. This sample form may be photocopied as necessary. Copies may also be obtained from the Traffic Operations Division. In the on-line version of this manual, an MS Word version of this form may be opened and printed out by clicking on the following file name: TFF-IDS.

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Procedure

Performing a delay study involves counting the vehicles stopped in the intersection approach at successive intervals.

Selecting the Interval. The typical duration for the interval is 15 seconds. Other values can be selected; however, if a larger interval is selected, the amount of overestimation of delay increases. Conversely, if a smaller interval is selected, the amount of overestimation of delay is lower, but the amount of data collected increases. So the 15 second interval represents a good compromise.

Preparing the Form. Before the start of the study, the identifying information is entered in the appropriate places on the Intersection Delay Study Form. The first column is completed to indicate the starting times in minutes for the indicated succession of sampling intervals.

Counting and Recording. When the starting time arrives, the observer counts and records the number of vehicles stopped on the approach for each observation time indicated. As a vehicle arrives, it is recorded on the “Total Number of Vehicles” section in the column corresponding to the 15 second interval when the vehicle arrived. For example, if a vehicle arrived at 8:00.08 am, it is recorded in the column “+ 0 sec.” If this same vehicle is still waiting at the stop line at 8:00.15 am, it is recorded in the column “+ 15 sec.” Thus, a vehicle is counted more than once in the delay determination if it is stopped during more than one sampling time.

A separate tabulation of the approach volume is made for each time period by classifying vehicles as stopped or not stopping. (Note: the vehicles not stopping column is typically used for a delay study of an existing signalized intersection.) The number of stopping vehicles is always equal to or less than the total number of vehicles stopped on the approach for a specific time interval, because vehicles can be delayed for more than one sampling period.

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Study Results

Each vehicle counted in the delay study is assumed to be stopped for the duration of the selected interval (typically 15 seconds). Each column is added up in each of the subtotal blocks, and the total is recorded in the “Total” block. The total number of vehicles delayed is then multiplied by the interval (15 seconds) to get total vehicle-seconds of delay. Then the highest four consecutive 15 minute time periods are added together. This sum is then divided by 3600 to convert the value to vehicle-hours of delay. The result is then used to determine if Warrant 10, “Peak Hour Delay,” is met.

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