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.