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Sub-problem 1a - Page 4 of 8

ID# C101A04

Sub-problem 1a: Analysis of the existing TWSC intersection

What default values should be used? Driver behavior at a TWSC intersection is described by two parameters, the critical gap and the follow up time. The HCM provides default values that represent average values from a number of measurements made at sites throughout the United States for critical gap and follow up time. However, conditions at the site (unusual geometric features, high volumes causing more aggressive driver behavior) that you are studying may produce values that are different than these default values. It is always better to use values that are estimated from the site that you are studying, if these values can be measured.

However, it should be pointed out that critical gaps cannot be directly measured in the field. Rather, they are estimated using statistical procedures based on the distribution of gaps that are accepted and rejected by drivers in the field. An appropriate but somewhat complex method for doing this was presented by Troutbeck in 1992. A rough method to check the validity of the critical gap would be to measure the follow-up time, which can be done easily, and to estimate the critical gap using the approximate relationship, tf/0.6.

What time periods should be analyzed? The data that have been collected for this site represent the volumes for the afternoon peak period. If there are other peak times during the day, such as the morning peak or sometimes a midday peak, these should also be included in an operational analysis.

What measure should be used to determine the performance of the intersection? The level of service for a TWSC intersection is based on control delay. Control delay is the primary measure of effectiveness for TWSC intersections and will be used as the parameter to compare the performances of the various alternatives that we consider in this problem. In fact, control delay is also used as the primary measure of effectiveness for signalized intersections. In addition, we will consider two other performance measures, each describing another aspect of the operation of the intersection. The v/c, or volume/capacity ratio is useful for showing how close the intersection is to capacity operation. The queue length provides a way for the analyst to determine the adequacy of the geometric design of the facility by examining the projected length of a queue compared with the length of turning or storage lanes.

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