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

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. 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, t_f/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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