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Sub-problem 1a - Page 7 of 8

ID# C101A07

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

Level of service (LOS) for a TWSC intersection is determined by the control delay and is defined for each movement, but LOS alone does not tell the whole story! Besides LOS, other performance measures that should also be considered include the volume/capacity ratios and the estimated queue length. Let's investigate each of these measures of effectiveness in greater detail (see Exhibit 1-9).

First, note that the left turn movements on the major street (NBLT and SBLT, movements 1 and 4), experience acceptable delay. The HCM methodology estimates that both movements will experience level of service A, with less than 10 seconds of control delay per vehicle. We can also see that the movements on the minor street approaches (westbound on Styner and eastbound on Lauder) experience moderate to high levels of delay. Both left turn movements (movements 7 and 10), for example, operate at level of service E with delays of 36 and 47 seconds, respectively.

The volume/capacity ratio is also important to consider because it tells us how close we are to capacity for each movement. Here, the v/c ratio is less than 0.60 for all movements. Thus, there is ample capacity available for the existing conditions. 

Queue length is always an important consideration at an unsignalized intersection, and especially when it is necessary to determine the adequacy of turning bays or when there is the possibility of a queue spilling back into the adjacent upstream intersection. In the case of the Styner-Lauder/U.S. 95 intersection, the 95th-percentile queue length estimates are less than four vehicles, meaning that there is sufficient space to store vehicles as they are waiting to enter the intersection. Even so, the queue length requirements for the right turns from the minor street (movements 9 and 12) are 3 vehicles. This is a length that some drivers might consider to be excessive (even though it only has a 5 percent probability of occurring), and especially so for drivers in a smaller community like Moscow.

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