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Sub-problem 1a - Page 5 of 8

ID# C101A05

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

What other factors should be considered? One of the important elements in determining the capacity of each minor movement at a TWSC intersection is the conflicting flow, which is defined as that flow to which the minor movement must yield the right of way. For this problem, we might pay particular attention to the northbound right turn movement. If you study the aerial photograph for this approach (link), you will note that there is a flare, providing a short lane for right turning vehicles.

We should also study whether or not right turning vehicles commonly used this lane, and, if so, whether this results in a lower conflicting flow faced by the minor street movement. At first glance, you might conclude that the right-turning traffic shouldn't even be included as one of the conflicting traffic streams since there is no physical conflict with any of the minor movements. However, field studies have shown that minor street drivers are reluctant to initiate their movement until they can be sure that the approaching vehicle is in fact going to make a right turn. It is for this reason that the HCM includes the major street right-turn volume as a potential conflict; the 0.5 factor associated in some instances with this right-turn volume was determined through field studies to be an appropriate "calibration factor" that accounts for the hesitancy minor street drivers often exhibit when confronted with an apparent right-turning vehicle. It should be noted that the HCM suggests that the major street right turn movement from a separate lane be entirely excluded from the conflicting flows for the minor street movement because, in such cases, the minor street drivers are usually quite confident that the approaching vehicle is actually going to make a right turn. 

In the case of this particular sub-problem, the flare or right-turn lane is very short, and so we will assume that the effect of the right turning vehicles is accounted for with the 0.5 factor assumed in the HCM procedure. For a complete discussion of the conflicting flow rates, see Exhibit 17-4 of the HCM.

Review the traffic data to see the existing afternoon peak period volumes for this site.

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