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Sub-problem 1b - Page 2 of 9

ID# C201B02

Sub-problem 1b: Maxwell Drive PM Peak Hour - With Conditions

Configuration Issues
Finding the best lane configuration for a new or revised intersection is one of the most exciting things a traffic engineer does. When a plan you develop works well, itís invigorating to see it in action.

For Maxwell Drive, with the new site traffic, we have an opportunity to see what designs will work best. There isnít going to be just one right answer; most likely, there will be several. Thatís usually the case with design problems.

Weíre going to start by examining the volumes for the "with" condition (presented in Exhibit 2-14), independent of what actually exists on the ground right now. For example, the intersection was originally constructed wide enough to accommodate a southbound through lane if a fourth leg were ever to be added to the intersection. In the interim, the extra width has been used to accommodate dual southbound left-turn lanes. But, are dual left-turn lanes really going to be needed? The volumes suggest that weíre likely to need at least one southbound left-turn lane, and maybe two. Two would produce a per-lane match between the northbound and southbound lefts, ignoring lane utilization. If we donít use two, we might be able to convert the second one into a through lane or a through-and-something lane. Although the northbound lefts are not as heavy (150) as the southbound lefts, they are significant. A left-turn lane (opposing the southbound left-turn lane) would be wise. The through volumes are small both north and southbound. That means one through lane should be plenty. We could even combine the throughs and rights.

Now, letís study lane configuration options. At the same time, we need to think about the signal timing plan. There are two tools to do this. One is the HCM planning method for signalized intersections. The other is critical lane analysis. Itís a simple, back-of-the-envelope technique for seeing what geometric/signal timing combinations might work. (The critical lane technique is discussed in most traffic engineering textbooks and handbooks, and itís the method for signal timing presented in the HCM.)

In each of these two analyses, weíre going to look at different lane configurations, phasing, signal timing, and cycle length. Acceptable cycle lengths range from 40-120 seconds, although longer and shorter values are possible.

Discussion:
It's important to note in this sub-problem that several analysis techniques are being applied/explored. You should think about whether there are others you might have used and the results you would have gotten. 

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