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Sub-problem 4a - Page 4 of 5

ID# C204A04

Sub-problem 4a: Clifton Country Road AM peak hour - Existing Conditions

Lane Group Definitions

In most analyses, itís easy to correctly define the lane groups. In the case of Moe Road, for example, there are left-turn lanes, through lanes, and through-and-right lanes. On the eastbound and westbound approaches, we considered the through lane and the through-and-right lane to be a two-lane through-and-right group. In these latter situations, we assume that 1) the right turning vehicles are in the right-most lane and 2) the through traffic distributes itself between the right-most lane and the next inner lane to balance the per-lane flow rates.

The HCM is capable of analyzing many different lane groupings: exclusive lefts (one, two, three, etc. lanes), shared left-and-through lanes, through lanes, shared right-and-through lanes, exclusive rights, etc. But it cannot do lane-by-lane analyses, and there are some lane groups that it doesnít accommodate easily. One of those is the southbound approach.

The southbound approach has the following lane configuration: left, left/through, and through/right. The HCM doesnít provide for an exclusive left-turn lane in conjunction with a left/through lane. That means you have to decide how this approach should be modeled. Two criteria must be satisfied. First, the innermost lane gets as much use as the center lane and the outermost lane gets very little use. Second, the queue lengths on the innermost lane and center lane are about balanced.

We compared and contrasted three ways to represent the southbound approach. In Dataset 32, the base case, which we prefer, we assume the innermost lanes are used only for left turns, and the outermost lane is used for throughs and rights, shown as the first condition in Exhibit 2-42. This simplification is not a major misrepresentation of the way the approach works, but it is a simplification. There are through vehicles that use the middle lane. That option produces equal delays for the lefts and the throughs and rights, and the queue length estimates (average and 95th percentile) for the left-turning lanes are double those of the through-and-right lane. That is consistent with the field observations.

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