Sub-problem 3a: Weaving Analysis

When you’re studying a weaving section, it is important to consider 1) type of weave, 2) weaving length, 3) distribution of flows within the weave, 4) speeds of the weaving and non-weaving movements, 5) peak hour factor, 6) percentages of trucks, buses, and recreational vehicles, and 7) passenger car equivalents for each of these.

Weaving sections are type A, B, or C, depending on how many lane changes are required for the weaving movements. Page 13-13 in the HCM 2000 tells you how these weaving types are defined. A Type A weave involves at least one lane change for both weaving movements. If two freeways both enter and exit a weave, to get from one to the other, at least one lane change is required. In a Type B weave, one of the two weaving movements doesn’t have to change lanes and the other changes only one lane. In a Type C weave, one of the two weaving streams has to shift at least two lanes.

Another necessary fact is whether or not the weave is constrained. In general, vehicles have difficulty changing from one lane to another in a constrained weave; whereas, in an unconstrained one, they don’t. The same analysis must be conducted for both weaves to determine which condition pertains. That will be determined by the number of lanes required for the weave. The minimum number required for unconstrained operation varies by type of weave. For Type A weaves, 1.41 lanes are required. Consequently, if the formula says more than 1.41 lanes are required for the weave, the weave is constrained.

For weaving sections, level of service is determined by what the formulas predict is the density of vehicles (passenger cars per mile per lane, or pcpmpl). The formula requires the volumes in weaving traffic streams, the number of lanes in the weaving section, and the speeds for the weaving and non-weaving vehicles. You can read Chapter 13 in the HCM for a more complete discussion about how this performance measure is defined for weaving sections. The breakpoints for LOS are the same as for basic freeway sections.