Sub-problem 2b: What are the Levels of Service in the Weaving Sections Located in the I-87/Alternate Route 7 Interchange?

Let's now consider each of the questions posed on the previous page, referring again to Exhibit 4-28.

How does the length of each weaving section affect its operation? Let's compare the results for Weaves A and B. In general, providing additional length in a weaving section allows drivers more time to complete their maneuvers (the intensity of lane changing decreases), often resulting in higher speeds in the weaving section. Thus, all other factors being equal, the degree of turbulence in Weave B should be lower than Weave A. However, since the volumes in Weave B are much higher than in Weave A, the overall weaving intensity is higher in Weave B, even with its greater length.

The volume ratio, VR, is more than twice as high in Weaves B and C as in Weave A; is this significant and if so, why? The volume ratio is the ratio of the weaving flow rate to the total flow rate in the weaving section. As the proportion of weaving traffic increases, the degree of turbulence also increases. Two key results follow from this increased turbulence: speeds decrease and density increases. The volume ratio for Weaves B and C (ranging from 0.68 to 0.76) shows than between two-thirds and three-quarters of the total traffic is required to change lanes. This higher degree of turbulence in the traffic stream lowers vehicle speeds in the section, and you can see this result directly in the table shown on the previous page.

What is the significance of the predicted speeds for the weaving and non-weaving traffic? The weaving speeds are approximately 16 to 18 mi/hr less than the non-weaving speeds for five of the time periods presented in the table; is this important and if so, why? Safer traffic flow always results if all vehicles in the traffic stream are traveling at the same speeds. While speed differentials are expected in a weaving section, the differences that we observe here are quite high, between 16 and 18 mi/hr. One of the factors that mitigates this speed differential in weaving sections is the degree of separation of the weaving traffic from the non-weaving traffic. Because of the nature of a Type A weave, all of the lane changing activity occurs in the two lanes adjacent to the crown line, with little or no spillover effects in the outer lanes.