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

Why is the weaving traffic constrained? What is the practical implication of this finding? (see Exhibit 4-28). The determination of whether a particular weaving segment is operating in an unconstrained or constrained state is based on the comparison of two variables: the number of lanes that must be used by weaving vehicles to achieve equilibrium or unconstrained operation (N_w) and the maximum number of lanes that can be used by weaving vehicles for a given configuration (N_wmax). As we have discussed previously, most, if not all, of the lane changing activity associated with weaving occurs in the two lanes adjacent to the crown line. In fact, for a Type A weave, the number of lanes that can be used by weaving vehicles is 1.4. It is less than 2 since some of the non-weaving vehicles also use these two lanes. Our results show that Weave A requires 1.5 lanes (fairly close to the number required for unconstrained flow), while Weave B requires from 3.3 to 3.6 lanes. Clearly, the volumes and proportion of weaving flow associated with Weave B requires much more space than is present in this type of weave, so the weaving traffic is definitely constrained.

What happens when the weaving flow rate exceeds the model limit? When weaving flow rates exceed the model limits (in this case, 2,800 pc/hr for a Type A weave), it is likely that the weaving section will fail, regardless of the results from the other parts of the weaving section methodologies. For Weave B, during the PM Peak, the weaving volume is 4,105 pc/hr, a rate significantly higher than than 2,800 limit cited above. Again, the practical result is a likely breakdown of flow in this segment during this time period.

In Weaves B and C, the volume ratio, VR, exceeds the model limit; what is the likely result that you would observe in the field? For weaving sections with five lanes, as in Weave A in this sub-problem, the volume ratio limit is 0.35. Both the AM and PM peak results are less than 0.35 for Weave A. However, for Weave B, the limit of 0.20 is exceeded in both the AM and PM peak periods. In fact, the values of 0.70 and 0.68 are significantly higher than the limit with a likely result of poor operations and local areas of queuing.

The questions that we discussed above are important in helping us to understand how the three weaving sections will operate under the given conditions. As you consider all of the data together, how would you summarize the operations of the three weaving sections? After you have considered this question, proceed to the next page for a further discussion of this issue.