Sub-problem 2b - Page 3 of 10

ID# C402B03

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

What are the limitations of the models used to analyze the performance of a weaving section? The weaving analysis procedure consists of five models or algorithms. Each of the models was developed and calibrated from several data sets collected from actual freeway operations. It is important when applying these sub-models to make sure the data that you are using fall within the following limits, which also define the limitations of the data sets upon which the models are based:

	The weaving flow rate, V_w, or the total weaving volume in the section, must be less than 2,800 pc/hr for Type A weaves, for example.
	The flow rate through the weaving section must be less than that allowable for a basic freeway segment.
	The volume ratio, VR, or the proportion of the total flow that is weaving, must be less than 0.45 for a three-lane section and 0.35 for a four-lane section.
	The total length of the weaving section must be less than 2,500 feet, or the section should be considered only as unconnected merge and diverge (ramp junctions) points.

If your data are outside of these limits, it may imply that poor operations will result and that local queuing should be expected.

Why are these weaving sections considered to be Type A weaves? The weaving section type is based on the number of lane changes that each of the weaving traffic streams must make in order to reach their final destination. In both of the cases here, each weaving traffic stream must make one lane change to reach its desired destination. For more discussion of the nature of the weaving traffic in the segments we are evaluating, refer to Exhibit 4-20 (Weave A), Exhibit 4-21, (Weave B), and Exhibit 4-22 (Weave C).

What measures of effectiveness are used to evaluate the performance of a weaving section? Typically, the weaving analysis procedure is used to determine the level of service of the section, the number of lanes required to meet a specified level of service, the required length of the weaving section to meet a given level of service, or the type of weaving section configuration required to meet a given level of service. An operational analysis will produce the level of service, while a design analysis can be used to produce the other three outputs. For a weaving section, the level of service is defined by the traffic density.

What is meant by the terms "constrained operation" and "unconstrained operation?" 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).

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