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Sub-problem 5b - Page 1 of 1

ID# C205B01

Sub-problem 5b: I-87 Interchange: Route 146 Weaving Section

Between the southbound-to-eastbound off-ramp and the eastbound-to-northbound on-ramp there is a short weaving section. All of the traffic leaving the off-ramp heads east on Route 146, while all of the traffic for the on-ramp comes from Route 146 to the west.

Weaving sections can occur on freeways and on arterials. This is one example. The HCM methodology for analyzing weaves makes provision for considering weaves on arterials.

In the context of the HCM, the inputs you need to provide are: the type of weave (A, B, or C); the length of the weaving section; the free-flow speed; the terrain; and the four weaving volumes and characteristics of those volumes like the peak hour factor, the percent trucks, and the percent recreational vehicles. The LOS performance measures are the density within the weaving section (passenger cars per mile per lane) and the weaving speed. The density determines the level of service.

Exhibit 2-60. I-87 Interchange Performance for the Eastbound Weave on Route 146

Case Density Speed LOS
AM Existing 14.69 43.95 B
AM Without/With 15.35 43.69 B
PM Existing 22.96 43.68 B
PM Without 24.06 43.38 C
PM With 26.43 41.74 C

Exhibit 2-61. I-87 Interchange Prediction of Delays at the Terminus of the Southbound-to-Eastbound Off-Ramp

Case Delay v/c 95% Q LOS
PM With (6.2 sec) 31.7 0.66 4.56 D
PM With (4.1 sec) 15.0 0.41 1.98 B

Exhibit 2-60 shows the performance of the Route 146 weaving section under a variety of conditions. The density ranges from 14.69 to 26.43 pcpmpl and the speed ranges from 41.74 to 43.95 mph. The LOS is either B or C. In the PM Peak, we move from low B to high C as we progress from PM Existing to PM Without conditions and the PM With conditions are just slightly worse. You can click here to see the datasets that produced these results.

To study the terminus of the southbound-to-eastbound off-ramp as an unsignalized intersection, we looked at the PM With condition (Dataset 65). Exhibit 2-61 shows that we found. If we stick to the 6.2 second critical gap thatís assumed for typical right turns, we get LOS D, a delay of 31.7 seconds, and a 95th-percentile queue of 4.56 vehicles. This is inconsistent with the conditions we see today. The critical gap used by the drivers at this location must be much shorter. We looked at what would happen if we assumed a much shorter gap, like the 4.1 seconds assumed for left turns from the major street. The delay then becomes 15 seconds and the LOS is B. Thereís a need to be accurate about the critical gap value being used at this location. We think 4.1 seconds is more realistic than 6.2 seconds.

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