Sub-problem 3c: The Southwestern Quadrant

Route 7 Eastbound Diverge
Note two things about the initial diverge from Route 7EB: 1) There’s no deceleration lane. The exit ramp leaves Route 7 as soon as it appears. This means that for the ramp analysis, we need to set the length of the deceleration lane at zero. 2) Since the exit ramp leads to the collector/distributor road, we need to include the traffic taking the loop ramp to I-787 north as well as the traffic taking the right-hand ramp to I-787 south. This will considerably affect the performance prediction. When we include both flows, the influence area for the ramp has a density of 32.8pc/mi/ln, which is LOS D. Although this poor level of service could be producing the queues on Route 7 EB during the AM peak hour, there may be another problem somewhere.

The Short Connector
There’s a short, 150-foot long single-lane ramp between Route 7 east and the beginning of the lane for the collector/distributor road. An analysis of this roadway segment will tell us if this might be the bottleneck. We’ll compare the volume it handles with the capacity it ought to have per the HCM. The exiting volume is 2,020 veh/hr, while the suggested capacity for a single-lane ramp is 2,000 veh/hr; so the v/c ratio is 1.01. This segment is a likely source of the congestion. Once the second lane is added for the collector/distributor road, the congestion eases. Now we have double the capacity for the same volume; however, the density doesn’t drop by half, because the lane distribution isn’t even. About 155 veh/hr are in the left-hand lane, because they’re going to the collector-distributor road, and 1,865 veh/hr are in the right-hand lane, because they’re going to I-787 south. The v/c ratios by lane are 0.077 and 0.923, respectively. So the congestion is eased, but the v/c ratio isn’t cut in half. The right-hand ramp after it diverges from the collector-distributor road might have a higher v/c ratio, since it has a sharp curvature and a low speed limit.