Sub-problem 3c: The Southwestern Quadrant

The Right-Hand Ramp
The volume on the right-hand ramp is 1,865 veh/hr in the AM peak. The HCM says capacity for a single lane ramp with a free flow speed of 30 mph should be 1,900 veh/hr; so the v/c ratio is 0.98, which is higher than the 0.923 we found for the short, two-lane connector. Therefore, the ramp will have poorer performance than the two-lane section of the short connector but won’t have a v/c ratio greater than the 1.0 for the single lane section. A v/c ratio this close to 1.0, combined with the high v/c ratio on the short connector, that are causing the queues on Route 7 EB.

Additional Capacity Analysis
The previous analyses showed that the v/c ratios on the short connector and the Route 7 EB/I-787 SB right-hand ramp were very close to 1.0. We will analyze two more ramps and three basic freeway sections to determine if they might also be contributing to congested operation. The ramps are 1) the single lane ramp from Route 7 WB to I-787 SB and 2) the combined (Route 7 EB/I-787 SB, 7 WB/I-787 SB) ramp after these two separate ramps merge. The three freeway sections are 1) the segment of I-787 SB upstream of the on-ramp, 2) the four lane section of I-787 at the point where the ramps join, and 3) the three lane section of I-787 just downstream of the lane drop.

The Route 7 WB/I-787 SB ramp sees relatively low volumes during its peak hour – 1,420 veh/hr - in the AM peak. The ramp free flow speed is 40 mph, so the capacity is 2,090 veh/hr and the v/c ratio is 0.68, indicating the ramp’s operation is not congested. At the point were the Route 7 WB and Route 7 EB ramps join, the total peak hour volume is 3,285 (AM peak again). The HCM suggests this two-lane, 40 mph section of ramp should have a capacity of 4,100 veh/hr, which means the v/c ratio should be 0.80. Congestion should not be an issue here.

For the I-787 SB basic freeway section just before the merge, the free flow speed is 65 mph and the AM peak hour volume is 2,005 veh/hr. The HCM analysis of this basic freeway section says the density should be 15.7 pc/mi/ln, which is LOS B. Therefore, prior to the point where the combined ramps merge with the freeway, the operation is adequate.