Sub-problem 5a: I-87 Interchange - Merges and Diverges on the Freeway

Merging and diverging points on the freeway are always important places to study in terms of capacity and performance. They’re also confusing spots to study because several ramp combinations must be examined (involving upstream and/or downstream ramps) to get an accurate sense of how the ramp is performing or will perform. Having entered information for the subject ramp and the upstream ramp does not mean that both have been studied. If two ramps are close to one another in a given location (here there are three), each must be examined to reach closure. You can study the upstream ramp first, entering the information about the downstream ramp as required; then study the downstream ramp, entering the information about the upstream ramp as required. In this case you need to study the middle ramp two ways, first treating it as a subject ramp with an upstream influencing ramp, then as a subject ramp with an influencing downstream ramp. The analysis result that predicts the worse ramp performance is the one to report.

The level of service measure at a ramp junction is the density of vehicles within the ramp influence area. As shown in Exhibit 2-53, this is an imaginary box encompassing the ramp and the two lanes closest to the ramp. This is the area at the merge or diverge location that is most congested or most affected by the presence of the merge or diverge. Here the freeway speeds will be the slowest and the amount of turbulence the greatest.

The inputs you have to provide are the freeway volume upstream of the merge, the number of lanes on the freeway, the freeway free-flow speed, the volume on the ramp, whether the ramp is on the right-hand or left-hand side, the free-flow speed on the ramp, the length of the deceleration lane (or lengths if there are two lanes), the volume on the upstream or downstream ramp (if there is one), the location of that ramp (upstream or downstream), and the volume on that ramp. You also have to indicate the peak hour factor, percent trucks and percent RV’s for all three volumes.

We will look at the six ramps starting upper left and moving counter clockwise: the southbound-to-westbound off-ramp, the southbound-to-eastbound loop ramp, the southbound on-ramp, the northbound off-ramp, the eastbound-to-northbound loop ramp, then the westbound-to-northbound on-ramp.