Problem 6: Route 146 Arterial Study

Findings

Our first run involved simulating the network using the signal timings developed in the HCM analyses. Complete datasets for the two main runs we performed are available for T7F1 and T7F2 signal timings. The output datasets for each of these runs are in Dataset 81 and Dataset 82, respectively. Except for Maxwell Drive, the timings we developed in the HCM were adequate. Exhibit 2-68 shows a comparison among three sets of timings for the network. The first are the timings we obtained from the HCM analysis. The second are timings we hand-generated in TRANSYT7-F to get acceptable v/c ratios for all of the lane groups. The third are the timings developed by TRANSYT-7F when it optimized coordinated operation across the network.

As you can see, at the Shen entrance, we didn’t need to adjust the signal timings very much to get acceptable performance from TRANSYT-7F at that intersection. The same is true at Moe Road and Clifton Country Road. Only at Maxwell Drive, and only for Phases 1 and 3 (the left-turn phases), that we needed to adjust the signal timings (substantially) in TRANSYT-7F to get acceptable v/c ratios.

The third green times (T7F2) are the values developed by TRANSYT-7F in optimizing the coordinated performance of the network. To give you a sense of the performance improvement provided with the T7F1 signal timings, the network had 300 vehicle-hours of delay (out of 455 total vehicle-hours of travel), while in the optimized scenario (T7F2), there were 282 vehicle-hours of delay (8% less) out of 436 vehicle hours of travel. The original signal timings were already fairly well matched to the traffic flows.

One important point to note about the analysis pertains to the westbound-to-southbound left turn at the I-87 interchange. We specifically included the I-87 interchange as a node in the network so we could look at the performance of this movement. In the T7F1 run, this movement had a degree of saturation of 145%, well above the 95% that TRANSYT-7F sets as an upper limit. We saw this as a significant problem and determined that some action would have to be taken to mitigate the long delays that occur there.

It is interesting to note that in the T7F2 solution, that problem has been rectified. TRANSYT-7F found a way to coordinate the signals at Clifton Country Road and Fire Road so the left turn can have a degree of saturation equal to 101%. This is not as good as the 95% that’s desirable, but it is much better than the 145% we had in the first case.