“Why? Why? WHY?,” thousands of people have wondered. Where the Silver Line Waterfront lines (to/from Logan Airport, etc.) cross D Street in Boston’s Seaport district, buses can be delayed by almost 2 minutes at a red light … while scarcely any traffic goes by on D Street. Does it have to be this way?
The road and traffic signal owners apparently think so. They put first priority on maintaining coordination between traffic signals on D Street, an important artery in the Seaport. That’s important, they say, for service and for safety — to prevent the gridlock and chaos that could come from traffic backing up into upstream intersections. Buses get served, but have to wait their turn.
I teach my students that with smart traffic signal control, it should be possible to serve buses when they need it — which is only a small fraction of the time — and, with the remaining time, serve other traffic well. So I had my students in CIVE 7382 Advanced Traffic Control tackle the problem, aiming to find a solution that would vastly reduce bus delay while still serving traffic on D Street well. They counted traffic, made detailed simulation models of the whole D Street corridor, and tested a few ideas. The winning idea: with simple, detector-based (“actuated”) control, average delay per bus falls from 50 seconds to 5 seconds, while delay to D Street traffic is unchanged. Moreover, backups into the nearby intersection (the safety issue that coordination aims to avoid) would actually be lessened.
Download the report: Simple and Strong Transit SIgnal Priority for the Silver LIne at D Street in Boston’s Seaport District
Abstract: Silver Line buses in Boston experience notoriously long delays at a traffic signal in the Seaport District where Silver Line Way crosses D Street. As reported in the Boston Globe (2/20/2016), delays can last up to 100 s, even after the City adjusted the traffic signal in response to complaints. This study showed that a combination of passive and active transit signal priority could eliminate almost all of the bus delay with no increase in delay to other traffic. The passive priority action is to take the intersection out of coordination, letting it run free, because coordination involves a long cycle length with very long red times that are hard to overcome with active priority. Then, with early green (red truncation) and green extension, almost all buses can pass through without delay. Simulation shows that during the busiest peak period, average bus delay falls from 51 s to 4 s. Average delay to other traffic remains virtually unchanged in spite of taking the signal out of coordination, because the proposed operation increases green time for D Street. Sensitivity analysis finds that the same general results (nearly zero delay for buses, no significant increase in delay for other traffic) apply even if traffic volumes increase by 50%, or if Silver Line frequency doubles. The only investment needed is detectors on Silver Line Way. For the Silver Line, the reduction in round trip running time and unreliability is predicted to lower bus operating cost by roughly $200,000 per year, while offering substantial travel time savings to more than a million passengers per year.
Authors: Peter G. Furth, Yousef Alsharif, Ray Saeidi-Razavi, Howie (Zhao) Sha, and Danny (Yue) Wang