Using Traffic Signals to Decongest Cities — Distinguished Lecture Series

This presentation focuses on two well-known means of re-timing ordinary traffic signals to mitigate city-street traffic congestion. The first entails synchronizing green times on 4-way (i.e., all-directional) street grids. The second involves re-timing the signals that reside along cordon lines, to meter vehicle inflows during the morning rush. We show that, in both cases, simple strategies can perform remarkably well. The lecture will offer next steps toward real-world implementation.

To synchronize green times, Professor Cassidy's team use common phase durations for all signals on a grid, rather than optimize each signal’s timing plan to suit local conditions. This simple idea enables them to exploit features of perfectly orthogonal grids to synchronize all signals on the grid in two orthogonal (e.g., northbound, and westbound) travel directions. As a result, drivers headed toward clustered workplaces in the morning rush, and away from those workplaces in the evening, can enjoy exceptionally good signal progression over their entire trips, even when their paths entail two directions. One synchronization mode is used for undersaturated conditions when queues dissipate during green times; another is used when conditions become oversaturated with long residual queues; and the team's strategy toggles promptly between these two synchronization modes, as measurable conditions on the grid change over time. Simulations of numerous scenarios indicate that our simple, adaptive strategy dramatically outperforms other synchronization schemes, including complicated ones and those viewed as gold standards. Importantly, the presentation also shows that their strategy can be easily modified to perform well on irregular, real-world street grids.

As regards cordon metering, they fill a gaping hole in the literature by providing logical and easy-to-follow rules on how exactly to place cordons around clustered workplaces. Simulations show that the simple rules hold for varying congestion levels and regional configurations, and that following the rules can be impactful, especially when cities become severely congested.

---

About the speaker: Michael Cassidy is the Robert Horonjeff Professor and Chancellor’s Professor of Civil and Environmental Engineering at the University of California, Berkeley. He received a doctorate in Civil Engineering (majoring in Transportation Engineering) from Berkeley; served for nearly 4 years as an Assistant Professor in the School of Civil Engineering at Purdue University in West Lafayette, Indiana; and joined the Berkeley faculty in 1994. He is currently an associate editor of the journal Transportation Research Part B. He is a member of the International Advisory Committee for the International Symposium on Transportation and Traffic Theory; a former Director of the University Transportation Center for federal region 9; and a Vice Chair in Berkeley’s Civil and Environmental Engineering Department. His research interests focus on transportation planning and management, particularly in the areas of highway traffic, public mass transit and multi-modal systems.