Benjamin Coifman will be giving an ITSO TransTalk seminar on Friday April 24 in the Civil Engineering Building (500 Pillsbury Drive) Room 205 at 12:15. Food will be provided.

Title

What happens downstream of a bottleneck does not always stay downstream.

Abstract

In modern cities freeway traffic congestion degrades the movement of most persons and goods. The congestion is due to a small number of bottlenecks and just as a chain is only as strong as the weakest link, freeway flow along a corridor is restricted by the tightest bottleneck. Conventionally bottlenecks are modeled as a point along the roadway with queuing upstream and free flow downstream. Downstream of the bottleneck all signals are presumed to flow downstream with the traffic while within the queue many signals propagate upstream (e.g., stop and go traffic). This talk presents two detailed examples where this conventional wisdom fails to capture the microscopic details of the actual traffic dynamics where disturbances actually propagate upstream through the bottleneck from the supposedly free flow conditions downstream. Unfortunately the small misunderstandings have lead to large errors in the conclusions reached by many researchers. The first example presents empirical evidence of subtle flow limiting and speed reducing phenomena more than a mile downstream of a lane drop bottleneck. These phenomena reduce the maximum throughput measured at the lane drop bottleneck below actual capacity, so in this case conventional measures underestimate capacity.

The second example presents a simulation-based study of an on-ramp bottleneck. In this case the modeling incorporates driver relaxation whereby drivers will tolerate a truncated headway for a little while after an entrance but slowly relax back to their preferred speed-spacing relationship. The results show that flow downstream of the on-ramp bottleneck is supersaturated, so in this case conventional measures overestimate capacity. Thus, an empirical study or traffic responsive ramp meter could easily mistake the supersaturated flows to be the bottleneck’s capacity flow, when in fact these supersaturated flows are unsustainable and simply represent system loading during the earliest portion of bottleneck activation. Instead of flow dropping “from capacity”, we see flow drop “to capacity” from supersaturation.

Bio

Benjamin Coifman grew up in Minneapolis, graduated Suma Cum Laude from the University of Minnesota, earned a MS and PhD in Civil Engineering and a MEng in Electrical Engineering and Computer Science at the University of California, Berkeley. Currently holds a joint appointment in Civil Engineering and Electrical and Computer Engineering at the Ohio State University. Research emphasis on: Traffic Flow Theory, Traffic Monitoring, and Intelligent Transportation Systems.