Goddard Space Flight Center, Greenbelt, Maryland 20771

ENGINEERING COLLOQUIUM

Monday, October 17, 2005 / 3:30 PM, Building 3 Auditorium

Craig Davis

"The Physics of Traffic"

ABSTRACT -- In the modern description of traffic flow, there are three principal phases: (1) free flow, (2) synchronous flow, and (3) wide moving jams. Only the first is desirable. The other two phases are types of congestion frequently experienced on urban freeways. Physicists think of transitions between these phases as if they were first-order thermodynamic transitions such as the freezing of water to ice. For example, when a freely flowing vehicle encounters a wide moving jam, it abruptly comes to a standstill-in effect, freezes. The vehicle remains stopped or slowly moving until the downstream front of the jam passes and it resumes a normal highway speed-as if melting occurred. The jam may have been caused by an earlier accident that is not evident to drivers when they pass the site of the incident. Unlike a jam, synchronous flow results from a bottleneck such as an on-ramp, which is at a fixed location. Once a vehicle passes the bottleneck it resumes a normal speed. The name comes from the equilibration (or synchronization) of speed and flow rate across all lanes due to vehicle lane changes. Vehicle velocities are higher in synchronous flow than in jams. Simulations show that for a given incoming flow upstream of an on-ramp, there is an optimum rate of merging that maximizes mainline flow. If substantiated by empirical data, this result could have important implications for on-ramp metering.

SPEAKER -- L. C. Davis is an Adjunct Professor in the Physics Department at the University of Michigan. Prior to retiring in 2002, he was the Manager of the Physics Department, Ford Research Laboratory. His research includes traffic modeling and condensed matter physics. He received the Ph.D. degree in Physics from Iowa State University in 1966. He was a postdoctoral fellow at the University of Illinois before joining Ford in 1969. He was a visiting scientist at the DESY synchrotron (Hamburg, Germany) in 1981, at the Institute for Theoretical Physics (Santa Barbara) in 1989, at the Jet Propulsion Laboratory in 1989 and 1990, and an Adjunct Professor of Physics at Michigan State University in 2002. He is a Fellow of the American Physical Society.