Analytics Stories

CHAPTER 47Red Light, Green Light, or No Light?

Since 1999, I have made over 50 trips to Redmond, Washington, to consult or teach classes for Microsoft. During each trip, I drive on I-405 from Sea-Tac Airport to my hotel in downtown Bellevue. The ramps on I-405 have ramp meters, which are traffic signals that cycle between green and red. When the light is green, a car is allowed to attempt to merge onto I-405. The first ramp meter in the United States was installed in 1953 on the Eisenhower Parkway in Chicago. In this chapter, we will explain how ramp meters can lessen freeway congestion and why some people oppose them. The U.S. Department of Transportation Ramp Management and Control Handbook is probably the most complete reference discussing ramp meters (ops.fhwa.dot.gov/publications/ramp_mgmt_handbook/manual/manual/pdf/rm_handbook.pdf).

What Causes Traffic Jams?

Clearly, if there are too many cars on the freeway, traffic will have to slow down. By increasing the fraction of time that ramp meters are red, traffic controllers can reduce the inflow of cars to the freeway during times of peak congestion. As pointed out by Gabor Orosz, R. Eddie Wilson, Robert Szalai, and Gábor Stépán (“Exciting Traffic Jams: Nonlinear Phenomena Behind Traffic Jam Formation on Highways,” Physical Review, vol. 80, no. 4, 2009, 046205), a mile-long traffic jam (the article calls it a backward traveling wave) can begin when one driver hits the brakes hard, thereby causing the driver behind them ...