Michael Cassidy, Robert Cervero, and Donald Shoup
For over two decades ACCESS Magazine has published short, engaging summaries of transportation research conducted at the University of California. We ask our authors to write for lay readers rather than solely for professional peers, without overestimating the readers’ familiarity with the subject or underestimating their intelligence.
Michael Cassidy
You're driving along the freeway when suddenly everything slows down. A crash? A sudden overload of cars joining the freeway from on-ramps up ahead? Maybe. Sometimes the cause never reveals itself to you—inexplicably, everything just starts moving again. If this happens every day in the same spot, you may develop a theory or two as to why it happens. Would it occur to you that the congestion might be caused not by too many cars getting on the freeway but by too many cars trying to get off?
For decades, traffic engineers have been managing freeway congestion by using meters to restrict the rates that vehicles enter the freeway from on-ramps. A metering scheme can often keep cars moving faster on the freeway, and sometimes can even reduce traveler delay systemwide. Realizing these benefits requires metering that is suitably designed, but traffic engineers disagree about what constitutes a suitably designed plan.
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