Complications at Off-Ramps

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 everyday 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 b ytoo many cars trying to get off? For decades, traffic engineers have been managing freeway congestion by using meters to restrict the rates that vehicles enterthe 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 meter-ing that is suitably designed, but traffic engineers disagree about what constitutes a suitably designed plan.