Out of Control Policy Blog

Don't weave

    Impatience on two-lane roads actually improves traffic flow, as antsy drivers pass slowpokes rather than letting a convoy form. On highways, however, "passing, aggressive behavior and lane changing is greatly detrimental to the flow," says Prof. Polus.

    The reason is that chronic lane changing simulates the "weaving section" of a highway. If an off-ramp lies just beyond an on-ramp, entering drivers merge left (assuming ramps are on the right) and exiting drivers merge right, causing traffic to crisscross like mobile braids. When, in heavy traffic, many drivers change lanes again and again, trying to find the one that is moving faster, the same weaving effect kicks in, reducing the capacity of that section of road.

    "Weaving is the worst condition for traffic flow," says Prof. Polus. Because drivers in heavy traffic brake when a car pulls into their lane, and because it takes time to get back up to speed, there are larger and constantly-changing gaps between vehicles. That invites yet more cars to change lanes, propagating a wave of stop-and-go traffic ...

And what's the deal with gridlock that seems to have no cause?

    Lane closures, on ramps, uphill, chronic lane changing and other "inhomogeneities" in traffic flow can all trigger a density wave, Martin Treiber of Dresden University of Technology has shown in mesmerizing simulations (www.traffic-simulation.de/). One result can be "phantom" jams, which occur so far upstream of the bottleneck that the congestion there has long cleared by the time drivers at the back of the pack reach it. As a result, they never see the snafu that flipped smooth flow into a stop-and-go mess. By one estimate, three-quarters of traffic jams are phantoms.

Read the whole thing. Very interesting

Ted Balaker is Producer


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