More than 2.5 million people have tried to evacuate their homes in advance of Hurricane Rita, according to a government official in the Houston area. Traffic jams have extended for up to 100 miles. What's causing all this congestion?
Bottlenecks, breakdowns, and reduced highway capacity. Traffic engineers can calculate the "capacity" of a given stretch of road: the number of cars that may pass through every hour under peak conditions. Increasing the number of lanes on a highway adds capacity. Hills and slopes reduce capacity, as do out-of-state drivers who are less familiar with the local road system. Pretty much anything that could affect the speed of traffic gets included in capacity calculations: the number of interchanges, for example, or the number of trucks on the road, or the width of the shoulders and lanes. If conditions are less than perfect—if too many cars are on the road at the same time—then capacity can drop. Reduced capacity in one stretch of highway creates a bottleneck for the higher-capacity stretches nearby. A bottleneck creates congestion.
In Texas, evacuees from the coastal regions headed inland on the highways that pass through Houston. Roads that would likely be congested on a normal day were suddenly backed up against a flush of dense traffic waiting to get through. An increasing flow of cars will sometimes to lead to "speed instability:" People are more likely to slow down suddenly, perhaps to avoid hitting someone who's just cut in front of them. When one car decelerates, other cars must slow down, too, creating a wave of congestion that propagates backward. In this case, the congestion would have propagated toward the denser traffic from the coast.
It takes time for jammed highways to clear up under any circumstances. But the conditions in Texas only exacerbated the problem. Some cars that idled in traffic for hours ran out of gas, which created a new source of congestion. Jams that extended back into city streets might have caused problems at intersections.
Traffic planners try to ease congestion by upping highway capacity. Transportation departments in the Gulf Coast states have worked out special evacuation plans to do just that. As Mimi Swartz describes in her "Escape From Houston" dispatch, traffic was reversed on some stretches of I-10 so that cars in all lanes headed out of Houston. This strategy—known as "contraflow"—typically requires a great deal of advance planning. First, every single entrance to the contraflow side of the highway must be closed off. Then the cars leaving the city must have some way to get to the contraflow side, which can be tricky if there are dividers. Finally, planners must figure out how to tell motorists that they can use those contraflow lanes.
Signal lights can also be adjusted to allow more time for cars to pass through a traffic-laden thoroughfare. (Cities with advanced signal systems can do this remotely.) Temporary signs can direct cars to alternate routes or instruct them to use carpool lanes indiscriminately. A staged or nighttime evacuation can also be used to reduce congestion.
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Explainer thanks James Banks of San Diego State University and Valerie Briggs of the American Association of State Highway and Transportation Officials.