Traffic Jam: What Causes Gridlock

Sept. 11, 2011 — -- Everyday life enters a different phase after Labor Day, the unofficial start of autumn in the United States. As students and employees return from vacation, and vehicles fully flood roadways once again, drivers face an increased risk of what may be the worst hassle a commuter can encounter: traffic gridlock.

Gridlock occurs when vehicles cannot pass through an intersection -- even if they have a green light and the right of way. Vehicles that were unable to make it completely through the traffic signal before the light turned red now block the "box" -- the area of the intersection where both roads overlap -- causing delays and unnerving blares from car horns.

Greg Mitchell, a manufacturer's representative from the Bronx who has driven in New York City for 20 years, has experienced gridlock many times when driving between sales calls in Manhattan.

"It's frustrating when you're at an intersection and you see vehicles in the intersection that shouldn't be," Mitchell said. "On the other hand, I don't want to be the guy sitting in the middle of an intersection when the light changes to red and the cross-traffic is sitting there, aimed at my driver's side door, honking their horns. That's not a fun feeling either."

Striving to figure out how to prevent this urban nuisance, traffic-physics researcher Boris Kerner of the Daimler Automotive Group in Germany has developed an explanation for how gridlock occurs. A preprint of his new model can be viewed at the website arXiv. Surprisingly, his new model suggests gridlock can occur even when traffic flow is relatively light. The culprit? Someone in the line of traffic near a light signal slows down, triggering a chain of events that can reduce the speed of all traffic behind it, build up successively longer lines of vehicles with every green-yellow-red cycle, and eventually lead to gridlock.

Continuing a physics approach that originates from the early 1960s, Kerner and his colleagues developed a mathematical description that treats vehicles in traffic like objects in natural systems, such as a network of electrical signals traveling in the brain, or complex molecules in a thick liquid bouncing against each other as they are being sucked up through a straw. In all these cases, the objects can together make abrupt "phase transitions" from one state to another -- from a smooth liquid to a molasses-like one, from normal electrical activity to epilepsy, and from free-moving vehicle traffic to a jam. Unlike ice resting in a freezer, these systems are all dynamic, and far from equilibrium. Introduce a disturbance above a critical level, and like a roll of the dice, this can sometimes -- randomly -- cause the system to change its phase abruptly and dramatically.

In traditional models, traffic has been treated as having only two distinct phases -- either the cars are moving freely, or they are congested. However, in the mid-1990s, Kerner introduced a three-phase model. There is a "free flow" phase, plus two different phases of congestion -- an all-out "jam," and a state of "synchronized flow" in which vehicles are locked into a reduced speed, such as when vehicles in three lanes slow down together after merging into two lanes.