Oct 5, 2018

How level-1 AVs may reduce phantom traffic jams

A traffic jam on Harbor Freeway (Interstate 110) in Los Angeles, California. Photo: Irfan Khan/Los Angeles Times via Getty Images

Adaptive cruise control, which holds a vehicle's speed steady while maintaining a safe distance from traffic ahead, is now a feature in 16 of the 20 bestselling vehicles in the U.S., classifying them as level-1 AVs.

Why it matters: Phantom traffic jams — the ones that appear to have no obvious cause — result from human driving behavior. Adaptive cruise control replaces some of these jam-inducing behaviors with algorithms, using sensors to detect the vehicle ahead and adjust cruise speed accordingly. When designed correctly, level-1 AVs may help prevent such traffic patterns from developing.

Level-1 AVs hold promise for traffic jam reduction because the vehicles can follow their driving rules faithfully and consistently.

  • Driving tests performed on a closed track in Arizona using one research-grade autonomous car and 19 human-driven ones suggested that replacing even 5% of the traffic flow with AVs could significantly cut phantom traffic jams, reducing variance in traffic speed by 50-80%.
  • Another test at Ford’s Michigan Proving Grounds showed that adaptive cruise control reduced the impact of a temporary slowdown. The ACC vehicles were able to maintain a higher speed through the slowdown than the human-driven ones.

Yes, but: A limitation of the current technology is that it can respond only to the vehicle directly ahead, whereas the best human drivers can take in information about many nearby vehicles. These systems must make additional design and connectivity advances before they can prevent these phantom jams.

What’s next: More testing is needed to determine how consistently commercially available level-1 AVs will stack up against human drivers. But to surpass human driving capabilities, these systems will need vehicle communication technologies that enable them to respond to more vehicles around them.

Dan Work is an associate professor of civil and environmental engineering at Vanderbilt University.

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