Adaptive Cruise Control Doesn’t Prevent Phantom Traffic Jams

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From a logical standpoint, Adaptive Cruise Control (ACC) seems to hold the key to phantom traffic jams but in reality, can actually exacerbate the problem. A study from Vanderbilt University examined why this is the case.

  • Phantom traffic jams create traffic when there’s no actual cause.
  • Although Adaptive Cruise Control (ACC) is supposed to help alleviate phantom jams, it actually can make them worse. 
  • While humans can anticipate what cars are doing ahead of them ink order to make decisions, ACC lacks this essential information.
  •  Once automakers figure out how cars can see ahead and anticipate, ACC could significantly improve traffic.
  • When fully autonomous cars come online, ACC will be especially important to keep freeways moving.  

Caused by human drivers, phantom jams bring the flow of traffic to a slow speed or stop when there’s no actual cause.  Not only does it cause driver’s temper to flare, but it also wastes precious time.

Because humans miscalculate speed adjustments, Adaptive Cruise Control is designed to overcome driver error and adjust car speeds electronically. However, the system still has blind spots that defeat the goal of adaptively cruising in traffic. Where human still surpass the machine is in anticipation of speed changes several cars ahead. Sensors can’t evaluate information beyond the car ahead so adaptive cruise control lacks crucial information to make the correct adjustment.

Adaptive cruise is not better than the worst drivers — yet

“Our experiments show that today’s driver-assist systems are not yet able to overcome the worst driving behaviors of humans that lead to extremely frustrating traffic jams,” said Dan Work, associate professor of civil and environmental engineering at Vanderbilt University, who helped lead the research.

Since adaptive cruise control comes as a selling point for driver-assist systems, it would be beneficial if it was actually a value add. If drivers knew that using the system defeated the whole point, I doubt they would use it or even opt to pay for it as an option.

To demonstrate the issue, Work and his collaborators took seven different cars from two different manufacturers on a rural road in Arizona. They simulated traffic conditions with a car using adaptive cruise control behind a pace car alternating speeds.

Photo by Dan Gold on Unsplash

For one simulation, researchers drove seven identical vehicles in one lane running the same adaptive cruise control system as they followed a pace car. After all cars reached a cruising speed of 50mph, the pace car quickly slowed down by 6mph. You would think the ACC could respond to such a small increment of change.

In Tests ACC Slowed Traffic By About 50%

“In each test, the following vehicle slowed down more than the leader, which is a signature of the creation of phantom traffic jams,” said Benjamin Seibold, associate professor of mathematics at Temple University and another lead researcher.

More specifically, moving down the line of the seven cars, each one slowed down even more than the one in front of it until the last car fell below the minimum for the ACC to function. To put that in perspective, the minimum speed required for most cars to use the adaptive cruise control system is 25-30mph. That’s around half the original speed. Factored into drive time, it’s hugely significant.


While ACC works great for safety concerns, when it comes to traffic, it’s an epic fail. As ACC becomes standard on all vehicles, automakers must address this glitch. Especially as fully autonomous vehicles start to come online. Once working as designed for freeway driving, ACC could not only be a benefit to cutting travel time, but count as one of the real heroes. Then it will truly be a bonus add as advertised.

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