A prototype of Google's self-driving car. The straighter and smoother a road is, the less likely the jams are to form, since it means drivers won't be doing as much sudden braking. Most highways are already built to be as straight as possible, so it's mainly by better maintaining current ones that this can make a difference. A more innovative idea, Seibold says, are variable speed limits , which are already in place in a few places in the US but are mainly used to alter speeds based on weather conditions.
Using LED signs, speed limits could be decreased in the area leading in to a phantom traffic jam, causing cars to slow down gradually, rather than all at once. In some cases, this could break up the wave. Finally, Seibold believes a comprehensive solution will come in the form of self-driving cars. Because they'll be able to control their speeds with more precision and use data on traffic miles down the road, they'll be able to anticipate slow-downs much more effectively than any human.
One car suddenly braking, for instance, could send out a signal to all the cars within a mile behind it, instructing them to slow down gradually, rather than suddenly when they arrive at the nascent traffic wave. In theory, at least, this will smooth out waves of traffic before they're able to form. Our mission has never been more vital than it is in this moment: to empower through understanding. Financial contributions from our readers are a critical part of supporting our resource-intensive work and help us keep our journalism free for all.
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Share this story Share this on Facebook Share this on Twitter Share All sharing options Share All sharing options for: Why do traffic jams sometimes form for no reason? Reddit Pocket Flipboard Email. Why phantom traffic jams form If there are enough cars on a highway, any minor disruptions to the flow of traffic can cause a self-reinforcing chain reaction: one car brakes slightly, and the ones behind it brake just a bit more to avoid hitting it, with the braking eventually amplifying until it produces a wave of stopped or slowed traffic.
Inevitably, traffic waves formed: So who's to blame for these traffic jams? Every driver has experienced "phantom" traffic jams, in which dense traffic crawls to a halt for no apparent reason. Now, new research may have come up with a solution for this frustrating traffic pattern: maintaining equal spacing between cars on the road, rather than tailgating.
Horn said phantom traffic jams are an emergent property of the flow of vehicles down a highway. A phantom jam begins when a car in dense traffic slows down even slightly, which causes the car behind that vehicle to slow even more — and the slowing action spreads backward through the lane of traffic like a wave, getting worse the farther it spreads.
Eventually, the cars far behind are forced to stop completely or risk hitting the slower vehicles ahead —and so the traffic grinds to a halt over nothing, Horn said. Horn has been working on the problem of phantom traffic jams for years, and he has come up with a solution: By splitting the difference between the cars in front and the cars behind, the spacing of cars in a stream of traffic can act as a damper on phantom traffic jams.
That is, it can prevent the slow-down effect from being amplified by the cars that follow behind, he said. In essence, each driver would constantly make adjustments to keep his or her car roughly halfway between the car in front and the car behind.
Horn calls this method of vehicle spacing bilateral control, and he said it could be achieved with relatively simple modifications to the adaptive cruise control ACC that many new cars already have. Say one driver brakes slightly. Each successive driver then brakes a little more strongly, creating a wave of brake lights that propagates backward through the cars on the road.
These stop-and-go waves can travel along a highway for miles. It begins to display dynamic instability, meaning small disturbances are amplified. The instability is a positive feedback loop. Above the critical density, any additional vehicle reduces the number of cars per second passing through a given point on the road.
This in turn means it takes longer for a local pileup to move out of a section of the road, increasing vehicle density even more, which eventually adds up to stop-and-go traffic.
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