You’re cruising down the highway  when all of a sudden endless rows of brake lights appear ahead. There’s no accident, no stoplight, no change in speed limit  or narrowing of the road. So why the @#$%! is there  so much traffic? When traffic comes to a near standstill  for no apparent reason, it’s called a phantom traffic jam. A phantom traffic jam  is an emergent phenomenon whose behavior takes on a life of its own, greater than the sum of its parts. But in spite of this,  we can actually model these jams, even understand the principles  that shape them— and we’re closer than you might think to preventing this kind of traffic in the future. For a phantom traffic jam to form,  there must be a lot of cars on the road. That doesn’t mean  there are necessarily too many cars to pass through a stretch  of roadway smoothly, at least not if every driver maintains  the same consistent speed and spacing from other drivers. In this dense, but flowing, traffic, it only takes a minor disturbance  to set off the chain of events that causes a traffic jam. 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. With a low density of cars on the road, traffic flows smoothly  because small disturbances, like individual cars changing lanes  or slowing down at a curve, are absorbed by other  drivers’ adjustments. But once the number of cars  on the road exceeds a critical density, generally when cars are spaced  less than 35 meters apart, the system’s behavior  changes dramatically. It begins to display dynamic instability,  meaning small disturbances are amplified. Dynamic instability isn’t unique  to phantom traffic jams— it’s also responsible for raindrops,  sand dunes, cloud patterns, and more. 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. Drivers tend not to realize they need  to break far in advance of a traffic jam, which means they end up having  to brake harder to avoid a collision. This strengthens the wave of braking  from vehicle to vehicle. What’s more, drivers tend to accelerate too rapidly out of a slowdown, meaning they try to drive faster than the average flow of traffic  downstream of them. Then, they have to brake again,  eventually producing another feedback loop that causes more stop-and-go traffic. In both cases, drivers make traffic worse simply because they don’t have a good sense of the conditions ahead of them. Self driving cars equipped with data  on traffic conditions ahead from connected vehicles  or roadway sensors might be able to counteract  phantom traffic in real-time. These vehicles would maintain  a uniform speed, safety permitting, that matches the average speed  of the overall flow, preventing traffic waves from forming. In situations where there’s  already a traffic wave, the automated vehicle  would be able to anticipate it, braking sooner and more gradually  than a human driver and reducing the strength of the wave. And it wouldn’t take that many  self-driving cars— In a recent experiment, one autonomous  vehicle for every 20 human drivers was enough to dampen  and prevent traffic waves. Traffic jams are not only  a daily annoyance– they’re a major cause of fatalities, wasted resources,  and planet-threatening pollution. But new technology may help reduce  these patterns, rendering our roads safer, our daily commutes more efficient,  and our air cleaner. And the next time you’re stuck in traffic, it may help to remember that other drivers aren’t necessarily driving spitefully, but are simply unaware of road  conditions ahead— and drive accordingly.