Some traffic scenarios seem to defy logic, such as how roundabouts can actually be more efficient than traditional intersections, and how waiting to merge at a lane closure can be safer than merging sooner. There’s a science behind traffic patterns and why certain road conditions reliably create congestion and delays. Knowing what to expect can help you anticipate and avoid potentially dangerous traffic patterns. Here are three seemingly paradoxical traffic patterns and the science behind them.
The Rubbernecking Redux
You’re on the highway with your radio tuned to a traffic report. An accident up ahead is tying up traffic in eastbound lanes, but fortunately you’re headed west. Then traffic slows in front of you, and you slowly travel along until you pass the accident scene on the opposite side of the road, the traffic in front of you clears, and you’re on your way. The slowdown in traffic is due to the human instinct to look over to see the accident scene, and that has the potential to make the situation worse.
That’s because sudden fluctuations in speed, like a tap on the brakes for a quick peek at an accident scene, create uncertainty for drivers, and force everyone to go more slowly, according to Chris Hayes, a transportation safety professional with Travelers Risk Control.
If you’ve ever seen a line of cars in traffic start to slam on their brakes all at once, and then couldn’t figure out what everyone was braking for, you were probably part of a traffic pulse. A “traffic pulse” occurs when a vehicle brakes, which in turn causes the vehicle behind it to brake, continuing back down the roadway as long as there is dense traffic. A pulse can go for miles in particularly heavy traffic as vehicles brake and tighten up following distance, then accelerate into faster speeds.
Traffic pulses also increase the potential for distracted drivers to rear-end the cars in front of them, causing more accidents. If there’s a major accident up ahead, even in the opposite direction, it may make sense to avoid the area entirely if you can.
For some drivers, roundabouts may appear more dangerous than a typical signalized intersection. However, roundabouts are becoming more popular in the United States because of the benefits they can provide.1 While keeping traffic moving, the geometric shape of a roundabout can serves as a traffic calming effect.
“Roundabouts are experiencing an upswing because they can be faster than signalized intersections, and can help save fuel because they reduce the amount of starts and stops and time spent idling,” said Hayes. According to research from the Federal Highway Association, “by reducing the number and severity of conflict points, and because of the lower speeds of vehicles moving through the intersection, roundabouts can be a significantly safer type of intersection.”2
A safer, faster merge? Zipper merging, or waiting until the last minute and then taking turns, like the teeth of a zipper coming together, can be safer than merging as soon as an upcoming lane closure is announced. That’s because it allows drivers to make use of both lanes of traffic for as long as possible, which can ease congestion and driver frustration. Some states, including Colorado, post signs in advance of road closures and instruct drivers to wait to merge, and then to take turns when they do.
A common theme in all three traffic patterns is that creating reliable travel conditions leads to safer driving, Hayes notes. “But don’t assume drivers are going to act predictably,” said Hayes. Especially when it comes to changing accustomed roadway behaviors, some drivers may resist late merging out of a sense of fairness or enforcing off-road norms of not cutting in line.
In all traffic patterns, drivers should be prepared to react to unexpected conditions. Keeping other drivers in mind and driving predictably can also help keep you safer on the road.