Abolishing the Yellow Light Could Revolutionize the Way We Drive

The roads have changed a lot in the past 150 years. Where there was once the horse and buggy, there's now self-driving cars; what was once little more than a loose network of dirt paths, is now millions of miles of paved roads and highways. But despite all the innovations, one element of overland travel has been remarkably consistent: the traffic light.

Although traffic lights are far less likely to explode and burn people's faces off now, they still eat up about six months of the average human's life. Yet now that everything about our transportation infrastructure is getting smarter, some systems scientists have begun to wonder if it's possible to build a better traffic light—or just do away with them completely. This is the subject of new research published last week in PLOS One which seeks to make traffic lights obsolete by treating cars more like planes.

As detailed in the paper, the problem with traffic lights is they involve a tradeoff in vehicle throughput at the intersection and the amount of time vehicles will spend waiting at a red light. This is because a traffic light does not switch immediately from green to red—there is a "setup phase" involved (a yellow light), and during this setup phase the intersection is considered to be operating in a highly sub-optimal condition (i.e., fewer vehicles can traverse the intersection as compared to a green light).

So the tradeoff involved with traffic lights can be conceived thus: you can lower delays by making red lights shorter, but this will involve more suboptimal yellow lights, which will lower the overall number of cars that can pass through the intersection.

According to the researchers, a proposed solution to this conundrum is something called a slot-based control system, such as those used to coordinate aerial traffic at an airport. The idea here is to use a scheduling algorithm to assign individual vehicles their own time slot for safely traversing an intersection, thereby maximizing an intersection's vehicle throughput and minimizing delay times for individual vehicles.

Slot-based control systems for intersections have only become possible with the rise of vehicles which are able to communicate not only with roadside infrastructure, but increasingly with other vehicles on the road. This allows for the precision timing needed to make a slot-based system work.

As the researchers acknowledge in their paper, at first glance slot-based control systems seem to involve the same tradeoff as conventional traffic lights. Although precisely regulating the arrival of vehicles at an intersection would allow for a one-by-one service policy, the efficiency of this system severely drops for intersections with high vehicle arrival rates. So which is more efficient overall?

Although other research has suggested that slot-based traffic systems are more efficient overall, the researchers say the simulations used to arrive at these conclusions lacked a "comprehensive framework" for assessing these systems' performance as compared with the conventional traffic light. The researchers compared slot-based systems with conventional traffic lights using a rubric where safety was the paramount performance criterion, followed by the system's ability to maximize vehicle throughput and minimize delay times at the intersection.

After simulating a variety of traffic scenarios, the researchers found that implementing slot-based traffic regulation systems could result in up to a two fold increase of vehicle throughput at intersections compared to traditional traffic lights and "even more dramatic reductions" in delay times, an improvement the researchers chalk up to shorter setup phases (aka yellow lights). In the simulations run by the researchers, the slot based systems allowed for setup phases that were dramatically shorter than the traffic lights we have now—about 1.5 seconds compared to the typical 5 to 8 second yellow lights on traffic lights.

Although the researchers see "major, positive benefits in slot-based systems, autonomous transportation technologies and regulations still have awhile to go before they're ready to handle such sophisticated scheduling protocols, so it looks like we'll be seeing yellow lights for at least a few more years.