Autonomous vehicles will force authorities to reassess yellow-light formulas


Academics Brian Ceccarelli and Joseph Shovlin (below, left and center) of North Carolina’s Research Triangle, have conducted extensive studies on yellow change interval formulas. Here, in an update to their 2013 Traffic Technology International feature on the subject, they join forces with personal injury lawyer D Hardison Wood (below right) to look at how autonomous vehicles will affect traffic signal timings… and how they could finally force authorities to curb unfair practices.It is a very exciting time for traffic technologists. This June in Stuttgart, Germany, engineers will gather together at the Autonomous Vehicle Test & Development Symposium to discuss their research and findings, the end goal of which is to bring the advent of the autonomous vehicle – the self-driving car. In a just a few short years we will reach this milestone in manufacturing, bringing the consumer a possibility once only imagined in science fiction.

On the road to the self-driving cars are many challenges. One of the biggest problems is the yellow signal light. How will the engineer program his car to react to a light changing from green to yellow? Will the engineer program his car to stop? What deceleration rate will the car use to stop? Will the manufacturer program his car to go? If go, at what speed will the car proceed? Will the car sense the distance to the intersection and the distances to the vehicles ahead and behind the car, and input those distances into the car’s decision-making algorithm so as to avoid rear-end collisions?

Complicating this is the international standard used for setting the length of the yellow light. This standard is called the ITE Yellow Change Interval Formula. Under common circumstances the Formula violates a law of physics for frequent reasonable traffic movements, creating Dilemma Zones upstream from the intersection. One type of Dilemma Zone occurs when a driver does not know if he has distance to safely stop, or if he must stay at speed and enter the intersection.

An autonomous car will have to know its speed and relation to other vehicles and distance from the intersection to figure this out. The second type of Dilemma Zone is an even worse scenario. In this case, the driver neither has the distance to stop nor the time to reach the intersection before the light turns red. The driver will either need to stop with unsafe deceleration, or run a red light. Stopping the car quickly may cause problems for following vehicles. Entering the intersection on red may lead to a traffic citation or accident. An autonomous car will have to make the same decision. Just as this brings up potential legal consequences for drivers today, it will also do so for manufacturers of the autonomous vehicles of tomorrow.

In all jurisdictions, when one runs a solid red light, one breaks the law. “If you were close, you should have stopped.” This is the sort of hindsight bias baked into our existing legal systems. Yet this truth applies to drivers only. Ordinarily a passenger is not liable for the sins of the driver. Several States in the US are considering legislation which proposes holding the driver responsible for the conduct of the vehicle – regardless of whether he was actually driving or not. Sometimes known as the “driver as if driving” liability rule, it may create more problems than it solves.

For example, assume the vehicle’s computer commands the car to run the red because it has no other good alternative. While running the red, the vehicle slams into an elderly gentleman who was trying to cross the street, injuring or killing him. The vehicle didn’t stop because of a programming decision. If the State’s law where such an accident occurred uses the “driver as if driving” liability rule, the driver is now legally responsible for a wrongful death and a traffic citation, though no fault of his own.

While this may seem somewhat far-fetched, look no further than the recent Minnesota trial where Toyota was ordered to pay $11M for a sudden unintended acceleration. In that case, the driver was charged with vehicular manslaughter and served approximately 3 years in prison until finally having his conviction set aside and blame placed on the manufacturer. If Minnesota followed a strict “driver as if driving” rule, the man would likely still be behind bars.

Autonomous car manufacturers will not be able to overcome the problem with the ITE Yellow Change Interval Formula until the State’s Departments of Transportation adopt a deterministic formula that works for all prudent traffic movements. Such a solution exists (Traffic Technology International published it here). If the manufacturers do not work in tandem with the State’s DOTs and then roll out autonomous cars with the existing situation, the manufacturers could face costly lawsuits, and passengers of their cars could face long legal fights of their own. Traffic engineers are vulnerable as well. The statutes of every State require professional engineers to know and to properly apply the mathematical and physical sciences. Timing the yellow light with an incorrectly applied formula makes the traffic engineer susceptible to prosecution.

We believe that for Tesla, Google, Mercedes-Benz, Audi, Volvo and others to succeed with their plans, they will have to confront the State’s Departments of Transportation, the Institute of Transportation Engineers (ITE), the National Council Highway Research Program (NCHRP) and the National Council of Examiners for Engineering and Surveying (NCEES). By misapplying the ITE Formula, these organizations perpetuate these problems. If calculated with a proper formula, it can be seen that the length of the yellow interval will have to increase by 2 to 4 seconds to accommodate the time required for turning and crash-avoidance maneuvers. IN the end, the yellow intervals will take on lengths last seen in the 1960s – 1990s. When this happens, drivers will be safer and the autonomous car manufacturers will be able to release their vehicles with confidence.

For world-class discussion on the advancement of connected and autonomous vehicles, register now for the first ever Autonomous Vehicle Test & Development Symposium here

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About Author


Tom has edited Traffic Technology International (TTi) magazine and its Traffic Technology Today website since May 2014. During his time at the title, he has interviewed some of the top transportation chiefs at public agencies around the world as well as CEOs of leading multinationals and ground-breaking start-ups. Tom's earlier career saw him working on some the UK's leading consumer magazine titles. He has a law degree from the London School of Economics (LSE).