Robocars: Do they need speed limits?

Do Self-Driving Cars Need Speed Limits?

Do Self-Driving Cars Need Speed Limits?

The citizen’s guide to the future.
Oct. 15 2013 9:47 AM

Should Self-Driving Cars Obey Speed Limits?

The rules of the road will need an update for robocars.

A Google self-driving car is displayed at the Google headquarters on September 25, 2012 in Mountain View, California.
A self-driving car at the Google headquarters on Sept. 25, 2012 in Mountain View, Calif.

Photo by Justin Sullivan/Getty Images

This article originally appeared on Brad Templeton’s site Robocars.

In many countries, including the United States, the speed limit is a rather nebulous thing. It's posted, but on many roads hardly anybody obeys it. Almost every driver speeds regularly, and anybody going at or below the limit on a clear road outside the right lane is typically an obstruction to traffic—they will find themselves being tailgated or passed at high speed on the left and right.

Half of Germany's Autobahns have no specific speed limit, but they have a better safety record. For a brief time, Montana's highways also had no set limit. In France they take a different approach. The Autoroute limit is 130 kph (81 mph), and almost nobody exceeds it; in fact the vast majority go under it. Reportedly, this is because the police are serious about it and will ticket you for any excess.


In the United States, it's not that way. A ticket for going 1 mph over the limit is an extremely rare thing. It usually signals a cop with another agenda or a special day of zero-tolerance enforcement. In fact, many drivers feel safe from tickets up to about 9 mph over the limit. Tickets happen there, but the major penalties require going faster, and most police like to go after that one weaving, racing guy who thinks the limit does not apply to him.

The limit is a number, but it is not especially magic. It's not like one is safe at 65 mph and reckless at 66 mph, even though that's how the law is written. Rather, the risk from accidents increases gradually with speed. The risk of having an accident is harder to measure, but the severity of an accident is related to the square of the speed of impact.

There is a speed at which we may judge the accident risk is above acceptable limits. This speed is not a single number. It varies from driver to driver and from car to car. It varies from hour to hour, from weather condition to weather condition, and from road to road. As the Autobahn's lower accident rate shows, some drivers are safer at very high speeds on well-designed roads than other drivers are at 50 mph on lesser roads.

And while the Germans are content to do it, the United States is not prepared to officially let drivers decide what the right speed for acceptable safety is. Rather it is done unofficially and irregularly.

So how does a self-driving enter this world of few hard-and-fast rules? There are two common schools of thought:

  1. As with its ancestor, the cruise control, the operator of a robocar can set the car to operate at any speed within its general limits, regardless of the road speed limit. The moral and safety decisions rest with this person.
  2. The vehicle must be programmed to not break the speed limit, nor allow its operator to do so. It must be aware of all limits and obey them.

I believe the first choice is both better and more likely. It's more likely because the public has a strong love for having control of their cars, even if it is automated. Attempts to put in speed limiters by law have all been rejected, and cars are routinely sold able to go much faster than any allowed speed limit. Some cars feature speed limiters due to European New Car Assessment Program rules and high European ticket costs, and some cars with very large engines have speed limiters that block them from doing truly crazy speeds, though these can be disabled for racetrack use. (Some people also seek speed limiters for children or parking valets, but don't want them for themselves unless they are one ticket away from losing their license.) In Canada, a truck driver successfully sued to have a court declare a speed limiter put on him was an unconstitutional violation of his basic freedoms.

People generally don't want their technology to disobey them or enforce the law. It's similar to the digital rights management battle in music players.

I think it's more interesting to examine whether limiting vehicles to the speed limit is the right thing to do. At first blush, considering that slower generally means safer, it seems as though such limits would improve safety. This turns out not to be true if, as seems likely, the driver of a robocar can take the wheel and operate it manually—especially if conventional vehicles remain on the road.

A vehicle limited to the speed limit will be going much more slowly than traffic on most U.S. freeways and be forced to drive in the right lane. This is actually a poor lane to drive in, because it means interacting with many cars merging onto the highway. This also means greater variation in speed as cars brake for merges and exits. This is also where the below-limit drivers are found.

This less comfortable ride, plus the longer travel time, will create a great temptation to manually take the wheel on many highways. Here's where some math comes in. If we presume the robocars have an accident rate that is just 50 percent of the human driver rate, the driver is doubling her risk by taking the wheel. If we presume accident risk is 25 percent higher at 75 mph than at 65 mph, this applies to both the vehicle and the human driver moving at that speed. In fact, it is likely the vehicle would do a better job at the higher speed or it would not be rated for that speed at all. The vehicle will know the exact shape of the road and the capability of its tires, and adjust this as it travels to do a better job.

On many roads all lanes are moving faster than the limit. The limited car would become an obstruction to traffic.

Math of lives and hours

Another interesting piece of math revolves around the reason people speed—they want shorter trips. While a trip in a robocar is not nearly so much wasted time as having to drive yourself, consider that human drivers have a fatal accident every 180 million miles of highway driving (at 75 mph,) and let's presume robocars get good enough to have one every 360 million miles at 65mph, and one every 300 million miles at 75mph.

Let's consider time spent driving as 20 percent productive. People have conversations when not alone, do phone calls, and listen to audiobooks and news. Let's consider time in a robocar as 70 percent productive—you can do many productive things, but it's not quite as good as being at a desk or at home. These numbers are arbitrary guesses, but you can fill in your own. At 16 hours per day, there are 5,800 productive hours in a year.

If robocars are constrained to 65mph, then 360 million miles of driving takes 5.5 million hours and has one fatality. We have 660 years of productive time saved and 282 years nonproductive.

Humans at 75 mph would use 4.8 million hours and have two fatalities. That’s 82 years productive time and 740 years nonproductive.

A robocar allowed 75 mph also uses 4.8 million hours with 1.2 fatalities. That's 575 years spent productive and 246 years nonproductive.

In other words, comparing the 65 mph constrained robocar with robocars and people going 75mph:

  1. If nobody takes the wheel manually because they want to go faster, going 65mph saves 0.2 fatalities but costs 36 years of productive time, about half a lifetime.
  2. If everybody takes the wheel to go faster, there are 0.8 extra fatalities and more than 6 full 80-year lifetimes of productive time is spent driving.
  3. If just 20 percent of people take the wheel, we get 1.2 fatalities and 129 extra years of productive time lost.

Clearly the option where people take the wheel is inferior for all. People have many opinions about how they would contrast 0.2 fatalities with 36 years of life wasted. On a pure time basis, 0.2 fatalities probably maps to about 8 years since on average half a lifetime is lost in a fatality. But you may view a death as far more tragic than 80 years of 16-hour days spent wasted, since that is spread out over many people. But there is an argument that even the pure "nobody switches to manual" scenario is not necessarily better, or only marginally better, and the reality is that plenty of people would switch (or simply not get a robocar at all) making the move a clear loss. By the time 20 percent of people are taking the wheel because they are in a hurry, the speeding robocar is clearly the superior result by any metric—based on these assumptions.

Yes, math that puts values on human lives is complex and difficult, though I think that it's easier to see an equivalence between lost time and lives than it is to see one between dollars and lives.

Nonetheless, I believe the math and other arguments clearly show that robocars should be allowed to move faster than the speed limit so long as they are rated suitably safe in the particular conditions, and the bulk of other traffic is also doing this.

The French system is almost surely better. With the fixed 130 kph (80 mph) limit, there is no issue to resolve. Robocars would be able to easily move the speed of traffic, and people would not disengage just to drive faster than the limit with the strict enforcement.


It should be noted that higher speeds use more fuel, at least at these levels. This is an independent variable, however. People routinely value their time more than the fuel cost and want to go faster, no matter what the means of transportation. If their car won't go faster and waste energy when they tell it to, they will take the manual controls. The way to solve energy waste is to put the environmental concerns into the price of the fuel.

Minor code violations

Speeding is just one of the code violations almost everybody does. There are many other examples. One that Google reported early on was handling a four-way stop. They found that if they were not a little aggressive in asserting their turn at the stop, others would quickly grab the slot, and the car would sit waiting for a long time. So they programmed the car to try to go on its turn even if somebody else was also trying to steal the slot, though it will eventually yield in a true game of chicken. Fortunately, there is always somebody who will be polite and yield when it was not their turn.