Is driving the speed limit on a highway safer?

Question

I'd have thought this would have been asked already but I didn't see it. Is driving the speed limit safer on a highway? By "highway" I mean high-traffic areas, such as 4-lane divided thoroughfares.

Having commuted for some time, I've noticed that people who clearly drive at excessive speeds seem to be dangerous. However, more often than not the danger I see is from people who are driving slower than the prevailing rate (where the prevailing rate is generally more than the speed limit). This seems more dangerous because it disrupts the flow of traffic, irritates drivers and people behind the slow driver end up tailgating.

Is there any evidence to suggest that:

1. speed limits on highways improve the safety of people driving on those highways?
2. people driving the speed limit and those around them are safer than those travelling at the prevailing rate of traffic - even where that prevailing rate is faster than the speed limit?

Answer 1

Higher average speed will lead to more accidents. This is because braking distance doesn’t just double from 100 km/h to 200 km/h. In fact, it quadruples. It’s nonlinear, but people think it’s linear. Insufficient following distance is the second-biggest cause of accidents (the biggest is excessive speed). Translate this link.

You might have thought that big traffic jams are mainly caused by accidents. But in fact, it’s the difference in speed that will on higher speed increase non-linearly probability of abrupt braking, insufficient car-to-car distances and longer braking distance of cars. Modern traffic systems try to decrease the human factor as much as possible in order to reduce braking distance. Then you can increase speeds. This is because several cars will behave like a railway train with fixed distances between train carriages.

The second point you mention to adapt to avg. speed (> speed limit) to drive safer is kind of misleading and mirroring this linear thinking (esp. for very old drivers with reduced reflexes), as people will tend to choose non-linearly too small distances to the car in front with increasing speed. Trucks in the right-hand (slow) lane are very unlikely to cause traffic jams. This is because they behave mostly like the "road train" that was described in the hyperlink in the previous paragraph.

We do a lot of traffic research here in Germany. This is because some of our speed-limit signs are computer-controllable, depending on traffic density, average speed, and traffic-jam messages from drivers. These limits get adapted temporally on highly frequented highways, but only downward from the maximum speed limit. Also you probably know of new car features like "Autonomous cruise control system", that will use computer control to control the speed and distance to cars in front to you.

So of course if the speed limit is 100 km/h, one shouldn’t drive 50 km/h in the rightmost lane. Small speed-limited two-wheeled motor vehicles (max. 50 km/h) are forbidden on German highways. As far as I know, in Europe the highway speed limit of around 120 km/h and an advisory speed limit around 80 km/h is the best compromise of fast traffic and human factor. In Germany we have, in some areas, no general speed limit, since we are car fanatics and have a strong lobby :) However, most European countries have a general speed limit of around 120 km/h.

Answer 2

This is a near-miss answer: it addresses only 60 km/h roads, not highways. Nonetheless, it addresses some of the attitudes to driving described in the question, so it may shed some light on the answer:

For 60 km/h speed-limits:

• Driving faster than the speed limit is associated with a huge increase in the chance of being involved in an accident involving a casualty.

• Driving slightly slower than the speed limit doesn't make much difference.

• Driving very slowly (e.g. 40 km/h in a 60 km/h zone) is associated with a moderate increase in the chance of being involved in an accident involved a casualty.

This is illustrated in Table 4.3 of a 1997 report from the NHMRC Road Accident Research Unit of The University of Adelaide: Kloeden CN, McLean AJ, Moore VM, Ponte G, Travelling Speed and the Risk of Crash Involvement, Volume 1 - Findings.

Their headline finding:

In a 60 km/h speed limit area, the risk of involvement in a casualty crash doubles with each 5 km/h increase in travelling speed above 60 km/h.

Correlation is not causality. Correlation is not causality. Correlation is not causality.

Answer 3

Some of the answers from non-U.S. countries may not be factoring in what might be a U.S. cultural phenomenon where governing bodies have strong financial incentives abetted by the insurance lobby to set speed limits artificially low. This results in a majority of traffic traveling faster than the posted speed limit (an odd situation for a democracy where the majority should be setting the law). So while the other posts on braking distance and energy of collision are true, they do not consider the primary effect in the U.S. case.

Research by the University of Michigan demonstrated that a proper speed limit should be set by measuring traffic over a period of time and posting a limit at approximately 1.7 standard deviations above the mean speed of traffic. In other words, the posted speed limit should be set well above the "natural" average speed of drivers on the road. This improves safety by reducing the number of car-car interactions through passing, speeding up, slowing down, etc. which is the primary controllable cause of crashes.

So the questioner is correct for many cases in the U.S.; people driving at the speed limit when the mean speed of traffic is higher cause a greater risk of accident and injury than those who are driving faster than the speed limit but within 1.7 sd of the mean. In the State of Michigan, the state police have been forcing municipalities to increase posted speed limits in order to improve safety.