How to build a better speed limit

Sometime in November, Texas will open a stretch of toll road south of Austin where the speed limit will be 85 miles per hour.It will be the highest speed limit in America. (Montana used to have no speed limit at all during the day, but that changed in 1999.)

Naturally, one of the big arguments against this is that higher speeds lead to more accidents. And there is some data to back this up. For instance, the Insurance Institute for Highway Safety makes a pretty good case for lower speed limits in a Q&A posted on their site:

In 2010, a total of 10,395 deaths, or nearly a third of all motor vehicle fatalities, occurred in speed-related crashes. Based on a nationally representative sample of police-reported crashes, speeding – defined as exceeding the speed limit, driving too fast for conditions or racing – was involved in 16 percent of property-damage-only crashes and 20 percent of crashes with injuries or fatalities. The National Highway Traffic Safety Administration (NHTSA) estimates that the economic cost of speed-related crashes is more than $40 billion each year.

...The National Research Council attributed 4,000 fewer fatalities to the decreased speeds in 1974 compared with 1973...

A 2009 study examining the long-term effects of the 1995 repeal of the national speed limit found a 3 percent increase in road fatalities attributable to higher speed limits on all road types, with the highest increase of 9 percent on rural interstates. The authors estimated that 12,545 deaths were attributed to increases in speed limits across the U.S. between 1995 and 2005.

There is definitely a relationship between speed and safety. It's there consistently in individual studies and you see it when you start looking at lots of studies all at once, too. But the meta-analyses—research that compares and analyzes the results of many studies—also show that the speed/safety connection is probably more complicated than it first appears. Speed limits matter. But maybe we need more options to pick from than a simple, static "faster" or "slower".

People and the environment both have a big impact on the relationship between speed and safety. There are a couple of meta-analyses available to read for free online. Check them out, and you'll see how psychology and road conditions play a big role.

For instance, a 1998 publication from the Federal Highway Administration found that the type of road matters. If you raise the speed limit on a road where people are already driving slowly, it won't affect safety at all.

In general, changing speed limits on low and moderate speed roads appears to have little or no effect on speed and thus little or no effect on crashes. This suggests that drivers travel at speeds they feel are reasonable and safe for the road and traffic regardless of the posted limit. However, on freeways and other high–speed roads, speed limit increases generally lead to higher speeds and crashes.

Here's another weird fact that turns up in both the 1998 report and a paper published by the Transportation Research Board in 2001: You're actually safest when you're traveling with the speed of the traffic around you. Speed-related accidents tend to happen when people are traveling faster or slower than the other cars on the road.

In fact, the 1998 report says that most speed-related accidents happen because an individual is driving too fast for the conditions of the road—that's the current weather, the width of the specific road, and how fast other people are driving.

The conclusion that both reports come to: We don't necessarily need lower speed limits. What we need are speed limits that adjust to the current conditions and the specific needs of a specific road. A variable speed limit would reflect the reality that a lot of drivers already see and respond to, and it might be more easily accepted by the drivers who ignore one-size-fits-all speed limits today. Plus, the variable speed limit would allow the law to match up with what's actually safe. If traffic is flowing at an average of 60 mph, it doesn't make sense to have 70 mph posted—somebody is going to try to keep up with the speed limit and create an unsafe condition.

It's an interesting idea. So far, there's not a lot of good data available to show whether or not it actually reduces accidents and fatalities. Variable speed limits have been tested out around the world, but they remain rare and, in North America, are mostly relegated to stretches of rural highway in places with a history of extreme weather—for instance, a road in Tennessee that gets a lot of heavy fog.

But the basic story is that we need more data. To know whether or not variable speed limits actually make sense, we need them to be implemented in more places with more traffic.

Synthesis of Studies on Speed and Safety - 2001 meta-analysis
Synthesis of Safety Research Related to Speed and Speed Management - 1998 meta-analysis
Driving Speed and the Risk of Road Crashes, A Review - 2006 meta-analysis that is behind a paywall
Wikipedia on variable speed limits
US Federal Highway Administration research on variable speed limits in construction zones

Image: Driving Cars in a Traffic Jam, a Creative Commons Attribution (2.0) image from epsos's photostream

Image: Speed Limit 35, a Creative Commons Attribution (2.0) image from 71966930@N00's photostream