Speed limits on some US roads, particularly rural interstates, have been going up. Texas is the latest example, raising speed limits to 80 mph on more than 500 miles of roadway. This has been the trend since the late 1980s — higher and higher speed limits followed by faster and faster travel speeds.
Is speeding a safety problem?
Yes. The federal government reports that speeding is a factor in 31 percent of fatal crashes, killing an average of 1,000 people per month on US roads. One problem is that speed increases the distance it takes to stop in an emergency, so crash likelihood increases. When a crash does occur, speed worsens its severity by increasing the energy of the impact.
Do speed limits matter? Do motorists heed them?
Speed limit laws date back to 1901. Traditionally the limits have varied from state to state. Yes, many drivers tend to go somewhat faster than the posted limits, whatever they are, but drivers don't ignore the limits. They choose speeds they perceive as unlikely to result in a ticket, so the limits influence travel speeds. In particular, they reduce the proportions of motorists going at very high speeds. For example, the proportion going 70 mph or faster on rural interstates in New Mexico increased from 5 percent to 36 percent after speed limits were raised from 55 to 65 mph in the late 1980s. When the limits subsequently were raised to 75 mph, the proportion of motorists going faster than, this increased from 29 to 55 percent.
What's the effect of setting lower limits?
In the 1970s all states adopted the 55 mph national maximum speed limit. The National Research Council attributed 4,000 fewer crash deaths in 1974 to slower speeds under this policy, compared with 1973. When the 55 mph policy was loosened in 1987 and repealed in 1995, posted limits were raised and speeds increased. Deaths on rural interstates went up 25- 30 percent in 1987 on roads where limits were raised to 65 mph, and about two-thirds of the increase was attributed to faster travel.
Isn't speed variation the real culprit?
No. The risk of death and severe injury is a direct exponential function of speed, not differences among vehicle speeds. Besides, many crashes including nearly half of those resulting in deaths are single-vehicle impacts in which vehicle speed differences don't contribute at all or only in a minor way. Even if speed variation were a major problem, it wouldn't make sense to allow motorists to go very fast. This is because slower speeds often are unavoidable (turning, merging, etc.), so allowing very fast speeds exacerbates the problem of variation.
Do police enforce speed limits?
The number of drivers and the miles traveled have increased faster than the availability of officers to routinely enforce traffic laws. Plus it's hard on congested roads to pull over speeding drivers without creating a hazard. So while police do pay attention to speeders, they cannot keep up.
What are speed cameras? Do they lower travel speeds?
These are electronic devices that police use to enforce speed limits. Also known as photo radar, the cameras monitor speeds and photograph vehicles going a set amount (often 10 mph, not 1 or 2 mph) over the speed limit. Citations are issued by mail. The effect is to reduce speeds. Average speeds went down 14 percent within 6 months of beginning to use cameras in Washington, DC. The proportion of motorists exceeding the speed limit by more than 10 mph declined 82 percent. For answers to 34 questions about speeding, go to iihs.org/research.
