DamianJ

Is speeding really a safety problem? Yes. "It is clear that speeding does represent a significant traffic safety problem," the National Highway Traffic Safety Administration (NHTSA) reports. Deaths on rural interstates have increased as a result of most states raising speed limits to 65 mph on these roads during the late 1980s. Because speeding reduces the time drivers have to avoid crashes, it increases not only the likelihood of crashing but also the severity of crashes that do occur. According to NHTSA, speeding is one of the most prevalent reported factors associated with crashes. The agency estimates that speeding is involved in 12 percent of all crashes and one-third of all fatal crashes. (These estimates are likely to be conservative because of the difficulty of establishing precrash travel speeds after crashes.) NHTSA estimates that 13,909 fatalities and 77,277 moderate to critical injuries occurred in speed-related crashes in 1991 and that the cost of all speed-related crashes was more than $18 billion.

Roadway design factors, including how far ahead a driver can see, are compromised if vehicles travel faster than circumstances warrant. Other vehicles and pedestrians are put at risk by speeding drivers whose distances they may not be able to judge accurately. Recent studies have shown that drivers who run red lights are likely to be speeding and that motorcyclists who crash with other vehicles making left turns are also likely to be speeding.

What is the relationship between how fast a car is going and what happens in a crash? The higher the travel speed, the greater the risk of serious injury or death in a crash. Vehicles and their occupants in motion have kinetic energy that is dissipated in a crash. The greater the energy that must be dissipated, the greater the chances of severe injury or death. Motor vehicle crashes are complex events with multiple causes, and individuals have a wide range of tolerance to injury, but the amount of energy that must be dissipated - thus the probability and severity of injury in a crash - is related to a vehicle's speed at impact.

The laws of physics tell us that the energy of impact delivered to vehicle occupants in collisions increases nonlinearly with impact speed. In other words, crash severity increases disproportionately with vehicle speed. A frontal impact at 35 mph, for example, is one-third more violent than one at 30 mph. Using data from the National Accident Sampling System, which is based on field investigations of a national sample of police-reported crashes, NHTSA researchers compared the relative severity of injuries in crashes with the estimated crash impact speed. The percentage of occupants with serious injuries consistently and dramatically increases with increasing impact speed. For example, the rate of severe injury for people involved in crashes at impact speeds of 21-30 mph is 11.1 - a rate that increases to 27.9 at impact speeds of 31-40 mph and to 54.3 at speeds of 50 mph or more. (The rate is calculated as the number of occupants at a certain impact speed with severe injuries, divided by the total number of occupants in crashes at that impact level times 100.)

Who speeds the most? Young drivers speed more often than older drivers. In a study of drivers on limited access highways, high-speed drivers were more often male and more often judged to be younger than 30. Studies in California have found that the rate of speeding violations per mile traveled is at least three times as high for drivers 16-19 years old as it is for drivers age 30 and older. Although speeding is a problem among all driver age groups, the crashes and violations of young drivers are much more likely to be related to speed than is the case for drivers of other ages - and the motor vehicle crash death rate per 100,000 people is especially high among 16-24 year-olds. A NHTSA analysis found that the relative proportion of speed-related fatal crashes decreases with increasing driver age. About 37 percent of all drivers age 14-19 involved in fatal crashes were in speed-related crashes, but the percentage among drivers 70 and older decreased to 7 percent. At all ages, male drivers are more likely than female drivers to be involved in speed-related fatal crashes.

Isn't speed variation - not speeding - the real problem? No. Although research conducted in the 1950s on two-lane rural roads did indicate that vehicles traveling much faster or much slower than average were more likely to be involved in crashes, this issue is not relevant on today's high-speed highways with controlled access. The authors of this early study acknowledged that their findings could not be extended to controlled access freeways, but some proponents of higher travel speeds have attempted to do so. Many differences in travel speeds are unavoidable because of the slower speeds of turning or merging vehicles. Many crashes, and nearly half of those resulting in occupant deaths, are single-vehicle impacts in which differences among vehicle speeds play no role or only a very minor one. Finally, the risk of death and severe injury is a direct exponential function of speed, not speed differences.

What is the role of speed in crashes? Speed influences crashes in four basic ways:

It increases the distance a vehicle travels from when a driver detects an emergency until the driver reacts.
It increases the distance needed to stop a vehicle once an emergency is perceived.
Crash severity increases by the square of the speed so that, when speed increases from 40 to 60 mph, speed goes up 50 percent while the energy released in a crash more than doubles.
Higher crash speeds reduce the ability of vehicles and restraint systems to protect occupants.
Does the speed limit matter? Don't drivers speed anyway? Many drivers tend to drive somewhat faster than posted speed limits, no matter what the limits are. Although people often opt to travel somewhat faster than the posted limit, they do not completely ignore it but choose a speed they perceive as unlikely to result in a ticket. The more important speed-related safety issue on freeways involves the proportion of vehicles traveling at very high speeds, not the proportion violating the speed limit. The Institute's frequent monitoring of free-flowing travel speeds on interstate highways where the 55 mph speed limit was retained and speeds on roads where limits have been raised to 65 mph shows that, in general, higher speed limits lead to greater proportions of cars traveling faster than 70 mph. For example, in New Mexico, the first state to raise its limits to 65 mph on rural interstates, the proportion of motorists exceeding 70 mph grew from 5 percent shortly after speed limits were raised to 36 percent in 1993. On urban interstates that stayed at 55 mph, only 13 percent of cars and 2 percent of tractor-trailers traveled faster than 70. In Maryland, which retained 55 mph limits, the proportion traveling faster than 70 mph remained virtually unchanged at 7 percent during 1988-93. By 1994, 12-15 percent of cars were exceeding 70. In neighboring Virginia, which switched to 65 mph limits, the percentage exceeding 70 mph went from 8 percent in 1988 to 29 percent by 1992 and 39 percent by 1994.

Why 55 mph? Isn't this just a holdover from the gas crisis? Speed limit laws, which date back to 1901, have traditionally been the responsibility of the states. Then Congress responded to the oil shortage of 1973 by directing the U.S. Department of Transportation to withhold highway funding from states that did not adopt a maximum speed limit of 55 mph. The National Research Council attributed 4,000 fewer fatalities to the decreased speeds in 1974, compared with 1973, and estimated that returning the speed limit on rural portions of the interstate highway system to pre-1974 levels would result in 500 more fatalities annually, a 20-25 percent increase on these highways. As concerns about fuel availability and costs faded, however, speeds began to gradually climb on U.S. highways. By the mid-1980s, a substantial majority of vehicles on rural interstates were exceeding 55 mph. In response to claims that the 55 mph limit had made the United States a nation of law breakers and assertions that deaths and injuries would not increase because people were already traveling at the speeds at which they felt comfortable, Congress allowed states to increase speed limits on rural interstates to 65 mph in 1987.