Yes. Speeding is a factor in about one-third of all fatal crashes, killing more than 1,000 Americans every month. In 2006, more than 13,500 people died in speed-related crashes. The National Highway Traffic Safety Administration estimates that the economic costs of speed-related crashes is more than $40 billion each year.¹

In a high-speed crash, a passenger vehicle is subjected to forces so severe that the vehicle structure cannot withstand the force of the crash and maintain survival space in the occupant compartment. Likewise, as crash speeds get very high, restraint systems such as airbags and safety belts cannot keep the forces on occupants below severe injury levels.

Speed influences the risk of crashes and crash injuries in three basic ways:

• It increases the distance a vehicle travels from the time a driver detects an emergency to the time the driver reacts.
• It increases the distance needed to stop a vehicle once an emergency is perceived.
• It increases the crash energy by the square of the speeds. When impact speed increases from 40 to 60 mph (a 50 percent increase), the energy that needs to be managed increases by 125 percent.

For practical reasons, there are limits to the amount of crash energy that can be managed by vehicles, restraint systems, and roadway hardware such as barriers and impact attenuators. The higher the speed, the more likely that these limits will be exceeded in crashes, thus limiting the protection available for vehicle occupants. To put speed into perspective, remember that government crash tests for occupant protection are conducted at speeds of 30-35 mph, and these are severe impact speeds.

Many drivers exceed speed limits by some amount at least occasionally. In a study of drivers on limited access highways, high-speed drivers were more often male and more often judged to be younger than 30.² An observational study conducted on roads with speed limits ranging from 40-55 mph compared drivers traveling at least 15 mph above posted speed limits with drivers of adjacent vehicles traveling no more than 5 mph above the speed limit.³ Speeders were younger than drivers in the comparison group and had more violations and crashes on their driving records.

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.⁴ Studies in California found that the rate of speeding violations per mile traveled was at least three times as high for drivers 16-19 years old as it was for drivers 30 and older.⁵ A National Highway Traffic Safety Administration analysis found that the proportion of fatal crashes in 2006 deemed to be speed-related decreased with increasing driver age. For example, 39 percent of male drivers ages 15-20 in fatal crashes were speeding at the time of the crash, compared with 21 percent of male drivers ages 35-44 and 16 percent of male drivers ages 45-54. At all ages, the fatal crashes of male drivers were more likely than those of female drivers to be speed-related.¹

No. Both variation and speed are important. Although research conducted in the 1950s on two-lane rural roads indicated that vehicles traveling much faster or much slower than average were more likely to be involved in crashes, severe crash involvement increased with speed.⁶ The risk of death and severe injury is a direct exponential function of speed, not speed differences. Many differences in travel speeds are unavoidable because of the slower speeds of turning or merging vehicles. Higher speeds of the other vehicles exacerbate this problem. Besides, 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 minor one.

Speed limit laws, which date to 1901, traditionally have been the responsibility of the states. Before 1973, when Congress responded to oil shortages by directing the US Department of Transportation to withhold highway funds from states that did not adopt a maximum speed limit of 55 mph, speed limits on rural interstates in most states ranged between 65 and 75 mph, with the majority of states setting rural interstate speed limits of 70 mph.⁷ In urban areas, most states maintained 55 mph speed limits before the national maximum speed limit was established.

By March 1974, all states adopted the 55 mph national maximum speed limit. As concerns about fuel availability and costs faded, however, and speeds began to gradually climb on US highways, Congress in 1987 allowed states to increase speed limits on rural interstates to 65 mph.

The National Highway System Designation Act of 1995 repealed the maximum speed limit, allowing states to set their own limits for the first time since 1974. Many states quickly moved to raise speed limits on both rural and urban interstates and limited access roads. As of September 2008, 33 states had raised speed limits to 70 mph or higher on some portion of their roadway systems.

Maximum posted speed limits for passenger vehicles by state

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.⁸

Most states raised speed limits in response to the 1995 abolishment of the national maximum speed limit, and the higher limits were associated with immediate increases in travel speeds. Within one year after speed limits were raised from 55 to 70 mph on three urban freeways in Texas, the percent of passenger vehicles traveling faster than 70 mph increased from 15 to 50 percent; the percent exceeding 75 mph increased from 4 to 17 percent.⁹ On California urban freeways where speed limits were raised from 55 to 65 mph, the percent of motorists traveling faster than 70 mph increased from 29 to 41 percent.⁹ A more recent Institute study examined longer-term changes in traffic speeds on roads where limits were increased following repeal of the national speed limit.¹⁰ Ten years after speed limits were raised from 65 to 75 mph on rural interstates, the proportion of passenger vehicles exceeding 80 mph tripled in Nevada and nearly tripled in New Mexico. By comparison, declines in travel speeds were observed on urban freeways in California and Nevada, where there also were large increases in traffic volume and development of surrounding areas.

In 2007 the Institute monitored travel speeds on interstates in 8 metropolitan areas (Albuquerque, Atlanta, Boston, Denver, Los Angeles, Omaha, Tampa and Washington, DC).¹¹ On urban interstates, the average speed of passenger vehicles exceeded the limits in all 8 metro areas. On suburban and rural interstates, average speeds were faster than the limits in half of the metro areas. The proportion of passenger vehicles exceeding 70 mph on urban interstates ranged from 1 percent in Denver and Tampa to 38 percent in Albuquerque, while the percentage exceeding 75 mph on suburban and rural interstates ranged from 6 percent in Los Angeles to 49 percent in Tampa. The same study examined segments of rural interstates located 30-50 miles outside 3 of the metro areas (Washington, DC, Atlanta, and Los Angeles). Outside Washington, where limits are 70 mph, 19 percent of passenger vehicles were logged exceeding 70 mph, and 3 percent surpassed 75 mph. Speeds were much faster on the Los Angeles intercity segment, where 86 percent of passenger vehicles surpassed the 70 mph limit and 35 percent traveled faster than 80 mph.

Institute studies showed that deaths on rural interstates increased 25-30 percent when states began increasing speed limits from 55 to 65 mph in 1987. In 1989, about two-thirds of this increase — 19 percent, or 400 deaths — was attributed to increased speed, the rest to increased travel.^12,13,14

A 1999 Institute study of the effects of the 1995 repeal of the national maximum speed limit indicated this trend had continued. Researchers compared the numbers of motor vehicle occupant deaths in 24 states that raised speed limits during late 1995 and 1996 with corresponding fatality counts in the 6 years before the speed limits were changed, as well as fatality counts from 7 states that did not change speed limits. The Institute estimated a 15 percent increase in fatalities on interstates and freeways.¹⁵

Researchers at the Land Transport Safety Authority of New Zealand also evaluated the effects of increasing state speed limits from 65 mph to either 70 or 75 mph. Based on deaths in states that did not change their speed limits, states that increased speed limits to 75 mph experienced 38 percent more deaths per million vehicle miles traveled than expected — an estimated 780 more deaths. States that increased speed limits to 70 mph experienced a 35 percent increase, resulting in approximately 1,100 more deaths.¹⁶

Statutory speed limits established by state legislators generally provide maximum speed limits by type of road (e.g., interstate highway, two-way undivided highway) and land use (urban or rural). For example, Minnesota law establishes statutory maximum speed limits of 10 mph in alleys, 30 mph in urban districts and rural residential districts, 65 mph on urban interstates, 70 mph on rural interstates, and 55 mph in locations other than those specified in the statutes.¹⁷ Statutory speed limits also can be established for special situations such as school zones. For example, California law establishes speed limits of 25 mph in school zones when children are present and on streets (other than state highways) adjacent to senior centers posted with "SENIOR" warning signs.¹⁸

Where statutory limits are not appropriate for specific road or traffic conditions, modified speed limits or speed zones can be established by administrative action. Such limits generally are based on traffic engineering studies, which consider factors such as the design of the road (e.g., lane width, pavement type and condition, terrain, parking conditions), commercial and residential development, the number of driveways and intersecting streets, prevailing vehicle speeds, traffic volumes, and crash history.

It has been argued that measuring the speed distribution of a roadway and then setting the speed limit so that 85 percent of motorists would be in compliance reduces the need for enforcement and, at the same time, reduces crash risk by narrowing variation among vehicle speeds. However, numerous studies of travel speeds on rural interstate highways have shown that 85th percentile speeds increased when speed limits were raised to 65 mph and then continued increasing. The 85th percentile is not a stationary point. It is rather a moving target that increases when speed limits are raised. If speed limits are raised to meet a current 85th percentile speed, a higher new 85th percentile speed will soon result.

Some claim that because interstate highways meet rigid design standards for sight distance and roadway geometry, they should be posted at their design speeds. But a design speed is not necessarily a safe travel speed. The American Association of State Highway and Transportation Officials defines design speed as "the maximum safe speed that can be maintained…when conditions are so favorable that the design features govern."¹⁹ In other words, it is the maximum speed at which drivers can travel safely under ideal conditions. Speed limits are set somewhat lower because conditions are not always favorable, and design features do not always warrant higher speeds.

No. Advocates of higher speed limits have claimed that research on speed variation indicates faster speeds are not hazardous. They cite David Solomon's research from the mid-1950s that seems to show an increase in crash likelihood among drivers traveling slower than the average speed and a minimum of crashes at 5-10 mph above the average speed.⁶ However, the speeds of the crash-involved drivers in these studies were based on self-reports. It is well known that drivers exceeding speed limits are likely to deliberately underestimate their speeds. Such underestimation can account for much of the apparent underinvolvement of moderately high-speed drivers in crashes. Later research found that simply removing the crashes involving intersections and turning maneuvers from the Solomon data eliminated the overinvolvement of slower drivers in crashes.²⁰ More important, the Solomon research addressed only speed variation, not speed limits.

Although there is no definitive research showing crash effects associated with posting lower (differential) speed limits for large trucks, many safety experts favor differential limits because large trucks require much longer distances than cars to stop. Lower speed limits for trucks make stopping distances of trucks closer to those of lighter vehicles. Slower truck speeds also allow passenger vehicles drivers to pass trucks more easily. And because on average, large trucks tend to go slower than passenger vehicles on expressways with uniform speed limits¹⁰, lower speed limits for trucks reflect a natural speed differential between larger trucks and passenger vehicles.

Institute research published in 1991 found that lower speed limits for trucks on 65 mph highways reduced the proportion traveling faster than 70 mph without increasing variation among vehicle speeds. The percentage of trucks traveling faster than 70 mph was twice as large in states with uniform 65 mph limits (14 percent in Arizona, 9 percent in Iowa) as it was in states with different passenger vehicle (65 mph) and large truck (55 mph) speed limits (4 percent in California, 3 percent in Illinois).²¹