Speed

Overview

More than 9,000 deaths — 26 percent of all crash fatalities — occurred in speed-related crashes in 2017. High speeds make a crash more likely because it takes longer to stop or slow down. They also make collisions more deadly because crash energy increases exponentially as speeds go up.

Raising speed limits leads to more deaths. People often drive faster than the speed limit, and if the limit is raised they will go faster still. Research shows that when speed limits are raised, speeds go up, as do fatal crashes.

Enforcement of speed limits helps keep speeds down. Traditional enforcement, which relies on police officers to measure speed with radar or other technology, has been joined recently by speed cameras. Speed cameras have been shown to reduce speeds and crashes.

Latest news

Speed limit increases tied to 37,000 deaths

Rising speed limits over the past 25 years have cost nearly 37,000 lives, including more than 1,900 in 2017 alone, a new IIHS study shows.

April 4, 2019

Drivers brake for lower speed limit

A 5 mph drop in the speed limit makes city streets safer for motorists, pedestrians and bicyclists alike. A look at Boston's experience.

August 28, 2018

Dangers of speed

Speed has a major impact on the number of crashes and injury severity (Elvik, 2005). It 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 the driver starts to brake.
  • It increases the risk that an evasive steering maneuver will result in loss of control.
  • It increases the crash energy exponentially. For example, when impact speed increases from 40 to 60 mph (a 50 percent increase), the energy that needs to be managed increases by 125 percent.

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.

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 crash cushions. The higher the speed, the higher the likelihood that these limits will be exceeded in crashes, limiting the protection available for vehicle occupants.

Some people contend that speed variation, not speeding, is the real danger. This idea is rooted in research conducted in the 1960s on two-lane rural roads, which found that vehicles traveling much faster or much slower than average were more likely to be involved in crashes (Solomon, 1964). However, that same research found that involvement in severe crashes increased with speed.

While less speed variation is associated with fewer crashes because it cuts down on passing maneuvers and lane changes (Transportation Research Board, 1984; Garber & Ehrhart, 2000), 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 other vehicles exacerbate this problem. Besides, many crashes and nearly half of those resulting in occupant deaths are single-vehicle impacts in which differences in vehicle speeds play no role or only a minor one.

By the numbers

In 2017, a total of 9,717 deaths, or 26 percent 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 10 percent of property-damage-only crashes and 13 percent of crashes with injuries or fatalities in 2017.

The National Highway Traffic Safety Administration (NHTSA) estimates that the economic cost of speed-related crashes is about $52 billion each year (Blincoe et al., 2015).

The majority of drivers exceed posted speed limits on many different kinds of roads, but the problem is greatest on freeways and expressways. National surveys have found that on freeways and interstates 48 percent of free-flow traffic in 2007 and 72 percent in 2009 exceeded posted speed limits (Huey et al., 2007; Huey et al., 2009).

Fourteen percent of all vehicles traveling on limited-access highways exceeded posted speed limits by 10 mph or more during 2007. The percentage rose to 20 percent during 2009.

On other types of roads, proportions of drivers exceeding speed limits fell slightly in 2009 compared with 2007. Still, 13 percent of vehicles on major arterials and 15 percent on minor arterials and collectors traveled at least 10 mph over posted speed limits during 2009.

In a 2017 national telephone survey conducted by the AAA Foundation for Traffic Safety, half of drivers said they had exceeded the speed limit by 15 mph on a freeway in the past month, and nearly half reported exceeding the speed limit by 10 mph on a residential street (AAA Foundation for Traffic Safety, 2018).

Who speeds?

Drivers who speed tend to be younger than drivers who don't, and male drivers are more likely than female ones to speed (Schroeder et al., 2013; Preusser et al., 1988Williams et al., 2006).

Crashes and violations of young drivers are much more likely to be related to speeding than those of drivers of other ages (Williams et al., 1995). A study in California found that the rate of speeding violations per mile traveled was at least 3 times as high for drivers 16-19 years old as it was for drivers 30 and older (Janke et al., 2003). In a 2011 national telephone survey, the percentage of drivers who reported having at least one speeding-related crash during the past five years was higher for the youngest drivers, those 16-20 years old, than for any other age group, even though the youngest drivers may not have been driving for all of the past five years (Schroeder et al., 2013).

These trends hold true for fatal crashes involving speeding. In all fatal crashes in 2017, male drivers were more likely to be speeding than female drivers. Nineteen percent of male drivers involved in fatal crashes were speeding at the time of the crash, compared with 11 percent of all female drivers. The proportion of drivers that were speeding in fatal crashes decreased with increasing driver age.

Speed limits

Speed limit laws, which date to 1901, traditionally have been the responsibility of the states, but the national maximum speed limit in place in the 1970s and 1980s effectively established maximum speed limits of 55 mph everywhere in the country. Since its complete repeal in 1995, speed limits have trended up.

Currently, 22 states have maximum speed limits of 70 mph, and 12 states have maximum speed limits of 75 mph on some portion of their roadway systems. On some sections of interstates in seven states, speed limits are 80 mph. In October 2012, a 41-mile stretch of Texas State Highway 130 opened with a speed limit of 85 mph.

Maximum posted speed limits for different road types, by state



State laws set maximum speed limits for each type of road (e.g., interstate highway, two-way undivided highway) and land use (urban or rural) (Federal Highway Administration, 2009). Statutory maximum speed limits also can be established for special situations such as school zones. The posted speed limit for a particular road or section of a road can be below the maximum speed limit allowed, however, if the local or state agency in charge of the road decides a lower limit is warranted.

Common factors considered in setting speed limits include the 85th percentile speed — which is the speed that 85 percent of vehicles are traveling at or below in free-flowing conditions — land use, roadside environment, roadway design, crash experience and the prevalence of pedestrians.

Advocates of using the 85th percentile speed to set limits argue that it reduces the need for enforcement and, at the same time, reduces crash risk by narrowing variation among vehicle speeds. However, raising the speed limit doesn't always reduce speed variation (Hu, 2017), and numerous studies of travel speeds have shown that 85th percentile speeds on rural interstate highways increased when speed limits were raised and then continued increasing (Retting & Greene, 1997; Retting & Teoh, 2008; Retting & Cheung, 2008; Najjar et al., 2000). 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 the current 85th percentile speed, a new, higher 85th percentile speed will likely result.

Effects of speed limits on safety

The establishment of the national maximum speed limit and its subsequent repeal provided ample opportunity to study the effects of lowering and raising speed limits.

Congress established the national maximum speed limit in 1973 in response to oil shortages. The U.S. Department of Transportation was directed to withhold highway funds from states that did not adopt a maximum speed limit of 55 mph. Before that, speed limits on rural interstates in most states ranged from 65 to 75 mph, with the majority of states setting rural interstate speed limits of 70 mph. In urban areas, most states maintained 55 mph speed limits before the national maximum speed limit was established.

By March 1974, all states had adopted the 55 mph national maximum speed limit. Concerns about fuel availability and costs faded, however, and 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 freeways.

Although the national maximum speed limit was imposed to conserve oil, its greatest effect was on safety. The National Research Council attributed 4,000 fewer fatalities to the decreased speeds in 1974 compared with 1973 (Transportation Research Board, 1984).

Not surprisingly, higher limits established after the 1995 repeal were associated with immediate increases in travel speeds. For example, 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 (Retting & Greene, 1997). 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.

As limits continued to rise to 70, 75 and 80 mph, travel speeds continued to go up (Retting & Teoh, 2008; Hu, 2017).

Fatalities also went up. Deaths on rural interstates increased 25-30 percent when states began increasing speed limits from 55 to 65 mph in 1987 (Baum et al., 1991; Baum et al., 1989; Baum et al., 1990). In 1989, about two-thirds of this increase — 400 deaths — was attributed to increased speed and the rest to increased travel. 

An IIHS study examined longer-term changes. During 1993-2017, a 5 mph increase in the maximum state speed limit was associated with an 8 percent increase in fatality rates on interstates and freeways and a 3 percent increase on other roads (Farmer, 2019). In total, there were an estimated 37,000 more traffic fatalities during these years than would have been expected if maximum speed limits in 1993 had remained in place. In 2017 alone, there were more than 1,900 additional deaths.

Speed enforcement methods

Police use a variety of methods and equipment to measure vehicle speeds.

Radar: Radar is the primary method of speed enforcement in the United States. Radar guns aim an electromagnetic signal at a target vehicle and pick up the return signal reflected off the vehicle. The Doppler effect causes the frequency of the return signal to shift by an amount dependent on the relative speeds of the source of the original signal and the target.

Radar is highly reliable and accurate. However, it can be difficult to pinpoint specific vehicles in heavy traffic, and some motorists use radar detectors to help them speed without getting caught.

Laser: Laser devices, also known as LIDAR (light detection and ranging), use a time/distance calculation to measure speed. The devices aim a narrow band of light at the target vehicle and measure the time it takes to receive the reflected light. Because the speed of both the original light pulse and its reflection are traveling at the same speed (the speed of light), differences in the time it takes the transmitted light to strike the target vehicle and return can be used to calculate the speed of the vehicle. Lasers can pinpoint specific vehicles in heavy traffic.

VASCAR: VASCAR stands for visual average speed calculator and recorder. It uses a portable computer to accurately clock, calculate and display speed based on the time a vehicle takes to travel a known length of road.

VASCAR provides an average speed measurement over a greater distance than is possible with radar. It enables police officers to identify specific speeding vehicles and can be used from patrol cars following speeders. VASCAR can detect speeding vehicles going in the opposite direction. When used correctly, it is very reliable.

Aerial speed measurement: Officers in light aircraft measure vehicle speeds based on the time it takes to travel between two or more pavement markings spaced a known distance apart. Information is transmitted to officers on the ground who then issue speeding citations.

Aerial surveillance can provide very accurate speed measurements and allow officers to focus on the fastest vehicles, but it is costly and can be difficult to use in locations with high traffic volumes.

Speed cameras

Among enforcement methods, speed cameras stand apart because they don't require the presence of an officer. By deploying cameras, communities are able to enforce speed limits much more consistently than they can using traditional enforcement.

Most speed cameras measure the speed of a vehicle at a single spot. Fixed cameras use either radar or detectors embedded in the road surface to measure a vehicle's speed. Mobile cameras are placed at the roadside in marked or unmarked police cars, containers, poles, etc., and use radar or laser to measure speeds. Some communities require mobile cameras to be manned. In either fixed or mobile systems, if a vehicle is traveling faster than a predetermined speed, the date, time, location and speed are recorded along with a photo of the vehicle.

More recent technology can measure average speeds over a certain distance. In this case, cameras located at two or more points record time-stamped images of all vehicles that pass them. Automatic license-plate recognition is used to match individual vehicles so that average speeds between the two points can be calculated. Time-stamped pictures of speeding vehicles are used as evidence of speeding. Point-to-point speed cameras have been used to enforce speed limits in countries such as Australia and the United Kingdom.

Speed cameras usually are programmed so they will not be activated unless a vehicle is traveling significantly faster than the posted limit — typically 10 or 11 mph faster, although in certain places such as school zones the tolerance may be lower.

Automated speed enforcement can substantially reduce speeding on a wide range of roads. IIHS studies of cameras on residential roads in Maryland, on a high-speed roadway in Arizona and on city streets in the District of Columbia found that the proportion of drivers exceeding speed limits by more than 10 mph declined by 70, 88 and 82 percent, respectively, six to eight months after cameras were introduced (Retting et al., 2008; Retting et al., 2008; Retting & Farmer, 2003).

An IIHS study in Montgomery County, Md., found that about 7½ years after the speed camera program began, the cameras were associated with a 10 percent reduction in mean speeds and a 62 percent reduction in the likelihood that a vehicle was traveling more than 10 mph above the speed limit on camera-eligible roads, almost all of which had cameras (Hu & McCartt, 2016).

A 2010 review published by the Cochrane Collaboration, an international public health organization, examined 35 studies from various countries. The authors concluded that speed cameras — including fixed, mobile, overt and covert devices — cut average speeds by 1-15 percent and the percentage of vehicles traveling above the speed limits or designated speed thresholds by 14-65 percent compared with sites without cameras (Wilson et al;. 2010).

Speed cameras have also been shown to reduce crashes and crash injuries. Speed camera enforcement in Montgomery County was associated with an 8 percent reduction in the likelihood that a crash on a camera-eligible road was speeding-related and a 19 percent reduction in the likelihood that a crash involved an incapacitating or fatal injury (Hu & McCartt, 2016). A corridor approach, in which cameras were periodically moved along the length of a roadway segment, provided an additional 30 percent reduction in the likelihood that a crash involved an incapacitating or fatal injury over and above the effect of the cameras.

In its 2010 review, the Cochrane Collaboration summarized 28 studies that reported the effect on crashes and found reductions of 8-49 percent for all crashes, 8-50 percent for injury crashes and 11-44 percent for crashes involving fatalities and serious injuries, in the vicinity of camera sites (Wilson et al;. 2010). Over wider areas, the review found reductions of 9-35 percent for all crashes, and 17-58 percent for crashes involving fatalities and serious injuries. Reviewed studies with longer duration showed that these trends were either maintained or improved with time.

Speed cameras were in operation in 137 U.S. communities in 14 states and the District of Columbia during 2018, according to media sources and other public information tracked by IIHS, up from only four Arizona and Utah communities in 1995. Peoria, Ariz., and Paradise Valley, Ariz., were the first two communities to implement speed cameras in 1987. Cameras are used statewide in highway work zones in Illinois, Maryland and Oregon.

Trends in the number of U.S. communities with speed cameras

U.S. communities using speed cameras

Despite some vocal opposition to camera enforcement, telephone surveys in jurisdictions with speed-camera programs show a majority of drivers support them.

A 2014 IIHS survey of 900 licensed drivers ages 18 and older residing in Montgomery County, Md., found that 62 percent of drivers favored automated speed enforcement on residential streets about 7½ years after camera ticketing began (Hu & McCartt, 2016). A 2017 national survey of drivers ages 16 and older indicated that 48 percent supported the use of speed cameras on residential streets (AAA Foundation for Traffic Safety, 2018).

A 2012 IIHS survey of 801 District of Columbia residents found strong support for speed cameras (Cicchino et al., 2014). D.C. has used speed cameras since 2001. In the survey, 88 percent of residents said that speeding was a serious threat to their personal safety. Seventy-one percent of residents who had driven a car in D.C. in the past month and 90 percent of residents who had not driven supported speed cameras.

In Scottsdale, Ariz., 63 percent of drivers surveyed prior to the start of automated enforcement said speed cameras should be used on an urban freeway where camera enforcement was planned. After speed cameras were operational, 77 percent of drivers supported their use (Retting et al., 2008).

NHTSA and the Federal Highway Administration have issued guidance for implementing speed camera programs that can increase a community’s support for them.

Intelligent speed assistance

Intelligent speed assistance (ISA), sometimes also called intelligent speed adaption, describes a class of in-vehicle systems that provide information to the driver on their speed relative to the posted speed limit. ISA systems work with either GPS devices linked to speed limits or on-board sensors or cameras that "read" speed limit signs and then integrate speed limit data with vehicle speed in real time.

ISA systems differ in how much control drivers have in deciding whether to speed. The least automated systems provide alerts to drivers (Biding & Lind, 2002). A second approach is to introduce resistance to gas pedals, making it harder but still possible to speed (Varhelyi & Makinen, 2001). The most aggressive automated systems limit gas flow to engines so drivers can accelerate up to but not over the speed limit. Some manufacturers offer built-in top speed limiters that must be set manually, whereas some systems will automatically limit the top speed to the posted speed limit (Carsten et al., 2008). A fourth option is to give drivers incentives, such as auto insurance discounts, to slow down (Reagan et al., 2013).

During the past 25 years, field assessments have indicated significant reductions in speeding when driving with ISA.

The largest research effort studied several thousand Swedish drivers using systems giving an advisory alert, pedal feedback or throttle constriction for more than a year. Researchers reported a decrease in speeding violations for each system (Biding & Lind, 2002).

A U.S. study indicated that the potential to earn a modest monetary incentive while driving with an alerting ISA system increased the percentage of time driving at or below the speed limit from 70 percent to 83 percent (Reagan et al., 2013).

European researchers have found large reductions in speeding over periods ranging from months to over a year (Carsten, 2012).

The largest technical barriers to ISA are the accuracy and breadth of coverage of digital maps with speed limits for GPS-based systems and the need for frequent speed limit signs for the camera-based systems. Digital maps may not include local roads and aren't always updated with speed limit changes in a timely fashion, and the camera-based systems will not know the speed limit until the vehicle passes a speed limit sign.

In 2019 the European Parliament approved legislation that will require all new vehicles sold in Europe to have standard ISA beginning in 2022. Several manufacturers offer optional advisory ISA systems for vehicles sold in the U.S., and ISA systems that limit gas flow to engines are available as options on a limited number of new vehicles. Manufacturers have begun to integrate camera-based speed limit recognition with cruise control to adjust the set speed based on the posted limit.