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Speed limits can be broadly understood to represent a society’s attempt to balance preferences surrounding traffic speed (Howard et al., 2008). Determining the best method, and the balance of priorities for safety, mobility for all types of road users, air quality, noise, and other livability and community values is an inherently complex process that has proven to be challenging (Forbes et al., 2012; Sanders et al., 2019). Public engagement can help determine the community’s priorities and well-crafted publicity can help to convey the reasons for lower speed limits (and the other countermeasures in this chapter). An important safety strategy involves setting appropriate speed limits using practices that take into consideration the road segment’s design, vulnerable users, traffic operations, land use, and environmental conditions (NHTSA, FHWA, & FMCSA, 2014). While speed limits form the basis for achieving the community’s desired balance of safety and other measures relating to mobility, speed limits should be supported by roadway design and operations and a level of enforcement necessary to achieve speed limit credibility and compliance among a vast majority of road users (NHTSA, FHWA, & FMCSA, 2014). (See Understanding the Problem section and Key Resources for more on this topic.)

Speed limits are set both by legislation and by administrative action. Default speed limits apply to all roads in a class such as rural interstates or local streets. Default limits are set by State, municipal, or, at times, Federal law. Speed zones apply to road segments where the default speed limit for that road type is thought to be inconsistent with safe driving. Speed limits in these zones usually are set by administrative action through engineering studies performed by the road managers or DOTs and may require approvals or consensus among partner agencies. Until recently, engineering studies for setting speed limits in speed zones in the United States have been based largely on the road segment’s free-flowing travel speeds, with crash experience, road function and land use conditions, presence of non-motorists and other factors often being considered to adjust the posted limit (Fitzpatrick et al., 2021).

Some urban jurisdictions and those with a Vision Zero framework have in recent years set lower speed limits to improve safety, including for people walking and other non-motorized road users (Sanders et al., 2019). Internationally, countries using a Safe Systems approach or having a Vision Zero framework often set limits based on an injury minimization principle to reduce the chances of fatal and injurious crashes. See the Key Resources section for information relating to methods and practices for setting speed limits, and the bullet on Vision Zero below.


A speed limit is in effect on all road segments in all States. For summaries of each State’s maximum speed limits see the GHSA (2018a) and IIHS (2019a). NHTSA (2013) provides a publication with each State’s complete speed limit laws. The highest speed limits may, however, only be in effect for a portion of rural interstates and other freeway miles.

Examples of cities that have widely lowered speed limits through both statutory actions by classes of road, and by administrative/engineering speed limit reviews include Boston and Cambridge, Massachusetts, Minneapolis, Minnesota, New York City, Portland, Oregon, and Seattle, Washington. Case examples are available at  Vision Zero Network and from NACTO.


The effects of maximum (highway) speed limits on speeds, crashes, and casualties have been studied extensively over the past 40+ years. In general, there is significant evidence that when limits are raised, speeds, crashes, and injuries rise, and when they are lowered, speeds, crashes and injuries usually decline (Farmer, 2019).

Effects of changing speed limits on highways

In 1974 the 55 mph National Maximum Speed Limit (NMSL) was enacted to conserve fuel. Travel decreased, speeds decreased on roads where the speed limit was lowered to 55 mph, and total traffic fatalities decreased by 9,100 compared to the previous year. The slower and more uniform speeds due to the 55-mph limit are judged to have saved from 3,000 to 5,000 lives in 1974 (TRB, 1984). In 1995 Congress repealed the NMSL and returned full authority to set speed limits back to the States. Again, increased speed limits produced modest increases in both average and 85th percentile speeds as well as increases in traffic fatalities (TRB, 1998; TRB, 2006). A 2016 study found that each 5 mph increase of State maximum speed limits was associated with an 8% increase in fatality rates on interstates and freeways and a 4% increase on other roads (Farmer, 2017). The study estimated there were 33,000 more traffic fatalities from 1995 to 2013 than would have been expected if State maximum speed limits had not increased since 1993. Another study found that each 1% increase in mileage posted at 70, 75, or 80 mph was, on average, associated with fatality increases of 0.2%, 0.5%, and 0.6%, respectively (Warner et al., 2019). Subsets of fatal crashes involving speeding and driver distraction increased even more.

Two high quality evaluations estimated the crash effects of increased freeway speed limits. A study using 15 years of data estimated the effects of increases in speed limits on sections of Michigan urban freeways from 55 to 70 mph that occurred from 2005 to 2010. Fatal and injury crashes, total crashes, and road departure crashes increased significantly (Kwayu et al., 2020). In a Virginia study, aggregate mean speeds only increased by 1.1 mph in response to the 5 mph increase in freeway speed limits (Himes et al., 2018). At an aggregate level, none of the crash types analyzed, including total, injury, truck, nor run-off-road crashes changed significantly in the analysis of 3 years before and 3 years after the change. However, crashes at interchanges increased significantly (19%).

In Minnesota, raising speed limits from 55 to 60 mph on some two-lane, two-way road sections increased mean speeds by only 1 mph, and 85th percentile speeds remained the same (Saleem et al., 2020). Total crashes still increased significantly by 7%, and injury crashes increased by a not statistically significant 5%.

No U.S. studies were identified where freeway speed limits were decreased, but a study from Korea found that reductions of posted speed limits from 100 to 80 km/h on selected high crash Korean expressways resulted in an estimated 14% reduction in total crashes (Park et al., 2010).

Lower limits in urban areas

Convincing evidence exists that lowering speed limits can reduce average travel speed and crashes in urban areas, even if no or few changes are made to the roadways. However, results achieved may depend on having sufficient enforcement and supporting publicity and communications that reinforce the lower limit and enhance speeding deterrence.

Several North American studies have evaluated the effects of lowering default city-wide speed limits, including from 30 to 25 mph in New York City.  A crash evaluation in New York City estimated total crashes were reduced by an average of 39% on streets that fell within the statutory default speed limit change (unposted streets) compared to streets where the speed limit was unchanged (those with posted limits) (Mammen et al., 2020). Vision Zero-related publicity was used to emphasize the speed limit changes in New York City. An evaluation also found that crashes declined in Toronto, Ontario, in response to lowering city-wide speed limits from 40 to 30 km/h (25 to 19 mph) (Fridman et al., 2020).

Several robust evaluations of the effects of reducing posted speed limits in Edmonton, Alberta, found that speeds and crashes on collector and local streets in residential areas decreased significantly when posted limits were lowered by 10 km/h (~6 mph). These changes in posted limits were supported with extensive publicity and enforcement, but no changes were made to the roadway. These reductions in speed limits were associated with a 50% reduction in fatal and injury crashes, a 26% reduction in total crashes, and an 18% reduction in PDO crashes (Islam & El-Basyouny, 2015).

Other international studies provide additional support that lowering urban limits can reduce speeds and road trauma. Crash-based studies (with comparison groups) from Bristol, U.K., estimated that crashes were reduced in response to lowering city-wide speed limits from 30 to 20 mph (Bornioli et al., 2020). More case studies (for Australia, Denmark, Hong Kong, Norway, and others) are presented in the International Transport Forum (2018). Conversely, when urban speed limits were increased from 50 to 70 km/h (from 31 to 43 mph) or from 70 to 80 km/h (from 43 to 50 mph) on urban road segments in Hong Kong, crashes increased by 20 to 30% (Wong et al., 2005).

Lower rural limits

More limited evidence is available on the effects of lowering rural speed limits. The Swedish Transport Administration carried out a system-wide review of speed limits on the national rural road network to assess whether speed limits were properly adapted to the road classifications and designs to align with Sweden’s Vision Zero principle (Vadeby & Forsman, 2018). This review resulted in speed limit changes on approximately 20,500 km of rural roads (mostly two-lane), with approximately 17,800 km (about 11,060 miles) receiving lower limits, and about 2,700 km (about 1,680 miles) receiving higher limits. A systematic evaluation of these changes found consistent increases in travel speeds on rural roads where limits were raised and decreases where limits were lowered (Vadeby & Forsman, 2018).

A multi-year crash-based evaluation with comparison group by the same researchers also estimated statistically significant crash reductions for some of the road types with lower speed limits. Fatal and serious injury crashes decreased 38% on grade-separated rural 2+1 roads, and 60% on other rural 2+1 roads where the limits were reduced from 110 to 100 km/hour (~68 to 62 mph). Fatalities were reduced by 41% on the most extensive set of rural two-lane highways where limits were reduced from 90 to 80 km/hr (~56 to 50 mph), although the decreasing trend in fatal and serious injury crashes was not statistically significant.

Victoria, Australia, provides an example of a comprehensive approach to managing speed to lower levels. It implemented a combined review and adjustment of speed limits, enhanced covert and overt forms of enforcement, a media campaign, penalty restructuring, and other efforts. An evaluation found these combined elements reduced injury crashes by 10% and fatal crashes by 27% (D’Elia et al., 2007).


Lower speed limits can reduce crashes and casualties on highways and in urban areas, and potentially in rural areas, when lower limits result in reduced speeds. In general, speeds tend to decrease, but to a lower degree than the reduction in limits. Similarly, when limits are raised, the reviewed studies show that speeds tend to increase by a smaller amount than the change in limits, and these changes may reflect speeds that existed prior to the change (Vadeby & Forsman, 2018).

If a lower speed limit yields reduced operating speeds, crashes and injuries are expected to decrease (Elvik et al., 2019). Small changes in average speed can, on average, yield significant changes in crash and injury outcomes, but results can vary, in part related to roadway infrastructure, enforcement levels and other factors. See more discussion on achieving compliance with lower limits below.


Costs associated with changing speed limits by administrative action/engineering study for a given location or locations include new signs, labor for installation, and for media efforts to publicize the new limit.

Time to implement:

Speed limit changes can be implemented quickly, as soon as signage is in place and the new limits are publicized. For lowering speed limits by statute on a class of roads, more time may be required to garner legislative approval, determine implementation parameters, and develop publicity. Conducting an engineering speed limit review and rezoning a particular corridor to a lower speed can be done more quickly.

Other considerations:

  • Vision Zero Speed Limit Resolutions: An increasing number of cities are adopting the objectives of Vision Zero to prevent reckless driving, increase safety for all road users, and mitigate injuries and fatalities. A range of measures can be used to achieve objectives through speed limit reductions, automated enforcement of speeding violations using an expansive network of speed cameras, media campaigns, and engineering measures such as speed humps. New York City is one of the early adopters of the program and enacted a law to implement City-wide speed limits of 25 mph in October 2014 (a decrease from the previous 30 mph) (New York City Mayor’s Office of Operations, 2015). This speed limit reduction potentially halves the fatality risk for a struck pedestrian. Similarly, Seattle lowered its speed limit to 20 mph on residential streets and to 25 mph on urban arterials in 2016 (Seattle Department of Transportation, 2017) and supports these limits with more signs and other engineering and enforcement strategies (Health Resources in Action, 2013).
  • Safe Systems: A systems approach to road safety encompasses understanding and recognizing how humans interact with a complex environment such as a road network (and other road users) and how the design of the system may contribute to crashes and injuries (Dumbaugh et al., 2020).
  • Obtaining Speed Compliance: Lowering speed limits can reduce average driving speeds, but it may be difficult to obtain broad compliance with lower speed limits on roadways designed for much higher speeds (TRB, 1998). As discussed in the Understanding the Problem section, both perceived level of enforcement and perceptions of speed limits can affect speed limit compliance. Gayah et al. (2018) found that the verified presence of heavy police enforcement reduced mean and 85th percentile speeds and helped increase speed limit compliance on rural highways in Montana where limits were set below 85th percentile speeds. However, setting speed limits much higher or much lower than current operating speeds may not achieve desired levels of compliance without other changes to the roadway or to enforcement (Gayah et al., 2018; Lee et al., 2017). Reviews suggest that it can be difficult to implement or sustain enhanced levels of enforcement to keep speeds close to the limit. In general, higher speed limits are very likely to lead to higher average speeds over time if nothing is done to the roadway (Hauer, 2009). These and other similar findings mean that the roadway environment should be considered and changed, if needed, along with lowering speed limits when the speed “messaged” by the road is higher than intended for safety. High visibility and speed safety camera enforcement might be used in the short term until such changes can be made.
  • Road designs: Besides speed limits and enforcement, road design factors such as more traffic lanes, presence of wide shoulders, long sight distance and more “open” land uses may encourage drivers to go faster, whereas on-street parking, sidewalks and pedestrian activity, and downtown land uses encourage drivers to slow down (Gargoum et al., 2016; Ivan et al., 2009; Lee et al., 2017). NCHRP Report 880, Design Guide for Low-Speed Multimodal Roadways provides information on road designs that may help DOT partners achieve lower speeds to support lower limits (Elizer et al., 2018; more design resources are mentioned in the Key Resources section). Safety offices and their partners can work with roadway planners, designers, and managers to reduce opportunities for speeding in the longer term.
  • Work Zone speed limits: If drivers perceive that work zone limits are too low, workers are not present, and other changes to the roadway do not seem to justify the lower limits, they may not comply and extensive enforcement may be needed to enforce the limit (Ullman et al., 2013). Sharma et al. (2017) collected data from nine construction work zones in Iowa during 2014 and 2015. The study found consistent speed reductions associated with work zone speed limits, compared to data collected from the time period when work zones were not in place at the same locations. Indiana DOT funded a project to develop guidance for police enforcement of work zone speed limits. Detailed deployment strategies to obtain the most effective compliance in several types of work zones are described in the report (Chen & Tarko, 2012).