Traffic calming
Traffic calming measures such as speed humps and tables (raised traffic calming devices, for use on lower speed roads), speed bumps (tall, narrow strips, typically in parking lots), pedestrian center crossing islands (also known as pedestrian refuge islands), and roundabouts strategically modify the built environment to affect traffic speed and patterns1. Traffic calming measures can be implemented independently or as a component of larger efforts to improve streetscape design2. Traffic calming measures are used in urban, suburban, and rural areas3. In rural communities, such efforts often reduce speeds as drivers transition from high-speed roads into lower-speed zones4.
What could this strategy improve?
Expected Benefits
Our evidence rating is based on the likelihood of achieving these outcomes:
Reduced traffic speed
Reduced crashes
Potential Benefits
Our evidence rating is not based on these outcomes, but these benefits may also be possible:
Increased pedestrian and cyclist safety
Increased active transportation
What does the research say about effectiveness?
There is strong evidence that traffic calming measures reduce traffic speed5, 6, 7, 8 and reduce crashes6, 9, 10, 11, 12. Traffic calming measures can also increase pedestrian and cyclist safety13, 14, 15 and increase bicycling and walking16, 17. Additional evidence is needed to determine which measures are most effective.
Traffic calming measures such as speed humps, single-lane roundabouts, and reduced speed limit zones reduce traffic speed5, 18. Speed humps are particularly effective ways to reduce pedestrian-vehicle crashes that involve children6. Area-wide traffic calming measures have been shown to reduce traffic injuries, as well as the frequency and severity of crashes5, 7.
Roundabouts reduce both fatal and non-fatal crashes9, 10, 11, 12 and improve road safety9. Intersections converted to single-lane roundabouts have been shown to reduce crashes with injuries, with the greatest reductions occurring at intersections converted to double-teardrop roundabouts10. A long-term study in Washington state found single-lane roundabouts reduced speed-related crashes and double-lane roundabouts reduced crashes, right-of-way crashes, and crashes with injuries19. Available evidence suggests crashes decline over time as drivers gain experience navigating more complex roundabouts19. An Arizona-based study suggests double-lane roundabouts can reduce the severity of crashes at a converted intersection20.
Traffic calming measures implemented with broader streetscape design efforts have been shown to reduce crashes8, 21. For example, pedestrian center crossing islands, sidewalks, crosswalks, advanced yield signs and markings, exclusive pedestrian signal phasing8, 21, 22, and increased lighting8, 22, can reduce the risk of pedestrian-vehicle crashes. Adding pedestrian center crossing islands or pedestrian plazas to intersections reduce pedestrian collision rates; even greater reductions occur when implemented with lane removals or narrowed lanes15. In Washington, D.C., one study found that hardened centerlines that extend into an intersection can increase pedestrian safety by reducing traffic speed and forcing 90 degree left turns, so drivers are more likely to see pedestrians in crosswalks14. Midblock crosswalks enhanced with rectangular rapid flash beacons may increase the likelihood that drivers yield to pedestrians23. Red light cameras and speed cameras have been shown to reduce casualty crashes18, 24. Individual traffic calming measures such as speed humps may be added as the result of partnerships between local organizations such as schools, hospitals, or community centers to improve safety at a specific location, often where children walk or play25.
Single-lane roundabouts can improve traffic performance (e.g., reduce intersection delays, queue lengths, and idling), which may reduce fuel consumption and greenhouse gas emissions11. In New Hampshire, a city located in a valley with limited air circulation replaced intersections with roundabouts and experienced reduced traffic delays, idling, and emissions, as well as local air quality improvements26.
Several design modifications can increase the effectiveness of roundabouts, such as clear right of way signs, elongated splitter islands, and lane markers, particularly for double-lane roundabouts. Experts also suggest using the road’s curvature and a built-up center island, preferably with landscaping, to limit views of the full roundabout and force vehicles to slow down19. Early notifications of approaching roundabouts can improve safety when roundabouts are used to transition vehicles from high speed roads to lower speed roads27. Double-teardrop roundabouts are especially recommended for highway exits and interchanges10. Traffic conditions (i.e., anticipated volume, speed, etc.) and the rate and severity of past accidents should be considered when deciding how many lanes to include in a new roundabout20. International studies and experts suggest that additional safety features are needed to protect cyclists using roundabouts. Rather than adding bike lanes to roundabouts, experts recommend creating separate bicycle paths nearby, especially near roundabouts that transition a great number of vehicles from high to lower speeds and include multiple lanes28.
How could this strategy advance health equity? This strategy is rated inconclusive impact on disparities.
It is unclear what impact traffic calming measures may have on disparities in safe and active travel between communities of different racial or ethnic backgrounds, with different income levels, or communities in urban or rural areas. Traffic calming effects vary substantially based on implementation and local community needs33. People of color, those with low incomes, and those who are uninsured are more likely to suffer pedestrian injuries and fatalities because they are all more likely to live in communities that are the least safe for walking34. Potential traffic calming programs should consider current issues with traffic flow and unintended traffic shifts that calming measures may create and should gather input from impacted communities to avoid exacerbating disparities between communities experiencing heavy traffic and unsafe speeds, especially along pedestrian routes to schools33. Case studies of smaller towns, as in Carlisle Borough (population of 20,000) in Cumberland County, Pennsylvania, suggest that stakeholder coalitions can support implementation of traffic calming measures that advance multiple community goals, including enhanced pedestrian and bicyclist safety, reduced vehicle accidents, and improved access to downtown businesses, employment, and recreation areas33.
To the extent that traffic calming measures manage to reduce idling and heavy traffic in neighborhoods suffering from concentrated poverty, they have the potential to reduce disparities in exposure to air pollution. However, traffic calming measures that divert additional traffic into neighborhoods with concentrated poverty have the potential to increase disparities in exposure to air pollution. People of color and people with lower incomes are exposed to higher levels of air pollution than white people and people with higher incomes. This higher exposure is associated with people of color and people with low incomes experiencing poorer health outcomes than white people and people with higher incomes35.
What is the relevant historical background?
The automobile-centric transportation network within the U.S. is based on more than half a century of building highways and expanding roads, rather than prioritizing access to public transportation systems and opportunities for active transit, as in other developed nations26. The growth and sprawl of cities in the U.S. was largely driven by discriminatory housing policies and car-centric urban planning which often prevents provision of equitable, consistent, and reliable public transit service36. The creation of the modern U.S. interstate highway system fed into a dependence on private vehicles, leading to low density suburban development, urban sprawl, and further decline in urban transit systems. Highways bypassed rural towns and at the same time cut through and destroyed low income and Black neighborhoods in cities37, while car-centric urban planning reduced access to public transportation options, increased residential segregation, and further embedded health disparities38. As a result, U.S. residents are more likely to experience traffic collisions or fatalities, waste time in traffic congestion, and have their health negatively impacted by vehicle emissions, than those in other developed nations26.
Traffic calming measures originated in the 1960s in the Netherlands as a way to increase safety for pedestrians, bicyclists, and drivers33 and in Great Britain with the creation of modern roundabouts to promote the safe, efficient flow of traffic39. The term traffic calming originated in Germany in the late 1970s, with practices soon spreading across Europe as well as to Canada and Australia33. Around 1980, roundabouts became common practice throughout Europe and Australia9. The first roundabout in the U.S. was constructed in 1990 in Nevada39, though roundabouts would not be more widely applied to intersections in the U.S. until about 20009.
Equity Considerations
- Which areas in your community experience a high number of traffic-related crashes? Who are the local stakeholders that can help identify opportunities for traffic calming measures in your community?
- How have historic zoning patterns and uses, community layout, and other built environment factors contributed to local traffic safety challenges?
- Can traffic calming measures be used to support efforts to increase active transit (i.e., walking, biking, public transit) and reduce dependence on cars in your community? How does this fit with existing land uses and pedestrian-oriented features?
Implementation Examples
Traffic calming programs are in place in urban, suburban, and rural areas around the country2. Carmel, Indiana has been replacing signalized intersections with roundabouts since the late 1990s; as of 2023, the city has approximately 140 roundabouts29.
As of February 2023, Alaska has 51 roundabouts; the Alaskan Department of Transportation and Public Facilities (Alaska DOT&PF) follows a “Roundabout First” policy that requires written justification for any traffic signal installation rather than a single-lane roundabout. The Alaska DOT&PF also offers guidance and safety tips for using roundabouts30.
City governments often conduct traffic studies of specific blocks or neighborhoods to determine which traffic calming measures may be most appropriate to improve safety. Philadelphia, Pennsylvania and Greenville, South Carolina both have formal processes through which residents can request traffic calming and safety studies31, 32.
Implementation Resources
‡ Resources with a focus on equity.
US DOT-Traffic calming - U.S. Department of Transportation (U.S. DOT), Federal Highway Administration (FHWA). Safety: Traffic calming ePrimer.
IIHS-HLDI-Roundabouts - Insurance Institute for Highway Safety (IIHS), Highway Loss Data Institute (HLDI). Roundabouts.
WeConservePA-Traffic calming - WeConservePA. Conservationtools.org. Traffic calming.
WSDOT-Roundabout - Washington State Department of Transportation (WSDOT). Traffic safety methods: Roundabouts. Learn about roundabouts, including how to travel through them as a pedestrian, cyclist or driver.
Footnotes
* Journal subscription may be required for access.
1 US DOT-PedSafe - U.S. Department of Transportation (U.S. DOT), Federal Highway Administration (FHWA). PedSafe. Pedestrian safety guide and countermeasure selection system: Countermeasures.
2 US DOT-Traffic calming - U.S. Department of Transportation (U.S. DOT), Federal Highway Administration (FHWA). Safety: Traffic calming ePrimer.
3 Gulden 2016 - Gulden J, De La Garza J. Traffic calming. In: Pande A, Wolshon B, eds. Traffic Engineering Handbook. 7th ed. Hoboken, NJ: John Wiley & Sons, Inc; 2016:501-540.
4 Hallmark 2013 - Hallmark SL, Knickerbocker S, Hawkins NR. Center island with raised curbing for rural traffic calming. Tech Transfer Summaries. Ames, IA: Center for Transportation Research and Education, Iowa State University; 2013.
5 Cochrane-Bunn 2003 - Bunn F, Collier T, Frost C, et al. Area-wide traffic calming for preventing traffic related injuries. Cochrane Database of Systematic Reviews. 2003;(1):CD003110.
6 Rothman 2015 - Rothman L, Macpherson A, Buliung R, et al. Installation of speed humps and pedestrian-motor vehicle collisions in Toronto, Canada: A quasi-experimental study. BMC Public Health. 2015;15(1):774.
7 Morrison 2003 - Morrison DS, Petticrew M, Thomson H. What are the most effective ways of improving population health through transport interventions? Evidence from systematic reviews. Journal of Epidemiology and Community Health. 2003;57(5):327-333.
8 Retting 2003 - Retting RA, Ferguson SA, McCartt AT. A review of evidence-based traffic engineering measures designed to reduce pedestrian-motor vehicle crashes. American Journal of Public Health. 2003;93(9):1456-1463.
9 Elvik 2017 - Elvik R. Road safety effects of roundabouts: A meta-analysis. Accident Analysis & Prevention. 2017;99:364-371.
10 Wang 2022b - Wang J, Cicchino JB. Safety effects of roundabout conversions in Carmel, Indiana, the Roundabout City. Journal of Safety Research. 2022;82:159-165.
11 Hu 2014 - Hu W, McCartt AT, Jermakian JS, Mandavilli S. Public opinion, traffic performance, the environment, and safety after construction of double-lane roundabouts. Transportation Research Record: Journal of the Transportation Research Board. 2014;2402:47-55.
12 Qin 2013 - Qin X, Bill A, Chitturi M, Noyce DA. Evaluation of roundabout safety. Transportation Research Board 92nd Annual Meeting. 2013.
13 CG-Physical activity - The Guide to Community Preventive Services (The Community Guide). Physical activity.
14 Hu 2020 - Hu W, Cicchino JB. The effects of left-turn traffic-calming treatments on conflicts and speeds in Washington, D.C. Journal of Safety Research. 2020;75:233-240.
15 Kang 2019a - Kang B. Identifying street design elements associated with vehicle-to-pedestrian collision reduction at intersections in New York City. Accident Analysis and Prevention. 2019;122:308-317.
16 Winters 2010 - Winters M, Brauer M, Setton EM, Teschke K. Built environment influences on healthy transportation choices: Bicycling versus driving. Journal of Urban Health. 2010;87(6):969-993.
17 Morrison 2004 - Morrison DS, Thomson H, Petticrew M. Evaluation of the health effects of a neighbourhood traffic calming scheme. Journal of Epidemiology and Community Health. 2004;58(10):837-840.
18 Mountain 2005 - Mountain LJ, Hirst WM, Maher MJ. Are speed enforcement cameras more effective than other speed management measures? The impact of speed management schemes on 30 mph roads. Accident Analysis & Prevention. 2005;37(4):742-754.
19 Hu 2019 - Hu W, Cicchino JB. Long-term crash trends at single- and double-lane roundabouts in Washington State. Journal of Safety Research. 2019;70:207-212.
20 Mamlouk 2019 - Mamlouk M, Souliman B. Effect of traffic roundabouts on accident rate and severity in Arizona. Journal of Transportation Safety & Security. 2019;11(4):430-442.
21 Zegeer 2017 - Zegeer C, Lyon C, Srinivasan R, et al. Development of crash modification factors for uncontrolled pedestrian crossing treatments. Transportation Research Record: Journal of the Transportation Research Board. 2017;2636:1-8.
22 Cochrane-Beyer 2009 - Beyer FR, Ker K. Street lighting for preventing road traffic injuries. Cochrane Database of Systematic Reviews. 2009;(1):CD004728.
23 Foster 2014a - Foster N, Monsere CM, Carlos K. Evaluating driver and pedestrian behaviors at enhanced, multilane, midblock pedestrian crossings. Transportation Research Record: Journal of the Transportation Research Board. 2014;2464:59-66.
24 Cochrane-Aeron-Thomas 2005 - Aeron-Thomas A, Hess S. Red-light cameras for the prevention of road traffic crashes. Cochrane Database of Systematic Reviews. 2005;(2):CD003862.
25 Arbogast 2018 - Arbogast H, Patao M, Demeter N, et al. The effectiveness of installing a speed hump in reducing motor vehicle accidents involving pedestrians under the age of 21. Journal of Transport and Health. 2018;8:30-34.
26 Garceau 2018 - Garceau TJ. Impacts of roundabouts on urban air quality: A case study of Keene, New Hampshire, USA. Journal of Transport and Health. 2018;10:144-155.
27 Isebrands 2021 - Isebrands H, Johnson MT, Van Parys L. Rural roundabouts save lives. Institute of Transportation Engineers ITE Journal. 2021;91(3):38-42.
28 Poudel 2021 - Poudel N, Singleton PA. Bicycle safety at roundabouts: A systematic literature review. Transport Reviews. 2021;41(5):617-642.
29 Carmel-Roundabouts - Carmel, Indiana. Roundabouts: Carmel is internationally known for its roundabout network.
30 Alaska DOT-PF-Roundabouts - Alaska Department of Transportation and Public Facilities (Alaska DOT-PF). Roundabouts in Alaska: Faster, safer, smarter.
31 Philadelphia-Traffic calming - City of Philadelphia, Pennsylvania. Services. Streets, sidewalks & alleys: Request a traffic calming and safety study.
32 Greenville-Traffic calming - Greenville, South Carolina. Traffic Engineering: Traffic calming.
33 WeConservePA-Traffic calming - WeConservePA. Conservationtools.org. Traffic calming.
34 Dodds 2017 - Dodds A. For the greater good: A Complete Streets approach can benefit low-income communities. Roads & Bridges. 2017.
35 Demetillo 2021 - Demetillo MAG, Harkins C, McDonald BC, et al. Space-based observational constraints on NO2 air pollution inequality from diesel traffic in major U.S. cities. Geophysical Research Letters. 2021;48(17).
36 NASEM-Thomas 2022 - Thomas DN, Heer N, Wyatt Mitchell I, et al. Racial equity, Black America, and public transportation, volume 1: A review of economic, health, and social impacts. Washington, D.C.: National Academies Press; 2022.
37 Tehrani 2019 - Tehrani SO, Wu SJ, Roberts JD. The color of health: Residential segregation, light rail transit developments, and gentrification in the United States. International Journal of Environmental Research and Public Health. 2019;16(19):3683.
38 McAndrews 2022 - McAndrews C, Schneider RJ, Yang Y, et al. Toward a gender-inclusive Complete Streets movement. Journal of Planning Literature. 2022;38(1):3-18.
39 IIHS-HLDI-Roundabouts - Insurance Institute for Highway Safety (IIHS), Highway Loss Data Institute (HLDI). Roundabouts.
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