Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis

BMJ 2009; 339 doi: http://dx.doi.org/10.1136/bmj.b4469 (Published 11 December 2009)
Cite this as: BMJ 2009;339:b4469

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13 December 2013

The use of speed bumps is widespread around the world. They are commonly used in residential areas where speeds are excessive, accidents have occurred, high pedestrian traffic or in areas with many children.. The height and length of a bump affects the amount of impact. UK regulations specify the height of bumps (1). However, the BBC (2) measured bumps in Islington and found bumps were up to 5 cm too high. According to a UK Government Report (3) bumps only work when they are uncomfortable.

Bumps can cause both acute and chronic injuries. A cyclist rode out of a workplace, turned left and struck a new bump at an angle. She fell off and was killed by a passing motor vehicle (4). Aslan et al (5) reported on five cases of spinal injuries in inner city buses due to bumps. In another case an ambulance crossed a bump causing a large impact, resulting in a complete spinal cord injury (4). Many people who suffer from back problems, recent abdominal surgery or other disabilities find them extremely painful. Problems exist for those with osteoporosis, post back surgery, kidney transplant recipient on peritoneal dialysis, dislocated jaw, brain cyst and spinal deformity. Wheelchair users frequently complain about discomfort (6).

Emergency vehicles are especially vulnerable to the impact of bumps. Apart from the major discomfort to ambulance passengers, they also substantially delay response times. This can be as much as 10s per bump. In a US study, (7) it was calculated that more deaths would arise from delayed arrival of ambulances than could ever be saved by any possible accident reduction. In Boulder, Colorado, an additional 6.6 cardiac arrest deaths are estimated per year due to bumps. The chairman of the London Ambulance Service claimed that delays due to speed bumps was responsible for up to 500 avoidable deaths from cardiac arrest each year in London (8).

Other traffic calming measures are available and bumps are not needed on 20 mph roads (9). Authorities should carefully consider alternative strategies.

1. Specifying the height of bumps, UK Regulations, 1999.
2. Speed bumps and the Islington Council, BBC Documentary, 29 October 2008.
3. Speed humps only work when they are uncomfortable. UK Transport Research Laboratory Report 417, 2006.
4. Speed bump injuries, The Daily Telegraph, 2006.
5. Aslan S,Karkloglu O,Katirci Y, Handis H, Ezermik N, Bilir O ,Speed bump- induced spinal cord injury, Am J Emerg Med, 23(4), 563-4, 2005
6. BBRAG policy on speed humps, Bromley Borough Roads Action Group London, UK, 2011.
7. Deaths Expected from Delayed Emergency Response Due to Neighborhood Traffic Mitigation, Submitted to the City Council of Boulder, Colorado, 3 April 1997.
8. Delay due to speed bumps is responsible for 500 SCA deaths each year in London, Transport Committee Minutes, London Assembly, Siguard Reinton, 2003.
9. Grundy C, Steinbach R, Edwards P, Green J, Armstrong B, Wilkinson P. Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis, British Medical Journal, BMJ 2009;339:b4469 10 December 2009.

Competing interests: None declared

Malcolm H Pope, Professor of Environmental and Occupational Medicine

Professor Daniel Chow (Interdisciplinary Division of Biomedical Engineering (BME), The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR China)

Liberty Safework Centre, Foresterhill, Aberdeen, UK

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Grundy et al.(1) recently reported that the introduction of 20 mph (32 km an hour) traffic speed zones in London during 1986-2006 was associated with a 41.9% (95% CI: 36.0% to 47.8%) reduction in road casualties, after adjusting for the underlying downward trend in traffic casualties. The reduction was greater for children (48.5%) and for deaths and serious injuries than for minor injuries; and there was no increase in casualties in adjacent areas. The authors estimated that 20 mph zones would prevent 200 casualties and 27 deaths a year, and recommended introducing 20 mph zones in major cities in Britain and elsewhere. Traffic deaths are projected to be the 5th leading cause of death worldwide by 2020, and excessive speed is considered the single most important contributor to road fatalities.(2)

Although a 20 mph speed limit will reduce casualties on roads with a mix of motor vehicles, bicycles and pedestrians, the use of speed limits can be questioned as an effective long-term strategy for preventing motor vehicle crash (MVC) casualties. Aside from the drawback of increased travel time, severe injury and death can occur even at low speeds. As Ralph Nader pointed out in 1965 his aptly titled book, cars are “unsafe at any speed”.

Crash fatality rates (per vehicle registered and distance travelled per vehicle) actually declined by 94% between 1921 and 2002,(3) despite increasing speeds and the fact that motor vehicles have been capable of speeds in excess of 100 mph for most of that time. Although many factors – environmental, behavioral, and medical – have contributed to declining MVC death rates, foremost among them are technological changes that have improved the safety (“crashworthiness”) of automobiles and highways;(4) indeed, it is well known that safety measures that require no human action and protect people automatically (“passive prevention”) are much more effective than those that do (“active prevention”).

These points suggest that the long-term solution to MVC deaths and injuries is to develop technology that not only enhances the crashworthiness of vehicles and reduces the severity of MVC injury but, more importantly, prevents crashes and injuries altogether. This will be achieved by completely automating vehicles and road traffic and thereby eliminating the human driver from the equation of traffic safety; that is, once the vehicle has been programmed to reach a certain destination. Vehicle speeds that are currently unimaginable will then be routinely possible on major highways, with minimal risk of crashes and of death or injury to occupants or pedestrians.

The central problem in traffic safety is not vehicle speed but the human beings who operate the vehicles. Safer cars are being driven on safer highways, but by humans who depend on each other to be alert, sober, and to make sound and rapid decisions but who may be sleepy, distracted, inebriated, dazzled by oncoming headlights, and/or incapable of making sound and rapid decisions. Speed limits are needed on today’s roads not because road traffic safety necessarily requires it but because human beings are unreliable as drivers at any given speed. Technological changes to vehicles that reduce and eventually eliminate human beings as drivers offer greater opportunities for reducing motor vehicle crashes and fatalities than can be achieved by reducing legal speed limits. Automatic braking systems that are already in development could be a precursor for cars that will automatically convey their occupants from departure to arrival at programmed destinations. Robotic vehicles are also in early production, indicating that self-operated cars will be widely available within a few years.(5)


1. Grundy C, Steinbach R, Edward. Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrupted time series analysis. BMJ 2009; Dec 10;339:b4469.

2. Global Road Safety Partnership. Speed management: a road safety manual for decision-makers and practitioners. 2008 www.who.int/roadsafety/projects/manuals/speed_manual/en/.

3. Evans L. Traffic Safety. Bloomfield Hills, MI: Science Serving Society, 2004.

4. Bolen JR, Sleet DA, Chorba T, et al. Overview of efforts to prevent motor vehicle-related injury. In: Prevention of motor vehicle- related injuries: a compendium of articles from the Morbidity and Mortality Weekly Report, 1985-1996. Atlanta, Georgia: US Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Injury Prevention and Control, 1997.

5. Ashley S. Crashless cars: making driving safer. Sci Am 2008; 299(6):86-94.

Competing interests: None declared

Competing interests: None declared

Anthony R Mawson, Professor

Department of Pediatrics, University of Mississippi Medical Center, Jackson, MS 39216, USA

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Grundy and colleagues discussed an important issue in their article on the effect of further speed limitation within urban areas in London [1]. This legislation has obviously resulted in a significant reduction in road casualties.

As mentioned in the accompanying editorial [2], changing the behaviour of individuals so that they drive more safely is a complex process. Television advertising campaigns are thought to have an effect on road safety [3], but perhaps do not remain at the forefront of our thoughts when driving.

Whilst on a recent trip to Ecuador we came across a novel public health campaign that addresses this problem. This government sponsored campaign, known as the Corazones Azules (blue hearts), involves painting a blue heart on the road at accident hotspots and where fatalities have occurred. These images are intended to remind drivers of the potentially fatal consequences of driving dangerously. The campaign is popular amongst residents and has also been implemented in other countries, such as Panama.

[1]Grundy C, Steinbach R, Edwards P, Green J, Armstrong B, Wilkinson P. Effect of 20mph traffic speed zones on road injuries in London, 1986- 2006: controlled interrupted time series analysis BMJ 2009;339:b4469

[2]Ameratunga S. Traffic speed zones and road injuries. BMJ 2009;339:b4743

[3]Powles JW, Gifford S. Health of nations: lessons from Victoria, Australia. BMJ 1993;306(6870):125–127

Competing interests: None declared

Competing interests: None declared

Paul G. Rainsbury, CT2 ENT

Karolina Z. Jankowska

Plymouth Hospitals NHS Trust, PL6 8DH

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The evidence provided by Grundy et al that a 20mph speed limit reduces the severity of injuries resulting from road traffic accidents is welcome when considering the impact of avoidable injuries and deaths . The measures discussed in this paper suggest reduced casualty counts of approximately 200 per year, which would result in substantial cost savings to the NHS, Criminal Justice system and social support. This highlights the need, in the current financial climate, to review the activity of our public services to identify areas where the application of appropriate primary prevention could prevent the ‘downstream’ costs of ‘upstream’ activity. For example, road traffic accidents, both for pedestrians and non-pedestrians, are partially attributable acute consequences of alcohol consumption . A trip into most local town or city centres at the week-end will depict a considerable number of individuals showing acute signs of alcohol intoxication. This is despite the provision in Law (s141 of the Licencing Act 2003) 6 prohibiting the sale of alcohol to those already intoxicated. In January 2009 Jacqui Smith (former Home Secretary) gave a written answer to the Dominic Grieve that only 7 prosecutions were brought under this act in 2007 with only 1 guilty verdict (reference). There are other potential areas where preventative measures will deliver significant cost savings. An example is that in 2005-6 only 9% of women used long acting forms of contraception . There are a wide range of reasons for this, and one is undoubtedly a lack of practitioners with sufficient experience and expertise in using Intra-uterine contraceptive devices (IUCD) and implants. This does not support the recent ‘worth talking about’ campaign that promotes the use to these contraceptive methods failing at the point of access . Given that the NHS has to find £15 - 20 billion in savings by 2014 7 ,it seems appropriate for public services to look collectively at what ‘quick wins’ it could make upstream to prevent the downstream costs (financial and social). It really doesn’t make sense to be spending on things that with some coordinated action could be avoided when other much needed services are threatened. 1 Grundy C, Steinbach R, Edwards P, Green J, Armstrong B, Wilkinson P. Effect of 20 mph traffic speed zones on road injuries in London, 1986- 2006: controlled interrupted time series analysis. BMJ 2009;339:b4469 Jones L, Bellis M A, Dedman D, Sumnall H, Tocque K. Alcohol-attributable fractions for England: alcohol attributable mortality and hospital admissions. North West Public Health Observatory June 2008 http://www.parliament.the-stationery-office.co.uk/pa/cm200809/cmhansrd/c... [accessed 21st December 2009] Taylor T, Keyse L, Bryant A (2006) Omnibus Survey Report No. 30: Contraception and Sexual Health, 2005/06. Office of National Statistics. http://www.nhs.uk/worthtalkingabout/Pages/sex-worth-talking-about.aspx [accessed 21st December 2009] 6 http://www.opsi.gov.uk/acts/acts2003/ukpga_20030017_en_10#pt7-pb2-l1g141 [accessed 21st December 2009]

Competing interests: None declared

Competing interests: None declared

Richard Puleston, Associate Professor of Health Protection

Glenda Augustine

University of Nottingham, NN6 7ET

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18 December 2009

It is predicted that pedestrian road deaths will increase by 65% in the next 10 years. (1) Currently, global initiatives promoting active lifestyles encourage more people to walk and cycle. This will increase the number of pedestrians and cyclists on roads, corresponding to increased human exposure to vehicle danger. Without intervention, this may result in increased incidence of pedestrian and cyclist fatalities and injuries, thereby countering the positive health effects of the active lifestyle initiatives.

Reducing vehicle speed has been proven to be effective in preventing crashes and reducing the severity of injuries.(2) At a speed of 20km/hr, vehicles and pedestrians are able to co-exist with relative safety, which means that drivers have sufficient time to stop for pedestrians, and pedestrians can make better crossing decisions.

Obesity prevention policies that should seek to alter the environment in order to foster increased levels of physical activity. Education, clinical, counseling interventions and other strategies alone are unlikely to be effective or sustainable to counteract the obesity epidemic, and there is widespread agreement that major changes to the current built environment that favor car travelling over walking. (3) Regular physical activity is a behavior that protects against obesity, but it is becoming difficult to adopt and maintain this behavior in the current hostile environment. (4) Virtually every major advance in public health has involved the reduction or the elimination of risk. (5) There are compelling reasons for policy makers to play a greater role in tackling and reducing these major risks. The ultimate goal is to develop government policies to create safe environments by eliminating vehicle danger by reducing vehicle speed, such that people can walk, cycle and carry out other leisure activities without compromising their safety. (6) The ideal speed reduction enables pedestrians to use the streets as a social space, meaning that children can play outside, neighbors can socialize and the local communities can take control of their own environments.


1. Peden M, Scurfield R, Sleet D. (2004). World Report on Road Traffic Injury Prevention. Geneva, Switzerland: World Health Organization.

2. Grundy C. Steinbach R. Edwards P. Green J.Armstrong A. Wilkinson P. Effect of 20 mph traffic speed zones on road injuries in London, 1986- 2006: controlled interrupted time series analysis. BMJ 2009 339: b4469

3. Swinburn BA, Egger G, Raza F. Dissecting obesogenic environments: the development and application of a framework for identifying and prioritizing environmental interventions for obesity. Prev Med. 1999; 29(6 Pt 1), 563-570.

4. French SA, Story M, Jeffery RW. Environmental influences on eating and physical activity. Ann Rev of Pub Heal. 2001; 22, 309-335.

5. World Health Organization (2005): Reducing Risks, Promoting Healthy Life. Geneva, Switzerland: World Health Organization.

6. Desapriya E, Pike I, Basic A, Subzwari S. Deterrent to healthy lifestyles in our communities. Pediatrics. 2007; 119(5):1040-2.

Competing interests: None declared

Competing interests: None declared

Ediriweera Desapriya, Research Associate

Department of Pediatrics, Faculty of Medicine, University of British Columbia Canada V6H 3V4

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It is pleasing that the effectiveness of traffic-calming has been established (1). However, what remains depressing is the motorist's attitude to speed that obligates such interventions. The problem has been further evinced in outrageous responses to speed-cameras (2).

The evidence suggests a bizarre paradox regarding speed perception. On the one hand, we know that the direct perception of one's own speed in a vehicle is poor. This is evinced in the phenomenon of vection, as occurs for example when one is stationary in a vehicle and views an adjoining moving vehicle: one perceives oneself to be moving opposite to the adjoining vehicle (3). Another example is the serious underestimation of speed after fast driving (4). The problem of misperceived speed has been exacerbated by the steadily improving ride and quietness of modern vehicles: vestibular and auditory cues are now less available to supplement unreliable visual information. How ironic that traffic-calming reintroduces vestibular information to counteract excessive speed!

On the other hand, each vehicle has a speedometer! Yet it has long been known that speedometers are often ignored: despite the evidence of misperceived speed, drivers seemingly think their own direct estimates are more accurate (5). When we add in official failure to inhibit the speed of modern vehicles - 200 mph is now available (6) - it is clear that the problem of excessive speed remains substantial.

Traffic-calming and speed-cameras, along with competently-designed cycle-lanes, have roles in developing a safe roadway environment for vulnerable modes of travel. However, we need a radical re-education of motorists: the opportunity may come if a 1970s style fuel crisis emerges and motorists are forced to eke out limited supplies of expensive fuel.


1. Grundy C, Steinbach R, Edwards P, Green J, Armstrong B, Wilkinson P. Effect of 20 mph traffic speed zones on road injuries in London, 1986- 2006: controlled interrupted time series analysis. BMJ 2009;339:b4469

2. Reinhardt-Rutland A H. Roadside speed-cameras: arguments for covert siting. Police J 2001;74:312-315

3. Howard I. Human visual orientation. 1982; New York:Wiley

4. Denton G G. The influence of visual pattern on perceived speed. Perception 1980;9:393-402

5. Denton, G G. the use made of the speedometer as an aid to driving. Ergonomics 1969;12:447-454

6. Cheetham, C. The encyclopedia of classics cars from 1890 to the present day. 2007;London:Amber.

Competing interests: None declared

Competing interests: None declared

Tony H Reinhardt-Rutland, Reader in Psychology

University of Ulster BT52 1SA

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