Reducing speed limits from 30mph to 20mph typically results in more than 20% fewer casualties

With post implementation results from more and more authorities that have already adopted wide-area 20mph limits, there is clear evidence of the benefits in casualty reduction.

  1. Why Reducing Vehicle Speeds in Built-Up Areas to 20mph Matters.

Speed reduction is significant to casualty levels because:

If average speeds reduced by 1 mph, the accident rate would fall by approximately 6% on urban main roads and residential roads with low average speeds”.[1]

  • Where speeds are reduced to 20mph through traffic calming, casualties fall by around two-fifths. The definitive study of the impact of these is the TfL commissioned study by the London School of Hygiene and Tropical Medicine in 2009 entitled 20 mph Zones and Road Safety in London[2].
  • The study looked at casualties in a total of 399 zones which had been implemented in London between 1990/91 and 2007/08. In summary this study found that “The time series regression analysis estimated a 42% reduction (95% CI 36%, 48%) in all casualties within 20 mph zones compared with outside areas, adjusting for an annual background decline in casualties of 1.7% on all roads in London”.
  1. The Impact of 20mph Speed Limits (alone).

2.1. Historic Research (pre-2018)

  • Brighton & Hove. The Council’s research found a reduction of 12% in all casualties (and 20% in the number of those killed and seriously injured) between the annual average of the preceding 3 years and the first year of operation of the 20mph limit[3].
  • In 2007 Newcastle introduced eight 20 mph speed limit areas for a trial period to gauge the effects of ‘sign-only’ schemes on residential roads. The number of car-related accidents on Newcastle's residential streets fell by more than half in some areas of the city following the council's introduction of 20mph speed limits.[4] The overall number of accidents reduced by between 24% and 56% in those streets where 20mph speed limits had been introduced.
  • Results indicate a reduction of 38% in annual road traffic collision rates (overall) and by level of severity on 20mph and 30mph streets post speed limit introduction (NB a background of falling casualties across Scotland)[5].
  • Portsmouth[6]. The scheme was implemented in 2007-08, and the final report on the scheme of September 2010 had two years worth of road traffic collision data to compare with the ‘before’ data to form a meaningful comparison. In the three years before the implementation of the scheme, there was an average of 183 casualties in road traffic collisions (rtcs) per year. In the two years following implementation, there was an average of 142.4 casualties per year; this is a decrease of 22%. Similar results from the DfT for the same time period show an underlying trend of 14% decrease in road traffic collisions; implying that the implementation of 20 mph limits have lowered road traffic collisions by a further 8% than would have otherwise occurred. More recent research released by Portsmouth City Council has shown that in the period since 2011 there was a 31% reduction of collisions in 20mph roads compared to a 10.5% reduction in 30mph roads and an 11% reduction for all roads.
  • Warrington[7]. In February 2009 Warrington established three pilot 20 mph speed limit areas (140 roads in total) for an experimental eighteen-month period. There were 40 ‘slight’ and ‘serious’ reported injury accidents during the study period, compared to 53.7 during the 18-month period prior to the start of the experiment (a 25% reduction).

2.2. Recent Research (early 2018). There have been three recent (early 2018) studies which have shed allowed us to understand better the impact of 20mph speed limits on casualties.

  • London – People Cycling. Research by Rachel Aldred at the University of Westminster (and others) on injury risk on London’s roads (across the whole city) finds that the introduction of 20mph limits (alone) is linked to 21% lower injury odds for people who are cycling compared to 30mph roads[8].
  • A study by the University of the West of England entitled The Bristol Twenty Miles Per Hour Limit Evaluation[9] found that the roll-out of 20mph speed limits across the city of Bristol was linked to:
    • statistically significant reductions in average traffic speeds of 2.7mph across the city.
    • Lower annual rates of fatal, serious, and slight injuries following the introduction of the 20mph speed limits compared to the respective pre-20mph limit rate, thus showing a reduction in the number of injuries.
    • An estimated total number of injuries avoided across the city each year is 4.53 fatal, 11.3 serious, and 159.3 slight injuries.
    • The number of residents who walk for 10 minutes or more in their local area most days has generally increased in every area.
  • The Council’s review of the impact of the introduction of 20mph limits across Calderdale[10] in West Yorkshire found:
    • A 30% casualty reduction over a 3-year period (and later schemes indicate a 40% reduction).
    • A 1.9mph mean reduction in speed (taken from 3.5 million+ readings with variations in some areas).
    • A rate of return of £3.65 for every £1 spent (with future benefits for a minimal ongoing cost).

Overall, therefore, it appears reasonable to assert that:

  • Where speeds are reduced to a maximum of 20mph in built-up areas a decline in casualties of more than 40% will occur.
  • In built-up areas, on non-arterial roads, where the speed limit is reduced from 30mph to 20mph there is typically an average decline in casualties of at least 20%.


Rod King MBE, Founder and Campaign Director for 20’s Plenty for Us commented :-

“The increasing evidence from local authorities who have already implemented wide area 20mph limits shows clear benefits on casualty reduction. This is coupled with evidence and recommendations from global organisations such as WHO, OECD and iRAP that 30kmh/20mph limits are the only safe speed limit where pedestrians and cyclists mix with motor vehicles.

There really can be little argument against 20mph as the limit for most urban and village roads. Local and national government should stop “kicking the can down the road” on replacing an 80 year old 30mph limit, that never had any scientific basis, with one appropriate for the 21st century. Our communities need a national default 20mph limit for urban and village roads now.”








[7] Ibid.





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  • Andrew Witts
    commented 2021-03-06 19:17:47 +0000
    How do we know that pedestrians in these areas are more knowledgeable and take more care crossing and walking these areas . So better pedestrian behaviour is also part of the equation rather than purely 20mph.
  • Tim Coote
    commented 2021-01-13 15:47:23 +0000
    I’m not sure that the reductions quoted by the different studies make sense. To quantify the discussion below, from a year ago, the annual reduction in accidents in 30mph speed limit areas is 2.9% (based on STATS19 data). This is in line with the changes quoted above, where a whole impact is described. Some of the examples constrain the impact to the area where the new limit was introduced, thus the impact of behavioural changes, which could reduce traffic levels, would also have an impact.

    It’s also worth noting that the original TRL reports (esp. 421) focus on the impact of speed change on accident rates, not speed limit. It also notes that a larger impact comes from changing the variation in driving speed on a given road. In fact, the reductions in accident rates observed between 2005 and 2019 exceed those projected as possible by reducing the speeds in 30mph zones across the country in TRL421, although hardly any changes were made between the report and now.

    Much of the original data comes from non-UK roads from late last century. The WHO comments also note that road accidents are mostly an issue in the developing world.

    (numbers and analysis available on request)
  • Rod King
    commented 2019-06-10 11:34:55 +0100
    Well Martin, you should note that the comparison was made to other areas without 20mph zones over the same period. It was comparing the same pool of cars. In addition, whilst there may have been some changes in brake, ABS, power steering and other technologies these really don’t come into the comparison between a 20mph driver and a 30mph driver. In the distance a 20mph driver can stop, a 30mph driver has only just exited his/her “thinking” time. Hence the brakes have only just been applied.

    As well as technology changes noted there has been a large increase in the technological distractions such as satnav, in-car entertainment, mobile phone usage (whether hands-free or not), etc.
  • Martin Barker
    commented 2019-06-10 11:25:24 +0100
    Your second point of section one, just proves that studies show nothing, you think in over 15 years there has not been massive innovation in car technology that would massively affect that percentage, Like oh I don’t know Brake improvements, Power Steering, ABS. where is the factoring in of that, where is the information about even researching the difference technology improvements has made to reducing crashes? because these are all massive factors that study should have addresses. If your going to have an article and references makes sure they are actually scientifically referenceable first.