The impact of speed limit on traffic casualties

Lowering of speed limit down to 30 or 40 km/h can be a contested decision. There is often skepticism about whether the act really reduces traffic casualties and worries it will hurt traffic flow and thus increase trip duration.

In this story we take a look at some cases from around the globe where a speed limit has been lowered and look at the impact on traffic casualties.

The interactive map below shows a selection of the relevant evidence available in the Urbanixm knowledge engine. The knowledge engine is in a proof-of-concept state of development and more evidence will be added over time.

In the following sections we briefly discuss the available evidence and formulate conclusions but encourage readers to follow links to the original content to get a in-depth understanding of the topic.


Many cities have seen a reduction in traffic casualties following a lowering of speed limit.

  • Seattle saw a 22% decrease in crashes after reducing speed limit to 25 mph [SDOT Blog].
  • Paris saw a 50% reduction in traffic fatalities between 2001 and 2007, attributing lower speed limits as one contributing factor [Vision Zero Network].
  • London experienced a 40% reduction in collisions when introducing 20 mph zones [Sustrans].
  • Several areas in Australia lowered speed limit from 60 km/h to 50 km/h and saw accident frequency drop by 13% [Livable Streets].

However, lowering speed limit is not always enough to reduce traffic casualties.

  • New York City, lowered speed limits in 2014 and experienced consistent drop in traffic fatalities between 2014 and 2018 [SDOT Blog]. In 2018 low speed limits were expanded in the city and cycling lines built but pedestrian fatalities rose in the following year [New York Times].
  • In Bath, UK, 20 mph speed limits lead to reduction of casualties in the city center but an increase in its outer lying rural areas [Telegraph].

While lower vehicle speeds have been shown to lower likelihood of pedestrian fatalities in a collision, speed limit reduction need not always be enough to lower traffic speed and should be considered in combination with road re-design and/or other measures.

Further reading

For a tailor made analysis of the topic, Börkur Sigurbjörnsson offers data storytelling consultancy services producing detailed reports or presentations.

Author: Börkur Sigurbjörnsson

Börkur Sigurbjörnsson is a data and information scientist who discovers and communicates data-driven insights through storytelling and helps early-stage startups put ideas into action.

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