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