What’s the impact of putting up 20MPH speed limit signs? Minimal according to a report from the London Borough of Hammersmith & Fulham. A report on traffic speeds after the introduction of wide-area 20 mph speed limits comparing measured traffic speeds in 2015 and 2017 showed an average reduction of just 1.3%.
Across 100 comparable roads, the 85th percentile reduced from 24.83 mph to 24.52 mph. Although some roads showed bigger reductions in speed, others actually showed substantial increases. But the data is difficult to interpret because those with the highest increases reported low speeds in 2015 while those with the biggest reductions reported high speeds in 2015. It’s possible these are statistical anomalies resulting from inadequate sample sizes.
Is there any data on accident impact? Too early to report apparently. But the Council is pushing ahead regardless with extending 20mph to all remaining side roads in the borough and on some sections of main roads.
You can read the H&F report here: http://democracy.lbhf.gov.uk/documents/s93226/20MPH%20Speed%20Limit%20-%20Speed%20Surveys%20Results.pdf
Comment: This surely demonstrates that the likely benefits of such schemes do not justify the cost. The money would be much better spent on other road safety measures.