Speed limits will always be a contentious topic, but there’s no denying that one of the places they’re most needed is on city streets. Despite that, most speed limits in the US are set according to state laws, and established based on prevailing traffic speed. This method, known as the 85th percentile rule, is implemented in the same way on urban roads as on free-flowing rural highways.
A few years ago, though, California passed Assembly Bill 43 (AB 43), a law that granted cities in the state the freedom to set certain speed limits themselves based on other, safety-based criteria instead, and San Diego could be about to reduce the speed limit on hundreds of miles of its roads as a result.
The 85th Percentile Rule
With some exceptions where blanket speed limits are applied, like residential areas, the US and many other countries set their limits based around the 85th percentile rule. This takes into account the speed that 85% of drivers on a stretch of road are travelling at or below, and uses it to determine the speed limit, usually to the nearest 5mph.
While essentially setting limits through the popular opinion of drivers, the system also has some obvious drawbacks. Namely, even if a piece of road has been identified as a danger spot, it can be difficult for local governments to get the speed limit lowered if the vast majority of drivers are still travelling within the posted limit. That’s what AB 43 aimed to change.
What AB 43 Changed
When it passed in 2021, AB 43 and its associated laws allowed Californian cities to set speed limits based on criteria outside the 85th percentile rule. On roads with a higher number of serious accidents and streets that are more likely to have a high concentration of pedestrians and cyclists, cities can now reduce speed limits by up to 5 MPH regardless of the 85th percentile…