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.Meanwhile, in ‘business activity districts’ – streets with high densities of business and therefore frequent parking and pedestrian movements – cities can now impose 25 or 20 MPH limits, and in school zones, limits can be set at as low as 15 MPH.What It Means For San Diego The city of San Diego plans to take advantage of these laws with its newly-released Comprehensive Speed Management Plan, which outlines that a total of 679.1 miles of roads have been identified as eligible for speed limit reductions under these new laws – over 20% of all the roads in the city.The document, though, makes it clear that this is all still in the planning stages, and that not every piece of road identified will necessarily get its limit reduced. It estimates that implementing the plan will take around a year and “will require significant coordination, signage updates, and staff resources.” It also accepts that lowering posted limits alone won’t help the city achieve its ‘Vision Zero’ goal of no deaths or serious injuries on its roads, and that public education and physical changes to street design and infrastructure will also need to play a role. In the shorter term, though, if you regularly drive in San Diego, you might soon have to start setting off for work that little bit earlier.