by San Diego plans to lower speed limits on hundreds of miles of roads
Roads can be dangerous, and this danger often comes from multi-ton vehicles moving at high speeds. To improve safety, San Diego is preparing to lower speed limits on over 20 percent of its roads. This will affect approximately 679 miles (about 1100 km) across the city, and only the number on the sign will change.
Plan receives committee approval
On Thursday, a city council committee advanced the Comprehensive Speed Management Plan. This initiative, aligned with the Vision Zero concept, uses new authorities granted under California laws AB 43, AB 1938, and AB 382. The full city council still must approve the changes before implementation begins.
However, everything indicates that the plan will be implemented: according to reports, $2.4 million has been reserved for sign changes. Traditionally, California, like many other states, has long used the so-called 85th percentile rule to set speed limits.
What is the 85th percentile rule?
Simply put, it uses the speed at which 85% of drivers travel to set the limit. However, critics argue that such a formula allows dangerous speeds to become the norm.
What specific changes are planned?
If the changes are adopted, cities will be able to set speed limits at 15 mph on two-lane streets within 500 feet of schools and 25 mph on streets 500-1000 feet away. In safety corridors, limits could be lowered by 5 mph. In areas with active commercial activity, the limit could drop to 25 or 20 mph depending on location. Importantly, the plan contains nothing that would change driver behavior beyond posting a new number on signs.
Can signs alone slow drivers down?
The plan repeatedly acknowledges that road design plays a role in driver speed, but never explains how the city or state can use tax dollars to influence speed through infrastructure changes.
Transportation studies consistently show that street geometry, lane width, curb extensions, traffic calming measures, and roundabouts often have a stronger and longer-lasting impact on driver speed than signs alone. Without physical changes, it is unlikely that speed will decrease without additional police enforcement.
“Clearly, much more needs to be done,” said Councilmember Stephen Whitburn. “And, as we heard today in the presentation and from many of those who spoke, vehicles moving at dangerous speeds are a big part of the problem.”
At what stage are the changes?
At this stage, these changes are still in the planning process. The city council must vote to implement them, and even then, engineering work and field checks will be needed before signs are installed.
San Diego’s initiative reflects a broader trend of rethinking road safety approaches, shifting the focus from adapting rules to existing driver behavior to proactively setting standards. The success of such measures often depends on a comprehensive approach, where regulatory changes are reinforced by physical transformations of the urban environment and ongoing public awareness efforts. The question of the effectiveness of sign-only regulation remains open, as the experience of many cities shows that drivers are often guided not by formal limits, but by a speed comfortable for them, which is directly shaped by road configuration and surroundings.
