San Francisco Commentary: For Vision Zero, Twenty mph is Plenty for our city’s default speed limit (War on Cars Watch)

Today, speed limits in California are set based on the 85th percentile rule: out of 100 drivers observed, the limit is set to the speed of the 15th fastest driver. That means that wide streets where traffic is unimpeded result in higher speeds, and higher speed limits. There are efforts to change that, but failure to change state law has been used as a crutch for San Francisco to do nothing. Furthermore, speed limits without corresponding design and enforcement changes may or may not be effective but will provide a false sense of security. In deadly situations, what matters is not the speed limit, but the actual driver speed, which is typically higher than the limit.