The Eighty-Five Percent Solution: Is Crowdsourcing Speed Limits the Best Approach to Traffic Safety?

Abstract

According to the U.S. Federal Highway Administration (FHWA), most places in North America set speed limits using the “85th-percentile rule.” If substantially more than 15% of urban drivers are found to regularly exceed the posted limit, the rule — originally intended for open rural roads — suggests that the limit be increased, safety considerations notwithstanding. Since 2017, the National Traffic Safety Board has called for “alternate methods” to set speed limits, especially those that consider vulnerable road users. With walking, public transit, ride-hailing, cycling, and emerging forms of micromobility increasingly competing with cars and trucks for urban street space in many places along with sidewalk cafes, parklets, and urban greenery, it may be time to reconsider whether the rigid “85% solution” is compatible with California’s commitment to Vision Zero and its goal of eliminating all traffic deaths.