Potential Safety Benefits Associated with Speed Limit Compliance in San Francisco and Phoenix

Objective

Safer Speeds represents one part of the Vision Zero and Safe System Approach to eliminate traffic fatalities and serious injuries. The objective of this paper was to estimate the potential safety benefits if all drivers in two U.S. cities complied with roadway speed limits on surface streets.

Methods

Sensor data from a fleet of automated driving system (ADS)-equipped vehicles operating a ride-hailing service were used to determine aggregate traffic speeds during free-flow conditions in Phoenix and San Francisco from over 1 million unique vehicle-road segment traversals. The current human driving speed distribution was estimated using opposite direction traffic speed observations to limit the influence of the ADS-equipped vehicle on surrounding vehicles’ travel speeds. The speed limit compliant driving fleet consisted of speed observations involving the
ADS-equipped vehicles. To estimate the potential safety benefits from reduced travel speeds associated with speed limit compliance, an exponential model relating the effect of speed reduction on fatal and injury crashes was applied, stratified by roadway speed limit. Recent fatality data from these cities was then used to quantify an estimate for lives saved simply through speed limit compliance.

Results

Across the roadway-location combinations considered, 33-49% of human drivers were observed to be speeding, with 85th percentile speeds 3.6-7.2 mph over the speed limit. Serious injury and fatality reductions associated with altering the current human-driven vehicle fleet speed distribution toward one that is speed limit compliant were observed to vary by roadway from 18-30% and 27-43%, respectively. When considering these fatality reduction rates in conjunction with available fatality data from FARS, an estimated 82 lives could be saved annually simply through speed limit compliance on surface streets, with 75 lives saved in the Phoenix metro area and 7 lives saved in San Francisco.

Conclusion

Using novel data from an ADS-equipped vehicle fleet to estimate the travel speed distribution of both the current human driven and a speed compliant fleet, in conjunction with the Elvik speed framework, this study estimated a 30% reduction in fatalities on surface streets in two U.S. cities, highlighting the impact of speed limit compliance on fatality prevention for all road users and building on the existing body of traffic safety literature capturing the deleterious effects of speeding.