On city streets, the most substantial risk comes from high-end speeding, even if it is typically only a small percentage of total traffic. As a result, changes in the number of high-end speeders is a primary metric for determining the efficacy of a speed limit change or safety project.
High-end speeding is measured as the number or percent of drivers exceeding specific, high-risk speed thresholds (e.g., over 30 or 35 mph, or greater than 10 mph over the target speed for most streets), in a typical 24 hour period.
Because high-end speeding is set to a specific threshold that does not change with the speed limit, this metric allows for apples-to-apples comparisons before and after a project or from site to site. The prevalence of high-end speeding is a better indication of risk than 85th percentile speed or the number of speeding vehicles, since there is sometimes a ‘long tail’ of high-end speeders. Well-done speed management can result in a dramatic change in high-end speeding, even when 85th percentile or median speeds do not change dramatically.
RAINIER AVENUE, SEATTLE
Speed management and street design changes can substantially reduce the amount of high-end speeding on a street. On Rainier Avenue in Seattle, a 4-lane-to-3-lane conversion resulted in up to a 16% decrease in median speed, and up to an 81% decrease in drivers exceeding 40 mph.
2015 speed limit: 30 mph
2016 speed limit: 25 mph