Determining the effectiveness of a speed limit change or safety project, and making further adjustments as necessary, is essential to reducing traffic fatalities. In addition, project data that shows how speed limit changes reduce speeding and can reduce fatalities is essential to making the case for future safety projects.
Cities should collect post-implementation data, mirroring the data that was collected before the project began, to conduct a full evaluation of their work. This data includes operating speeds, traffic incidents—paying special attention to fatal and serious pedestrian and cyclist injuries—conflict points, and speeding opportunities.
In collecting post-implementation data and conducting project evaluations, cities should remember that drivers typically adjust to speed limit changes slowly and therefore operating speeds may not change at all in the short-term. As tempting as it is to try to produce immediate results, cities should focus on reporting 6-month and 1-year after data for operating speeds to ensure a robust and accurate evaluation. Transportation department leadership should prepare elected officials, policy makers, the media, and the public for some “lag-time” between project implementation and evaluation and results.