Speeding is a significant safety concern and has contributed to almost one-third of road fatalities in the past 20 years. As roadside responders, it's important for the towing community to be aware of changes to roadway systems that may impact the frequency and severity of roadway incidents.

The AAA Foundation for Traffic Safety recently conducted and published a project titled “Uncovering the Spillover Effect from Posted Speed Limit Changes: A Tool to Examine Potential Safety Concerns” which employed a systematic exploration of the literature and spatial analysis to uncover speed-related crashes on roadways adjacent to Interstate highways with newly increased posted speed limits.

The goal of this work was to ensure roadways can be safe for users by identifying potential safety risks caused by the spillover effect. The following takeaways offer insights into the importance of evaluating safety impacts on nearby traffic networks when posted speed limits on Interstates were raised:

Key Findings

• After the posted speed limits were raised on Interstates, comparing ‘before' and ‘after' crashes along the roadway may not yield the true safety impact. Spatial analyses identified safety concerns (i.e., hot spots) on multiple adjacent roads along the Interstates examined in this project. Such an approach offers a repeatable and systemic way to examine unintended impacts from posted speed limit changes.
• When posted speed limits on Interstates were raised to accommodate operating speeds of vehicles and increase traffic flow and throughput, adjacent roads being operated and managed by local transportation agencies could see increases in traffic safety concerns because of the spillover effect. To minimize unintended safety consequences, it is important for transportation departments, at all levels, to coordinate and work closely when considering posted speed limit adjustments.
• The tool used in this project measures how speed-related crashes are correlated to each other in space across a study area and graphically presents areas of traffic safety concerns. When transportation agencies can visually identify hot spots on their roads, comprehensive plans can be developed with strategies and countermeasures to address them.

TRAA appreciates the AAA Foundation for Traffic Safety for conducting this valuable research. We look forward to seeing the results of their next study. For more information visit: AAAFoundation.org.

Romo, A., McDonough, J., Wei, A. & Yang, C.Y.D. (2024). Uncovering the Spillover Effect from Posted Speed Limit Changes: A Tool to Examine Potential Safety Concerns (Technical Report). Washington, D.C.: AAA Foundation for Traffic Safety.