Uncovering the Spillover Effect from Posted Speed Limit Changes: A Tool to Examine Potential Safety Concerns

This research 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.

April 2024

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Speeding is a significant safety concern and has contributed to almost one-third of road fatalities in the past 20 years. This research investigated whether changing to higher posted speed limits on Interstates can cause spillover effects, or a systemic propagation of speeding behaviors that results in more crashes at nearby road segments.

Findings presented in this study are useful in raising awareness of the safety impact after regulatory posted speed limits on Interstates were raised, especially among local agencies that may not have the tools to quantify any unintended safety consequences along adjacent roadways within their networks.


The study consisted of two main tasks. The first task was a systematic literature review on the spillover phenomenon as well as subjects including general speeding behaviors, speeding countermeasures, and alternative methods for measuring any unanticipated effects of implementing roadway treatments. The main product from this task is a detailed summary of findings regarding the presence and magnitude of spillover effects and other topics listed above.

The second portion of this project focused on understanding spillover from a geographic perspective and exploring patterns, trends, and relationships among speed-related crashes occurring on adjacent roadways near Interstates that have raised posted speed limits. Hot spot analyses were conducted on roads within a 1-mile radius of Interstates. This task allowed the project team to quantify and visualize differences in speed-related crash clusters in areas surrounding Interstates before and after a regulatory speed increase became effective. The sites selected for the spatial analysis included communities in Georgia, Michigan, and Oregon.

Key Findings

The goal of this work was to ensure our 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:

  • 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.

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Alicia Romo

AAA Foundation for Traffic Safety

Jessica McDonough

AAA Foundation for Traffic Safety

Anqi Wei

C. Y. David Yang

AAA Foundation for Traffic Safety