Impacts of Freeway Speed Limit on Safety and Operation Speed of Adjacent Arterial Roads

Date of Award


Degree Name

Doctor of Philosophy


Civil and Construction Engineering

First Advisor

Dr. Valerian Kwigizile

Second Advisor

Dr. Jun-Seok Oh

Third Advisor

Dr. Ron Van Houten


crash analysis, spillover effect, speed limit, arterial roads, speed analysis, driving simulator


Urban arterials carry the most traffic on urban road networks and experience the highest percentage of crashes in urban areas. Safety on the lower speed urban arterials that are adjacent to a higher speed freeway may be impacted by speed spillover or adaptation. In this context, speed spillover is defined as the tendency of drivers to speed on the lower speed facility after exiting the higher speed facility. The objective of this research is to investigate drivers’ speed choices and the relationship between speed and crashes on urban arterials adjacent to freeways. The study also examines potential countermeasures for speeding behavior in the area under the speed spillover effect.

To determine the effect of freeway speed limits on speed spillover, analysis was performed using two types of speed data: (1) point (i.e., spot speed) and (2) trajectory (i.e., continuous trip time series). While spot speed data were collected using a radar technology, the trajectory data were gathered from around 2,700 vehicles equipped with connected vehicle technologies and acquired from Intelligent Transportation System Research Data Exchange - Safety Pilot Model Deployment (SPMD) roadside equipment. Comparison of speed differences between motorists who exited the freeway and those who were already driving on the arterial road was performed. Results showed that the mean speed and the 85th percentile speed of vehicles exiting from the 70 mph freeway are significantly higher when compared to the vehicles that were already driving on the arterial road.

The effect of raising freeway speed limits to 70 mph on the frequency of speed-related crashes on urban arterial roads adjacent to freeways (i.e., spillover effects) was investigated. Crash data were collected from Michigan on 1,393 road segments of urban arterials before and after speed limits were altered. Before-and-after data was collected simultaneously on 1,470 comparison segments of urban arterial where speed limits did not change to control for the regression-to-the-mean bias. The mixed effects negative binomial (MENB) regression model was developed to analyze crash frequency. The results indicate that raising speed limits of freeways by as much as five miles per hour had a likelihood of increasing crash frequency on adjacent arterial roads by as much as 13.9 percent. This increment in crashes observed on arterial roads involved more different crash types comparing to those that occurred on freeway. There exist more potential motorist-to-motorist conflicts as well as motorist-to-other road users (e.g., pedestrians and bicyclists) on arterial roads than on freeways. Therefore, the potential for a speeding vehicle to collide with another vehicle or other road user is higher on urban arterials. To investigate if the safety impact of speed spillover changes with the distance from the freeway, influence areas (0-1 mile, 1-2 mile, and 2-3 mile) were used. Speed-related crashes were impacted more on the 0-1 mile and the 1-2 ranges by 41.1 percent and 17.5 percent, respectively, compared to the 2-3 mile range. The study finds that the influence of freeway speed on drivers’ speeding behavior on adjacent urban arterials fades away as the distance from the freeway increases.

To mitigate speed spillover, a driving simulator study was used to investigate potential countermeasures for speeding behavior. The study observed the behavior of 56 participants who drove a 5300 meters section of freeway, then exited to a two-mile urban arterial. The driving simulator results were validated using field data collected from trajectory speed data. The impact of Crash Fact Sign (CFS), a message that provides information about the number of crashes/fatalities that occurred on that particular road, and Warning Sign (WS) on areas under the speed spillover influence were studied. Each participant was asked to drive under a base scenario (without any countermeasure) and four test scenarios, namely(1) an additional speed limit sign (SLS-2); (2) Warning Sign (WS); (3) Crash Fact Sign (CFS); and (4) Crash Fact Sign With additional Speed Limit Sign (CFS&SLS-2). The study finds CFS&SLS-2 to be the most effective countermeasure because it produces significant speed reductions in the area under speed spillover effect. Furthermore, the effect of a traffic signal on speed spillover behavior was investigated. Speed spillover effect existed for longer distance when drivers did not stop at the signalized intersection compared to when drivers stopped.

This study provides researchers, policymakers, law enforcement officials, and engineers with a better understanding of the effects of speed spillover on adjacent roads when determining whether to increase the speed limit on freeways. The study suggests that urban arterial segments adjacent to high speed freeways need more attention in terms of speeding countermeasures. It also recommends potential effective countermeasures for speeding.

Access Setting

Dissertation-Open Access

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