Doctoral Defense - Sunday Imosemi

About the Event

The Department of Civil & Environmental Engineering

Ph.D. Dissertation Defense

Thursday, July 17, 2025 at 10:00 AM EST

3540 Engineering Building (Seminar Room)

ABSTRACT

ASSESSING THE IMPACTS OF POSTED SPEED LIMITS AND ROADWAY DESIGN ELEMENTS ON DRIVER SPEED BEHAVIOR

By: Sunday Imosemi

Advisor: Peter Savolainen

Recently, speed management practices have shifted away from a historical focus on the 85th percentile speed toward a safe systems approach that is focused on promoting safer speeds in all roadway environments through a combination of thoughtful, equitable, context-appropriate roadway design, appropriate speed-limit setting, targeted education, outreach campaigns, and enforcement. This includes the use of roadway design elements to reduce speeds and create a more accommodating environment for all users. One important geometric feature that could be used in this context is a roundabout.

Roundabouts have been shown to significantly improve safety due, in part, to fewer conflict points between road users as compared to other intersection designs. However, the integration of roundabouts into a roadway poses many challenges, especially where the accommodation of vulnerable road users (VRUs) like pedestrians and bicyclists is concerned. To that end, this study aims to inform the design process by advancing our understanding of how drivers adjust their speeds based on changes in the posted speed limit and other contextual factors related to the roadway environment. It also examines driver speed selection at roundabouts in the presence or absence of VRUs and how their speed profiles compare to those at signalized intersections and all-way stop controls, especially at the entry and exit approaches.

To examine the effect of roadway characteristics on driver speed selection at speed transition zones, field data were collected from 19 highway corridors across Minnesota using handheld lidar

guns to track drivers' operating speeds as they transitioned from high-speed rural highways to lower-speed rural and suburban communities. This resulted in the estimation of a series of speed reduction factors (SRF), which detail the impacts of various site-specific characteristics such as roundabouts, medians, and two-way left turn lanes on travel speeds. The results also show driver’s response to posted speed limit signs.

This study also assessed driver speed selection at roundabouts' entry in the presence and absence of pedestrians and bicyclists. Speed data were collected at 9 roundabouts in Minnesota using a combination of handheld lidar guns and video cameras. To capture driver yielding behavior, staged crossing events were conducted for pedestrians and bicyclists. The results showed differences in speed selection behavior based on whether a VRU was waiting to cross at the roundabout entry and the response of the driver to VRU presence. The study also recommended a minimum of 1.5 times the stopping sight distance (SSD) for the line of sight of the crosswalk and potential crossing points.

Lastly, this study examined driver speed selection at the entry and exit approaches of roundabouts, signalized intersections, and all-way stop-controlled (AWSC) intersections. Speed data were collected across 20 locations in Minnesota. Findings from this study show that roundabouts are more effective at traffic calming compared to the other two intersection control types.

Ultimately, this study will allow for a more proactive and data-driven approach to highway design that considers the needs of all road users. It also supports transportation agencies in achieving their goal of setting effective target speeds.

Persons with disabilities have the right to request and receive reasonable accommodation. Please call the Department of Civil and Environmental Engineering at 517-355-5107 at least one day prior to the seminar; requests received after this date will be met when possible.