Getting a grip: Research helps identify where roads need more traction

Every driver depends on pavement friction for safe maneuvering when stopping at an intersection, navigating a curve or taking swift action to avoid a crash.

But managing that grip across an entire roadway network is complicated. Pavement friction varies from place to place, changes over time and can be affected by traffic, weather, pavement materials and surface treatments. What’s more, appearances can be deceiving: A polished roadway segment with dangerously low friction could appear to be “smooth” and in good structural condition.

To address this issue, the Illinois Center for Transportation is helping the Illinois Department of Transportation and the people of Illinois by developing a statewide, data-driven way to use pavement-friction information in roadway safety and maintenance decisions.

The project, “R27-264: Development of a Pavement Friction Management Program,” helps IDOT connect friction data with crash history, pavement condition, roadway features and treatment options so the department can better identify locations where additional review or pavement treatment may be needed.

“Pavement friction gives drivers the grip they need to maneuver safely through traffic and across our roads,” said John Senger, IDOT bureau chief of research. “There is a direct connection between pavement friction and roadway safety.”

Yanfeng Ouyang, George Krambles Professor at the University of Illinois Urbana-Champaign, led the research. Ramez Hajj (Illinois), Priscilla Tobias (Arora and Associates) and Hao Wang (Rutgers University) also assisted with the project.

IDOT has collected and used pavement-friction data through specialized testing equipment, but this project addressed the need for a more comprehensive statewide approach. By combining friction measurements with related safety and roadway information, the research gives IDOT a more consistent way to identify where pavement friction may be related to safety risk.

“Friction changes over time and its safety impact varies across locations, so it is important to take a holistic look at all related sources of information,” Ouyang said. “This project helps turn those different data sources into a more consistent statewide process.”

To develop that process, the research team reviewed existing pavement-friction research, surveyed other transportation agencies and analyzed Illinois data on friction, crashes, roadway characteristics and pavement preservation.

The team also used statistical modeling, machine learning and life cycle cost analysis to examine how friction relates to crash risk and how cost-effective treatment decisions could be prioritized.

Pavement friction was strongly associated with crash risk, especially for wet-pavement crashes, run-off-the-road crashes, rear-end crashes and pedestrian-related crashes. The level of risk varied by roadway type, traffic volume, geometric design, pavement surface condition and environmental conditions.

High-friction-demand locations, such as ramps, curves and intersections, require particular focus because they can experience greater polishing effects from traffic. For drivers, better friction management matters most in the moments when roadway conditions leave little margin for error.

The analysis also showed that even modest, measurable increases in friction can be associated with meaningful safety benefits.

“At many locations, an increase of 10 friction units can lead to a crash reduction of more than 10%, which is a significant safety benefit,” Ouyang said.

These findings can help IDOT weigh safety, pavement condition, cost and long-term benefit when evaluating potential treatments.

“The goal is to implement the pavement friction management program as quickly as possible to help increase the safety of IDOT-maintained roadways,” Senger said. “The project supports safer and smoother travel for the public, especially under wet pavement conditions and at locations where drivers need more grip.”