Modified right-turn lane design improves road safety and operations

Exclusive right-turn lanes are usually associated with improved safety because they tend to separate the conflicts between turning vehicles and through traffic. However, right-turn lanes on state routes are designed in a way that accommodates large vehicles, such as semi-tractor trailer trucks. At these intersections, raised channelized islands with exclusive right-turn lanes are commonly used, and there is a large turning radii.

This configuration may cause reduced sight distance and increased rear-end and turning crashes, especially for intersection approaches where motorists have to turn their head over their shoulder to merge with oncoming traffic due to the large skew angle. Also, motorist–pedestrian conflicts may arise from the fact that drivers have their attention divided between watching for pedestrians in front and merging with on-coming traffic.

To avoid this adverse effect on intersection operations and safety by improving right-turn motorists’ view of oncoming traffic, the Illinois Department of Transportation (IDOT) reconfigured or removed raised corner islands from several intersections in the greater Peoria area. Subsequently, a research study, titled Effects of Intersection Right-Turn Lane Design and Skew on Safety and Operations, was initiated by the Illinois Center for Transportation in partnership with IDOT for the purpose of evaluating effectiveness of the redesigned right-turn lanes.

“Passenger vehicles require far less turning roadway width than large trucks and can get ‘lost’ in the excessive turning roadway width. If the intersection is skewed, the approach angle for right-turning passenger vehicles may pose further challenges related to reduced sight distance,” says Dan Mlacnik, Policy Engineer at IDOT’s Bureau of Design and Environment and chair of the Technical Review Panel that oversaw the research project.

He adds, “While these parameters affect all drivers in all vehicles, the effects of head-turn and skew angle can impose additional safety risks, especially on younger, less experienced, and older, typically less agile, passenger vehicle drivers.”

The modified right-turn lane approach was designed, implemented, and evaluated at ten locations in Illinois for its efficacy in reducing the effects of skew and excessive head-turn. The geometric changes—ranging from the reconstruction or elimination of corner islands and/or the restriping of the right-turn approach and stop bar to bring the approach angle closer to perpendicular—were intended to improve the line of sight of right-turning passenger vehicles by reducing the skew of the approach angle, while allowing large trucks to make right turns without encroachment. Traffic crash–based safety analyses were used, and geometric variables that influence right-turn crashes were identified.

Field observations of drivers’ behavior and the field data collected from 2012 through 2015 showed that drivers at the modified right-turn lane used fewer exaggerated head turns, roll-and-go stops, and stops past the stop bar and, therefore, they essentially traveled at slower speeds, had an improved line of sight, and approached the right turn with less skew. Additionally, a safety evaluation of the modified design indicated statistically significant reductions in total intersection crashes (estimated at 44%) and right-turn approach crashes (estimated at 60%).

Kerrie Schattler, professor in the Department of Civil Engineering and Construction at Bradley University, served as principal investigator of the study. She emphasized the benefits of modifying right-turn lanes to increase the approach angle of right-turn vehicles by means of geometric modifications to corner islands and pavement striping placed on the inside of the radius return.

“It is recommended that similar modifications to improve the right-turn approach angle/position be considered as potential improvements for intersection approaches on state routes that experience a pattern of right-turn crashes in Illinois,” Schattler says.

The final report summarizing findings of the study is available on ICT’s website. This report contains recommendations about the type of sites that would make good candidates for retrofit in Illinois. IDOT also held a webinar “Illinois DOT Safety Projects Series: Modified Right Turn Lane Design” on October 25,2016. The webinar recording will be made available online in November.

“IDOT is modifying its design policies regarding intersection design to incorporate results of the study and is encouraging its districts to utilize the intersection retrofit at high-crash intersections that meet the criteria for improvement,” Mlacnik said. “The findings are also being shared with the design community to enhance future intersection designs in Illinois.”