Research examines drivers’ mental workload to improve safety for left-turning buses

In the United States between 1999 and 2005, more than 40 percent of all pedestrian fatalities occurred as a result of collisions with transit buses. The risk of collision is highest for buses making left turns, which are four times more likely to collide with pedestrians than buses passing straight through an intersection.

According to researchers from the HumanFIRST Program at the Intelligent Transportation Systems (ITS) Institute, left turns are more dangerous because of the increased mental workload experienced by bus drivers during the turning maneuver. By conducting an in-depth analysis of the tasks completed by bus drivers, the research team aims to identify countermeasures to reduce bus–pedestrian collisions. The project was sponsored by the ITS Institute.

Led by HumanFIRST director Mike Manser and research associate Ensar Becic, the research team created a detailed list of everything bus drivers do when making a left turn. To compile the list, the team conducted several interviews with bus drivers and trainers from Metro Transit. 

In the United States, one pedestrian fatality occurs every two hours, and 24 percent of these fatalities occur at intersections. 

Tasks identified by drivers included both tangible, observable behaviors—such as changing lanes or completing the turn—and internal tasks such as determining the status of the traffic signal or deciding how much time is needed to merge successfully. The researchers then classified the identified tasks into five categories—visual, working memory, executive, motor, or unplanned—to better understand the different types of demands placed on drivers.

The research team also divided the left-turn process into six separate stages, beginning with intersection approach and ending with the post-turn exit of the intersection. They then created a chart illustrating the specific tasks performed by drivers in each segment of the intersection, allowing them to see exactly how much drivers were doing and where.

Results show that drivers have the highest mental workload when entering the intersection and when completing the turning maneuver. During those times, drivers must complete many tasks in all five categories—while also attempting to detect and avoid collisions with pedestrians.

Identifying driver tasks in this stage-by-stage analysis was the most valuable aspect of the study for Metro Transit, according to Steve McLaird, assistant director of garage operations.

“It confirmed that there was an overload concerning the brain’s ability to process information,” McLaird says. “Knowing that, the message we want to get to bus operators is that you cannot be distracted by conversation, radio use, or looking at pieces of paper at that point in time. You have to be fully concentrated on making that turn and observing your surroundings.”

Metro Transit also plans to use the task analysis results to evaluate its standard operating procedures and review training materials. The process will include an internal group of safety, instruction, and operations personnel.

“Can we take a second look and rethink the mechanics of making a left turn? Or we might identify tasks we can remove from a driver’s workload,” McLaird says.

The ultimate goal is to find ways to improve safety, heighten driver awareness, and reduce the likelihood of all types of collisions.

Reprinted with permission from the February 2013 issue of CTS Catalyst, a publication of the University of Minnesota's Center for Transportation Studies.