Driverless shuttles show potential as an economical and sustainable way to serve small community routes, inter-campus loops, first/last-mile connections, and other public transportation needs. Despite ongoing improvements in autonomous technologies, however, safety concerns remain a barrier to widespread use of these vehicles. The problem is not simply the technology; it’s also the behavior of the human drivers who share the road.
To explore this issue, researchers with the U of M’s Human Factors Safety Laboratory (HFSL) studied the behavior of human-driven vehicles around a driverless shuttle in Rochester, Minnesota. Med City Mover, a Minnesota Department of Transportation (MnDOT) pilot project, ran from August 2021 to August 2022 in a downtown loop surrounding the Mayo Clinic, covering a route of 6,000 miles and carrying 3,000 passengers to clinics, hotels, businesses, and neighborhoods. The six-passenger electric and autonomous shuttle included a human operator to assist riders and take occasional control of the vehicle.
Along the route, the operators observed that other vehicles followed the shuttle too closely, executed unsafe passing maneuvers, and failed to yield to pedestrians—including in high-risk “multiple threat pass” situations in which a vehicle passes the Med City Mover in a nearby lane while the shuttle is yielding to a pedestrian. Additionally, drivers overtook the Med City Mover while turning right at an intersection more frequently than other human-driven vehicles. In at least one collision, a driver hit the shuttle’s bumper during an unsafe pass.
“Operator concerns were a major motivator for the research study,” says Nichole Morris, research associate professor, HFSL director, and the study’s principal investigator. The study, Assessment of Pedestrian Safety and Driver Behavior Near Automated Vehicles, was funded by MnDOT.
Researchers collected data during normal operations in the summer of 2022 and measured the relative risks surrounding the shuttle. While the shuttle outperformed the nearby drivers in complying with the Minnesota Crosswalk Law and other traffic laws, its slow operating speed of up to 11 mph in a 25 mph zone appeared to provoke aggressive behaviors. A follow-up study conducted at the Minnesota State Fairgrounds found that participants were more likely to indicate that they would overtake slower moving vehicles (10 mph) at a crosswalk.
“The shuttle is capable of going faster, but it really isn't safe or advisable for it to do so given the current technology,” Morris says. The sensor technologies that enable autonomous vehicles to identify and respond to hazards are still advancing and the shuttle made frequent, abrupt hard stops in response to hazards, including false positives, she explains.
“Going faster and then braking [hard] would just increase the discomfort for riders,” says Morris, who experienced this as a passenger on a similar EasyMile shuttle in Florida. “I was quite thankful to be wearing my seatbelt during a hard brake, or else I would have been surely thrown from my seat.”
A more immediate solution than increasing shuttle speed may lie in better communication between the shuttle and other drivers. In a driving simulation study conducted at the HFSL, researchers again observed other drivers following too closely and trying to pass the autonomous shuttle. However, adding text and icon messaging on the back of the shuttle resulted in a significant reduction in dangerous passing moves. An LED screen with the message “PED X-ING” made all the difference, Morris says. Still, “I think we have much to learn from what the human is doing to smooth out the interactions between humans and automated vehicles,” Morris says. “There is a big need to incorporate that understanding into designs of the automated vehicles before we try to remove the human operators from their roles.”
—Amy Goetzman, contributing writer
