San Francisco – Researchers recently allowed 22 drivers to test retaking control of an autonomous car on a track and found that the transition could be problematic.

The participants drove a 15-second course consisting of a straightaway and a lane change, then took their hands off the wheel and the self-drive car took over, bringing them back to the start.

After going through the process four times, they drove the course 10 additional times with steering conditions that were modified to represent changes in speed or steering. It was noticeable under the researchers' watch that the drivers wobble the wheel to account for over- and understeering, according to a study published last week in the first issue of Science Robotics. 

These challenges bring up the possibility that, depending on the particulars of the driver, the driving conditions and the autonomous system being used, the transition back to driver-controlled driving could be an especially risky window of time.

"Many people have been doing research on paying attention and situation awareness. That's very important," lead researcher Holly Russell said. "But, in addition, there is this physical change and we need to acknowledge that people's performance might not be at its peak if they haven't actively been participating in the driving." 

Changing the steering ratio from the standard 15:1 to 2:1 simulated the more sensitive steering feel drivers experience at a higher speed. This modification made the car turn more sharply to simulate the way less steering wheel movement is needed to make a lane change at a high speed versus at a low speed. The drivers were given advance warning of the changes and had some opportunity to probe the difference during the straightaway. 

Regardless, during the altered steering ratio trials, the drivers' steering maneouvres differed significantly from their paths previous to the experimental modifications. "Even knowing about the change, being able to make a plan and do some explicit motor planning for how to compensate, you still saw a very different steering behavior and compromised performance," said Lene Harbott, co-author of the research. This is explained in neuroscience as a difference between explicit and implicit learning, said another co-author IIana Nisky. 

Even when a person is aware of a change, their implicit motor control is unaware of what that change means and can only figure out how to react through experience. Although the drivers were not so thrown off by the changes in steering that they drove off-course, the fact that there is a period of altered steering behavior is still significant, the researchers said. There are so many different variables involved in driving that anything that compromises driving performance could lead to an accident. "If someone is designing a method for automated vehicle handover, there will need to be detailed research on that specific method," said Harbott. "This study is tip of an iceberg."