On a clear morning in early May, Brian Lathrop, a senior engineer for Volkswagen's Electronics Research Laboratory, was in the driver's seat of a Tesla Model S as it travelled along a stretch of road near Blacksburg, Virginia, when the car began to drift from its lane. Lathrop had his hands on the wheel but was not in control of the vehicle. The Tesla was in Autopilot mode, a highly evolved version of cruise control that, via an array of sensors, allows the car to change lanes, steer through corners, and match the lurching of traffic unaided. As the vehicle--one of a fleet belonging to Virginia Tech's Transportation Institute, which Lathrop was visiting that day--lost track of the road markings, he shook the wheel to disengage Autopilot. "If I hadn't been aware of what was happening, it could have been a completely different outcome," Lathrop told me recently.

In catch-22 traffic emergencies where there are only two deadly options, people generally want a self-driving vehicle to, for example, avoid a group of pedestrians and instead slam itself and its passengers into a wall, a new study says. But they would rather not be travelling in a car designed to do that. The findings of the study, released on Thursday in the journal Science, highlight just how difficult it may be for auto companies to market those cars to a public that tends to contradict itself. Related: Statistically, self-driving cars are about to kill someone. "People want to live a world in which everybody owns driverless cars that minimize casualties, but they want their own car to protect them at all costs," Iyad Rahwan, a co-author of the study and a professor at MIT, said.