The devices are marketed for a variety of innocuous uses — a cellphone holder, for instance, or a safety hammer. One promises to relieve shoulder pain. Others ditch the pretext and list simply as “wheel weights” or “wheel knobs.”

They all have a common purpose: to let Tesla drivers take their hands off the wheel….

Tesla requires drivers to keep their hands on the steering wheel while using both of its driver-assistance systems — Autopilot, which can maneuver the cars from highway on-ramp to off-ramp, and Full Self-Driving, which can navigate city and residential streets without the driver’s physical input — and the systems are designed to issue periodic reminders. By replicating the pressure of a driver’s hands, the wheel weights silence the nagging.

“Elon Musk’s saying it’s supposed to drive itself. That’s what they’re going to hear,” said Carnegie Mellon University professor Philip Koopman, who has been studying autonomous vehicle safety for 25 years. “How do you think they’re going to behave?”