Steering wheel weights have become a popular commodity as Tesla has expanded its “Full Self-Driving” technology from around 12,000 vehicles to more than 400,000 over the past year. While the electric car manufacturer has adopted measures to discourage their use, the devices have been involved in at least two recent traffic incidents.

In March, a Tesla plowed without slowing into a teenager getting off a school bus in North Carolina, police said, causing severe injuries. And in December, a driver in Germany fell asleep at the wheel while a Tesla in Autopilot led police on a chase at speeds reaching nearly 70 mph, Bavarian authorities said.

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?”

As recently as Monday, sellers were marketing the devices widely on online shopping sites, including Alibaba’s AliExpress and Amazon, where they could be obtained in as little as a day. Wheel weights recently ranked as the top two releases in Amazon’s “automotive steering wheels” category. After The Washington Post flagged them, Amazon and Alibaba said they removed the listings, citing safety issues and violations of their policies.

The Post reported last month that the number of fatalities and serious injuries involving Tesla’s Autopilot has surged over the past year, likely due to wider availability of the features. Among those incidents was the North Carolina crash.

It was not clear from the incident reports and summaries from that crash or the police chase in Germany how the drivers obtained the weights and what form they took. In both cases, police say the steering wheel weights were intended to trick Autopilot.

The weights are not illegal, although federal regulators have cracked down on one such device, deeming it “unsafe.”

Musk and Tesla did not respond to requests for comment.

Both Musk and his car company have touted the safety of Autopilot compared with normal driving, and Musk has called the technology “unequivocally safer.” Tesla tells drivers they must pay attention at all times while using the technology and be ready to intervene.

Last year, people began reporting that the software had been trained to sense the presence of “defeat devices,” issuing a message reading “Hands-on defeat device detected,” according to owners online and in Tesla forums who have been dinged — sometimes erroneously — by Tesla’s safety monitoring.

That warning came on top of Tesla’s standard alerts when the technology senses that drivers are distracted: “an escalating series of visual and audio warnings, reminding you to place your hands on the wheel if insufficient torque is applied,” as the company’s website describes it. “If you repeatedly ignore these warnings, you will be locked out from using Autopilot during that trip.”

Tesla also has introduced camera-based driver monitoring to ensure drivers are paying attention — a long-standing request of safety advocates.

The defeat devices, which generally run around $30 to $75 but can cost far more, have been available for years, sold under names such as Autopilot Buddy, Buddy Steering Wheel Booster and AP Papa.

Listings on AliBaba’s Ali Express showed that more than a thousand have been sold. Two separate Amazon listings each showed more than 100 sold over the past month. Several of the devices were eligible for shipping via Amazon Prime.

Amazon said the wheel weights ran afoul of its rules for automotive products and have since been removed. “Products intended to circumvent driver assistance systems or other car safety features, including counterweight rings and autopilot nag reduction devices, are prohibited,” said Amazon spokesperson Samantha Boyd, noting that Amazon’s third-party sellers must follow applicable laws and regulations and Amazon policies.

“The products in question were evasively listed, have been removed, and we are taking corrective action. Amazon does not tolerate illegal or evasive behavior, and we enforce bad actors that make factual misrepresentations to customers,” Boyd said. (Amazon founder Jeff Bezos owns The Post. Interim CEO of The Post, Patty Stonesifer, sits on Amazon’s board.)

AliExpress said it removed the Tesla wheel weights from its site as soon as it learned about the potential safety issues associated with them, citing its emphasis on product safety.

One “nag-reduction device” promises in a sales listing to “extend the Autopilot detection interval from 30 seconds to 600 seconds” saying it is “adapted to the latest algorithm,” a sign that manufacturers are trying to skirt Tesla’s efforts to stymie the devices. A page promoting a version of that device was taken down from AliExpress this week.

On Amazon, the products were flooded with reviews from owners touting their effectiveness.

“If you use it in city streets then it’ll work without any alerts,” according to one Amazon review of a “Steering Wheel Glass Breaker” from May. “Use it at your own risk.”

“Just got this, and tested it, driving 38 miles. No Autopilot nag for the entire ride,” according to another from late last month, specifying it “Eliminates Tesla Autopilot nag.”

Safety experts have expressed concern about defeat devices since the early days when Tesla drivers jammed oranges into their steering wheels to trick Autopilot into sensing the weight of a driver’s hands.

In 2018, NHTSA cracked down “Autopilot Buddy,” issuing a consumer advisory calling the device unsafe and ordering the company behind it to certify that it had stopped selling and marketing it in the United States. The company took Autopilot Buddy off the market.

Due “to an influx of inexpensive substitutes and China imitations, we’re forced to cease operations,” a message on the Autopilot Buddy website says. Attempts to reach the makers of Autopilot Buddy were unsuccessful.

After that, defeat devices seemed to lose traction for a while. But “for some reason, they’re on the rise now,” said John Bernal, a former Tesla Autopilot employee who was fired in 2022 after posting videos of Full Self-Driving in action, which the company deemed an improper use of its technology.

The steering wheel weights may not always be needed to drive hands free. A hacker last month revealed the existence of a mode buried deep in Tesla’s software that appears to turn off the reminders to pay attention.

The hacker known as “Green,” who has spent years exploring Tesla’s driver-assistance software, interprets the data that the car sends and receives from Tesla’s servers, including vehicle permissions — which enabled him to learn that the special mode was off limits to normal users.

After he enabled the feature on his Tesla, Green said in emailed comments to The Post, “One of the first things I noticed was that the nags were completely gone.”

Jeremy B. Merrill contributed to this report.