Mods that control a car are selling well, with open-source hands-free driving systems like comma four grafting assistance onto more than 300 car models. Some owners install homemade versions of the software. The U.S. highway safety authority, the National Highway Traffic Safety Administration (NHTSA), is investigating after five accidents involving the comma four, two of which were fatal. These incidents involved collisions with stopped or slow vehicles in the same lane, resulting in three deaths and 11 injuries. Preliminary data suggest the comma system may not have correctly detected vehicles in the lane or responded appropriately. The NHTSA is also examining the marketing of the comma devices, which promote hands-free driving but require a driver to be ready to take control, as well as third-party forks of the open-source software. Mike LaBarbera, a Tesla Model Y owner, let his Tesla Full Self-Driving subscription expire in January 2026 and installed a comma four on his vehicle in February. He shared a video of the SUV staying in its lane without intervention, which went viral. The device, sold for $999, is part of a community variant of openpilot, the open-source software developed by comma.ai, a company founded by George Hotz. Comma claims more than 30,000 cars are equipped with its devices, with over 640 million kilometers driven. The NHTSA opened a preliminary evaluation on September 21, 2026, targeting all comma three, comma 3X, and comma four devices, as well as the openpilot software. The agency noted that the fatal accident in Louisiana in February 2026 involved a comma device running third-party software. In France, installing a comma four device may be a legal gamble, as the devices lack European certification. Level 2 driving assistance systems must comply with UN Regulation R171, which requires type and manufacturer approvals. Tesla had to secure exemptions in the Netherlands in April 2026 for its Full Self-Driving system. French law requires new approval for any vehicle that has undergone significant modifications, and a module controlling steering and braking is likely to fall into this category. Insurance companies may reduce compensation or cancel contracts if modifications that increase risk are concealed.