Exclusive: U.S. Talks With GM Over Deploying Self-Driving Cars Without Steering Wheels

"I think the complexity was far greater than what a lot of very optimistic advocates were thinking," Chao said. In GM's petition, NHTSA is for the first time looking at a vehicle in which all driving decisions are made by a computer rather than a human driver. Nuro, which partnered with Kroger Co last year to deliver groceries, seeks approval not to include a windshield in the vehicle. The petitions - formal applications for action by the agency - seek exemptions from U.S. vehicle safety rules largely written decades ago that assumed human drivers would be in control of a vehicle. The petitions are for up to 2,500 vehicles per manufacturer.