The letter came shortly after the report of a Google autonomous self-driving car sideswiping a bus. It cited Google’s so called “disengagement report” filed with the California Department of Motor Vehicles reporting that a human driver had to take control of test vehicles over 341 times, an average of 22.7 times a month. The car technology failed 272 times and ceded control to the human driver; the driver felt compelled to intervene and take control 69 times.

“These real-world results show there are many everyday routine traffic situations that cars simply can’t cope with yet,” the letter said. “NHTSA’s model policy must reflect this simple reality: Self-driving vehicles aren’t ready to safely manage many routine traffic situations without human intervention.”