PALO ALTO, Calif. — Federal safety regulators are asking automakers to help them figure out how to test self-driving cars to determine whether the vehicles are safe enough for consumers.

The effort comes as the National Highway Traffic Safety Administration prepares guidelines — due out in the coming months — to steer the development and rollout of autonomous vehicles.

Regulators want evidence that self-driving cars are safer than human-driven vehicles, but there remains no obvious way to test them aside from putting them on the road, as Google has done for more than 1.5 million miles under the supervision of trained drivers.

“The old model, of counting vehicle miles traveled and counting crashes and injuries, is not sufficient,” NHTSA Administrator Mark Rosekind said last week during a public meeting at Stanford University.

Rosekind asked autonomous-vehicle developers to pay special attention to how they measure their vehicles’ safety. And he wants an answer soon.

NHTSA vowed in January to release federal guidelines for self-driving vehicles within six months. The guidelines would not be legally binding, but could be adopted by states, preventing a patchwork of rules feared by the vehicles’ developers.

California drew a backlash from developers in December, when its department of motor vehicles released draft rules for autonomous vehicles.

The rules would require operators of self-driving cars to submit safety data. But California’s DMV admits it lacks the expertise to interpret them, Chris Urmson, director of Google’s self-driving car program, said during last week’s meeting.

“We would like NHTSA to be the expert authority for the nation on this issue,” Urmson said.

Google, Ford, Volvo and ride-sharing companies Uber and Lyft have formed a lobbying group called the Self-Driving Coalition for Safer Streets. The group, which named former NHTSA Administrator David Strickland as its counsel and spokesman, is advocating that NHTSA allow fully self-driving cars — not just autopilot systems that require a human driver to remain in the loop.

“We are years, not decades, away from this becoming a reality,” Strickland said during last week’s meeting.

It is difficult to develop objective tests to show that self-driving vehicles will perform as intended, James Kuffner, chief technology officer at Toyota’s new Silicon Valley research center, said during last week’s meting. He said Toyota encourages regulators to have self-driving vehicles run through a standard course to ensure they at least perform basic functions.

If the agency takes Toyota’s advice, it may end up encouraging a test regimen at a test center such as the University of Michigan’s new Mcity complex.

NHTSA plans to take written comment through Monday, May 9.