WASHINGTON -- Car companies are racing to get self-driving cars on U.S. highways. But federal regulators are playing catch-up.

Starting next year, a largely autonomous Audi hits the highways. This fall, students at the University of Michigan will be hopping a driverless bus across campus.

A driverless bus being rolled out at the University of Michigan. CBS News

Carmakers are promising mass-market, fully self-driving cars by 2021, and tech companies like Uber and Google could deploy them much sooner.

California is readying its roads, replacing raised lane markers with six-inch-thick solid lines, because they're easier for self-driving cars to see.

But what's not ready are the rules.

Scott Keogh CBS News

"We need definitions, we need the government and the states to set laws that are all united and unified around this, and there needs to be a little bit less PT Barnumism, when everyone's throwing around these terms," said Scott Keogh, president of Audi of America.

Right now, only 23 states and the District of Columbia have laws on the books about self-driving cars, but two senators are hoping to change that.

"As this technology gets out on the road we want to make sure it is safe, that it truly is ready for prime time," said Sen. Gary Peters, D-Michigan.

"The concept behind this bill is to try to create some certainty, some clarity so that people know what those rules are going to be," said South Dakota Republican Sen. John Thune.

Thune and Peters plan to present rare bipartisan legislation setting standards for safety, cybersecurity and guidelines to determine accident liability if no one is driving.

Sen. John Thune, left, and Sen. Gary Peters. CBS News

"It's only a matter of time, and that horizon is coming very quickly at us and I think we have to be prepared for that," Thune said.

"Hopefully we can all get around the fact that this is an incredibly exciting technology that's going to save literally tens of thousands of lives," Peters said.

They believe it will save lives because self-driving cars would eliminate the biggest factor in more than 90 percent of crashes: human error.