The federal government should play a greater role in helping states develop guidelines for the licensing of autonomous vehicles, the Eno Center for Transportation argues in a new study.

While the National Highway Traffic Safety Administration has developed guidelines for testing self-driving vehicles, licensing their use has been left to the states. A larger federal role could help ensure continuity between the states and could also help governments better pool their resources as they seek to address safety and operations standards facing the new technology, the study says.

Several automakers have said that self-driving vehicles could become commercially available by the end of the decade. Google's famous self-driving vehicle has reportedly already logged more than 500,000 miles.

Yet the states have taken a piecemeal approach to crafting policies addressing the technology.

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Nevada became the first state to authorize the operation of autonomous vehicles on its roadways in 2011; since then California, Florida and the District of Columbia have all passed laws that open the door for self-driving vehicle testing. Those places -- and other states considering bills -- have largely deferred many of the questions of how to regulate autonomous vehicles to their motor vehicle departments.

The paper says the feds already have a system in place they could emulate as they try to develop standards for autonomous vehicles. While states and localities install their own traffic signals, highway signs, and road markers, the details of how they should look and function is dictated largely by the U.S. Department of Transportation's Manual on Uniform Traffic Control Devices.

States can make some tweaks, but for the most part, all traffic control devices look the same because everyone follows the same manual.

If the United States took the same approach to autonomous vehicle licensing, the paper argues, manufacturers could efficiently use their resources to meet national requirements instead of a potential hodgepodge of 50 different rules.

The technology has prompted a series of policy questions beyond just licensing. As Governing reported earlier this year, issues about liability, training, and privacy, among others, remain largely unanswered by states, and the feds haven't done much to provide insight either.

Yet answering those questions will be key to moving the technology forward. And many government leaders hope to do so quickly, since the benefits of self-driving vehicles, transportation experts say, extend beyond simply making driving convenient.

The technology could potentially save thousands of lives. Around 90 percent of the 30,000 annual fatal auto collisions are attributed to human factors, according to federal estimates.

Because autonomous vehicles, working in conjunction with connected vehicle technology, may travel more closely together than cars currently do, the road network could become more efficient. That would mean reduced travel times and congestion without building new highway capacity.

There's also speculation that self-driving vehicles could ease congestion by reducing the need for on-street parking in dense cities, since many believe the technology will allow for widespread,on-demand car-sharing. That would ostensibly eliminate the need for many people to own cars that sit idle on the street the majority of the time.

Still, the paper notes, there hasn't been much research into the risks that the technology may carry. Daniel Fagnant, one of the authors, also notes that 1.6 million freight truck drivers and 240,000 taxi cab drivers could potentially find their jobs made increasingly obsolete by automated drivers.

Mary Lynn Tischer, director of transportation policy studies at the Federal Highway Administration, said while much work has been done to quantify the benefits of the technology -- like savings from reduced fuel costs and travel time -- there's a shortage of research on its costs.

There's also a debate about the impact that autonomous vehicles could have on urban planning. Some speculate the technology might be a boon to dense urban areas, since cities wouldn't need reserve as much space for parking and could instead use that land for other purposes

But Tischer says an unintended effect of the technology may be that it could actually facilitate sprawl, since long distance commutes would seem more palatable to drivers if they can do other things while they're in the car.

The paper urges more federal funding for research on how autonomous vehicles will impact the broader transportation system.