Imagine driving the Pennsylvania Turnpike, glancing out the driver's side window at the car a lane over, and seeing no human being inside.

It's a technological feat that may be coming soon. In that moment legislators and regulators will face more quotidian, yet crucial, concerns. What are the rules for this device? How do you ensure safety without stifling innovation?

If legislation passes next year, Pennsylvania will join nine states and the District of Columbia that already have driverless-vehicle legislation, says the National Conference of State Legislatures. Pennsylvania and New Jersey are among the 17 states that introduced driverless-vehicle legislation this year, though anticipating vehicles without a human in the driver's seat makes Pennsylvania's policy more far-reaching than most. New Jersey's bill addresses only self-driving vehicles with a human at the wheel.

This month, PennDot unveiled recommendations designed to strike a balance between protecting public safety and helping innovation. They could go into effect if driverless-vehicle legislation passes in 2017.

Officials point to a federal study finding that human error causes about 94 percent of traffic accidents; autonomous vehicles could make roads far safer.

There's also an economic incentive. Pittsburgh hosts technology developers such as Carnegie Mellon University, Google, and Uber, which in September began offering the public limited rides there. Such investment has helped revitalize business districts, the city's mayor has said.

PennDot's dual role is bound to be a tricky one.

"It's very hard to believe any regulatory agency will get it perfectly right out of the gate," said Nidhi Kalra, director at the RAND Center for Decision Making Under Uncertainty in Santa Monica, Calif.

Kalra was among the experts who worked on a Rand report for policy makers on self driving vehicles. She praised the state's approach, which allows PennDot to bypass the usual process and set standards. The field is moving too fast for regulations to keep pace.

The recommendations touch on vehicles' safety, precautions to prevent hacking, asking testers to share accident data, and saying when and where vehicles can be tested. But there are unanswered questions.

A big one: What teeth will PennDot's recommendations have? Because the vehicles won't fall under the normal regulatory process, PennDot's enforcement power is unclear.

Just this month, Uber ignored an order from California officials to stop operating driverless cars in San Francisco because they lacked a required permit. Uber finally complied Dec. 21, after being threatened with legal action. Uber participated in the expert panel that assembled PennDot's recommendations, and in the appendix questioned whether they should even exist before a bill passes.

"While legislation remains pending, we recommend setting aside the detailed policy recommendations," wrote Sheri Shapiro, Uber's head of public affairs in Pennsylvania and Delaware.

Uber did not respond when asked whether it would abide by PennDot's proposal.

The pending bill requires testers to enter into a contract with PennDot, and the department can withdraw those contracts. Yet it isn't clear whether that could be an enforcement tool. "Those are the questions we're having right now," said Leslie Richards, the state transportation secretary.

PennDot officials call the recommendations a work in progress. They are encouraging feedback at penndotav@pa.gov before Jan. 12.

Kalra also noticed what appeared to be a contradiction. Right now, Pennsylvania law requires autonomous vehicles to have a steering wheel and a human being in the driver's seat. Experts believe that Pennsylvania's laws will loosen. PennDot's proposal anticipates that and requests that testers prove the vehicle can handle, "under all trafficway and environmental conditions that can be managed by a human driver" before testing the vehicle on public roads.

But that's an impossible standard, Kalra says. It suggests permission to test a driverless vehicle on a public road relies on the vehicle having a track record of doing just that.

PennDot officials responded that the proposed policy reflects federal recommendations from NHTSA. "We also reserve the right to request a demonstration of the vehicle's capabilities before approval," said Rich Kirkpatrick, a PennDot spokesman.

That's not good enough, said Jamie Lincoln Kitman, Automobile Magazine's New York bureau chief. In a New York Times op-ed this month, he questioned whether autonomous vehicles would virtually eliminate car crashes and reduce congestion. Negatives could include erasing livelihoods for drivers, discouraging  investments in public transit, and the complications of a transition when self driving cars share the road with human drivers.

"There's no doubt they'll get good at it, but at what cost?" Kitman said. "The idea that there'll be no accidents, as some people seem to think, is ludicrous. The idea that it won't be subverted by criminals and terrorists, that's ludicrous."

He compared it to the introduction of cars about a century ago. They were hailed as a wonder, but some people warned about the pollution they could cause and the lives they would take.

"Nobody can deny the automobile conferred many benefits, and these may, as well," he said, "and there are a ton of questions we should be asking."