Will driverless cars make traffic better or worse?

In test runs for driverless cars, the gee-whiz moment comes when the person behind the wheel takes their hands off the wheel.

The testing phase still has years ahead, but what will happen if and when driverless cars are proven to be better than human drivers?

"Every day we're seeing technology advancing toward a fully autonomous system," said Bob Pishue, a transportation analyst for the Kirkland-based traffic data company INRIX.

Pishue is enthusiastic about autonomous vehicles and predicts they'll someday relieve traffic congestion.

Cars without people driving could travel closer together at high speed, making use of more of the roadway.

"It's an increase in capacity without actually pouring concrete or building rail lines," Pishue said.

The benefits of driverless cars are demonstrated well by C.G.P. Grey, the popular producer of online educational videos.

Pishue says the safety benefits of self-driving cars will also help traffic move.

"About 25 percent of congestion is due to accidents. If you can avoid the accidents, that's an immediate decrease in travel times, decrease in congestion," Pishue said.

But there are other experts who predict that instead of making traffic better, driverless cars could make it worse.

"Driverless vehicles will probably help us solve some of our problems and will probably introduce new ones," said University of Washington transportation engineering professor Don MacKenzie.

He predicts if people don't have to sit with their hands on the wheel, and can get other things done in the car, they won't see commuting as lost time and might move farther from work.

"We think people will drive more, with the assumptions we used, we think they could drive up to 60 percent more, that's a lot more travel," MacKenzie said of a UW study.

That means more energy use, more climate impact, and more traffic.

MacKenzie agrees self-driving cars will ease congestion, at first.

But he says there's a well-established pattern for what happens when we create new road capacity.

"Demand will expand to fill that capacity and congestion will return," McKenzie said. "This is what we call induced demand, if you build it they will come."

MacKenzie says following that pattern, after short-term traffic improvement, "the problem comes back, sometimes with a vengeance."

Researcher Mark Hallenbeck, also at UW, describes a scenario where traffic would get worse with driverless cars.

He imagines you sending your car by itself to the dry cleaner.

"He'll hang it in your car and the car will drive back. The fact that that car is stuck in real awful congestion with all those errands? What does the car care? It doesn't. What do you care? You don't you're doing work. But if you're in the car that's now stuck behind 18 other cars picking up groceries, going to the dry cleaner, dropping kids off, too bad for you, you're still stuck there," Hallenbeck said.

At INRIX, Pishue says he doesn't see that happening because he thinks vehicle ownership will drop.

Pishue predicts a future of shared car services like Uber, which is testing driverless technology in Pittsburgh.

You don't need as many shared vehicles because each car serves a lot of people.

"As things get more convenient, people will start adopting shared vehicles a lot more," Pishue said.

Whether the car is yours, or Uber's, your future commute could be without any hands on the wheel.

And it could be in a car with no steering wheel at all.