The first section of the Wattway road, a 0.6-mile stretch, opened this week in Normandy. The road will be tested for the next two years and is expected to generate enough power to light a city of 5,000, according to France's Ministry of Environment, Energy and the Sea.

Last week, France’s minister of Ecology and Energy announced that the country will pave 621 miles of road with solar panels over the next five years, with the goal of providing cheap, renewable energy to five million people.

Called "the Wattway," the roads will be built in collaboration with the French road-building company Colas and the National Institute of Solar Energy. The company spent the last five years developing solar panels that are only about a quarter of an inch thick and are hardy enough to stand up to heavy highway traffic without breaking or making the roads more slippery, David Rogers reports for Global Construction Review. The panels are also designed so that they can be installed directly on top of existing roadways, making them relatively cheap and easy to install without having to tear up any infrastructure.

"There is no need to rebuild infrastructure," Colas CEO Hervé Le Bouc told Myriam Chauvot for the French magazine Les Echoes in 2015. "At Chambéry and Grenoble, was tested successfully on Wattway a cycle of 1 million vehicles, or 20 years of normal traffic a road, and the surface does not move."

The panels are made out of a thin polycrystalline silicon film and coated in a layer of resin to strengthen them and make them less slippery. Because the panels are so thin, they can adapt to small changes in the surface of pavement due to temperature shifts and are sealed tightly against the weather, Fiona MacDonald reports for ScienceAlert. According to Colas, the panels are even snowplow-proof, although plows need to be a little more cautious so as not to rip the panels off the ground.

France isn’t the first country to kick around the idea of paving its roads with solar panels. In November 2015, the Netherlands unveiled a 229-foot-long bike path paved with solar panels as a test for future projects, and a couple in Idaho raised more than $2 million through Kickstarter in 2014 and received a 2-year contract from the Federal Highway Administration to develop their own solar roadways, Rob Wile writes for Fusion. However, this is the first time a panel has been designed to be laid directly on top of existing roads and the first project to install the panels on public highways.

For many environmentalists, paving roadways with solar panels sounds like a great idea. Colas says that 215 square feet of Wattway will provide enough energy to power a single French home (aside from heating), but some researchers are still skeptical that solar roadways will ever be efficient and cost-effective enough to compete with regular rooftop solar panels, MacDonald writes.

It will be difficult for photovoltaic glass to compete against the much cheaper asphalt, for example, and rooftop panels are better placed to get the best possible sunlight, researcher Andrew Thomson wrote for The Conversation. In addition, he writes, if solar roadways prove to be more slippery than traditional roadways, safety concerns could kill the burgeoning technology, regardless of how much power they may put out.

“For solar roadways to be effective, it needs a complete technological rethink,” Thomson wrote.

For now, French authorities are going ahead with the project, and will start laying down segments of Wattway this coming spring.