The sun’s rays can hit some roads for up to 90 percent of the daylight hours, so companies in Europe and the U.S. are experimenting with building solar panels along or above roads. But are such projects worth the cost? In France one company is hoping to distinguish itself—and reduce costs—with solar panels that are laid directly on the pavement.

Solar is popping up just about everywhere, even landfills and parka pockets. So why not roads? Indeed, solar road projects are gaining interest around the world, and some promise to even charge electric cars while moving.

The Netherlands built the first solar road, a bike path, in 2014. France announced a bolder move in January—over the next five years, it plans to install 1,000 kilometers (621 miles) of solar roads, designed to supply power to five million people.

That’s not all. German company Solmove aims to bring solar panels to German roads, and Idaho-based Solar Roadways has received three rounds of U.S. government funding (plus $2 million in venture capital) to test its technology.

“We have interested customers from all 50 states and most countries around the world,” says Julie Brusaw, who co-founded Solar Roadways with her engineer husband Scott. She says before hitting the open road, they’re testing their panels in non-critical areas such as parking lots, walkways, and their own driveway.

“We are in talks about some very interesting projects,” she says, noting the Missouri Department of Transportation wants to install the panels at a rest area along the I-70 highway. The couple say their tempered-glass panels offer asphalt-like traction, support the weight of semi-trucks, include LEDs for signage, and contain heating elements to melt snow and ice.

Could solar panels really pave the roads of the future? Proponents see endless possibilities, but others raise questions about cost, efficiency, and durability.

“We just place our solar panels on an existing pavement,” says Jean-Luc Gautier, inventor of the Wattway technology that will be tested this spring in France before its polycrystalline silicon layer is applied to actual roads. Gautier, technical director at construction company Colas, says he was inspired by the fact that roads look at the sky so they can collect solar energy.

“The sheer amount of surfaces each country devotes to roadways is enormous,” the Bursaws write on their website. “Allowing this space to double as a solar farm could have very positive implications in the battle to put a halt to climate change.” They estimate that their panels, if used in lieu of existing U.S. roads and walkways, could produce more than three times the electricity used in the United States.

Besides, they say their panels could charge electric vehicles, first on solar parking lots. With enough solar highways and cars with the right equipment (to pick up energy from induction plates in the road) they might even be able to charge vehicles while moving.

The Cost Challenge

“In theory, solar PV roadways sound great. The issue is cost.” says Mark Jacobson, an engineering professor at Stanford University who has promoted a plan for powering the U.S. solely with renewable energy. (Read about his blueprint for a carbon-free America.)

“Aside from road dust, particularly black tire dust and diesel exhaust, which will quickly cover a portion of each panel, the continuous traffic covering panels will reduce their solar output,” says Jacobson, adding they’ll likely suffer more wear and tear and need more repairs than other solar panels.

He also says that while they don’t require land acquisition costs, as do solar power plants, their panels cannot be rotated for optimal solar exposure. He expects a solar road won’t be able to compete on cost, but “I’m hopeful it will.”

“Installing photovoltaics in roads seems like a daft idea at first, “says a report last month by IDTechEx, an independent research and consulting firm. “A closer look reveals that most of the problems are easily overcome and even at poor efficiency, that local electricity has viable uses.”

Despite high costs, company chairman Peter Harrop says solar roads might work in places that are putting down roads for the first time. “They need early (technology) adopters like China that want to leapfrog in development.”

In contrast, “I can’t see solar roads in London,” he says, noting the city often digs up its roads for underground repairs.

So far, the Netherlands’ solar path is popular. In its first year, 300,000 bikes and mopeds rode the initial 70-meter (230-foot) stretch connecting two Amsterdam suburbs. Officials say the SolaRoad produced more energy last year than expected—enough to power three households. It’s made of crystalline silicon solar cells, encased in concrete and covered with a translucent layer of tempered glass.

In the U.S., Solar Roadways has received more than $1.5 million from the Department of Transportation over the last six years to develop and test its hexagonal-shaped panels.

“One of the shortcomings Solar Roadways has yet to resolve is the manufacturing process,” two DOT officials wrote in a December post, noting the solar cells are handmade and thus “very costly” to produce.

Still, they say the DOT has received “a lot of positive feedback” about the project; the company’s promotional video has 21 million views on YouTube. Because the panels can melt snow or keep water from freezing, even with high costs, the DOT officials say the innovation “could still be useful in smaller areas such as parking lots, sidewalks, driveways, and bike lanes.”