Solar power highways are hitting a roadblock. Nearly three years after France built a 0.6-mile stretch of photovoltaic road in Normandy, the government is deeming it a disappointing experiment.

In 2016, France announced its bold plan to “pave” 1,000 kilometers (around 620 miles) with photovoltaic panels, which would generate 790kWh per day. When completed, the road was supposed to power up to 5 million homes. But that first 0.6-mile stretch, which engineers had originally estimated would power up to 5,000 homes, hasn’t lived up to expectations.

After installation, it was clear that the panels produced by the manufacturer Wattway couldn’t hold up under the wear and tear of highway traffic. According to Global Construction Review, “the 2,800 square meters of solar panels have degraded, peeled away and splintered, and 100m of them have been removed after being declared too damaged to repair.”

The report claims that engineers didn’t account for the natural deterioration caused by thunderstorms, leaf mold, and heavy trucks and tractors that would be regularly using the road. At its peak, the road only generated 149,459 kWh in a year, making them far less efficient than regular tilted solar panels.

The outcome, while disappointing, isn’t necessarily surprising. Experts have lobbed skepticism at the glitzy promise of solar roadways since they were first announced. Other experiments in the realm have gone similarly awry. In China, one six-foot panel of a 0.6-mile solar highway was stolen, prompting the government to abandon the project. In Missouri, where the company Solar Roadways was meant to test a small patch of sidewalk at a rest stop along Route 66, negotiations between the Department of Transportation and the company broke down.

What does this mean for the future of energy generating roads? It’s safe to assume companies will continue exploring experimental roadways—they might just have to temper their expectations.