I am sorry, they are a dumb idea, and the data prove it.

It's SOLAR FREAKIN' ROADWAYS! TIME on Treehugger, as we have another look at the Colas Wattway solar road installed in France a few years ago, which Derek described in France to pave 1000km of roads with solar panels.

This TreeHugger has wondered why anyone would want to put solar panels in a roadway where they have to be made with materials strong enough to be run over by trucks, get covered in dirt, are not at an optimum angle, and cost a fortune. I thought that it was the dumbest idea ever, but they keep popping up and readers kept yelling at me to stop complaining: "This is an innovative idea. It's refreshing to see such original ideas out in the world."

But now the results are in from the French trial of the solar roadway. Dylan Ryan of Edinburgh Napier University writes in The Conversation that it has a maximum power of 420 kW. It covered 2800 m2 and cost 5 million Euros to install, which is a cost of €11,905 (US$ 14,000) per installed kW. (An average rooftop solar system in the US costs $ 3140 per installed kW)

© Brice Robert

It was originally supposed to be 17,963 kWh per day, but before it opened that estimate was downgraded to 800 kWh per day, and after a year it was found to have actually yielded 409 kWh per day. It also has not held up well; due to thermal stresses and joint sealing problems, 5 percent of the slabs have been replaced already.

It was always assumed that the panels would generate about a third less than panels set at the optimum angle, but results were worse than expected. As Xavier Lula writes on a French site looking at the road:

In other words, the concept of solar road, at best will lose "only" 33% of the producible photovoltaic cells compared to standard solar, and in the real world, in 2017, with a brand new road, it has lost 58.3%. What way? In a global context of increasing pressure on the resources of raw materials (especially metals), is this really a way forward?

Dylan Ryan also points out that the 300mW big Cestas solar plant near Bordeaux cost a tenth as much per installed kW. But the idea just doesn't go away; since Derek's post we have seen the installation of solar bike lanes in the Netherlands and a huge solar roadway in China.

In each post, readers continue to complain that I am missing the point and that I should be more optimistic.

Any new technology is always expensive. Yes, the cost is astronomical, but this is the stepping stone of a new electric revolution, my friend. Solar panels on houses are great but they cannot charge your car while driving through induction. Imagine a road that charged your car as you drove. This is something much larger than the cost it takes to build.

Fine. I always thought that is why we had wires. But I am happy to imagine that solar roadways can make sense, even if this French example didn't.