> and I would like at least some forest to remain forests
You're dramatically overestimating the actual land coverage that would be needed to power the entire world with solar panels (assuming one abstracts away the problem of actually running power lines everywhere). A square 340 miles on a side would do it, from the back-of-the-envelope calculations I've seen.
The forests are not cut down because of solar panels, but because of agriculture. The more land that is covered with solar panels, means less land for agriculture (I like the dual use concept, though).
The spaces here in germany, that are covered with solar panels, would have been adequate agriculture area, but is now ugly and in use. So if you could just use the road as the space, you could save that other land for other use. This is the idea behind it.
You're dramatically overestimating the actual land coverage that would be needed to power the entire world with solar panels (assuming one abstracts away the problem of actually running power lines everywhere). A square 340 miles on a side would do it, from the back-of-the-envelope calculations I've seen.