When I ran the numbers for the USA, it was something along the lines of adding a 1m wide strip of normal efficiency PV alongside the entire length of the interstate network. I’ve not looked at the size of any other highway system relative to population density but I do know that Denmark gets less sunlight than the USA average.

Like I said in my original comment: I’m optimistic, I just don’t think it’s trivial.

To put it another way, I think it’s something that requires a government rather than being achievable purely with the private sector. That might mean just incentives and template plans, or it might require government to commission things, but it’s that sort of scale.