I just can't comprehend how this is even a proposal in the first place.
That was my first thought too. My second thought was, "Some people who know a lot more about this than I do clearly think it's a good idea, so perhaps they know things I don't." After thinking about it more, I came up with some potential advantages: installation is very easy (just huge sheets of panels lying flat on the ground), it allows huge solar installations (the cost of hooking up to the grid isn't proportional to the number of panels, so the bigger your installation, the cheaper it is), it requires no new land, they're going somewhere that's owned by the government (you might choose to put solar panels on your roof, but that's completely your choice, not something the government can force on you), and it's potentially easier to maintain them and keep them clean (think of the work required just to wash an equal number of solar panels mounted on poles or rooftops, compared to just driving a street sweeper down the road).
The obvious disadvantage is that they need to be super sturdy to handle thousands of cars driving over them every day. But if they've already solved this problem, who knows? It might be a good idea.