There's another way: put solar panels
above the road. Especially for big, wide highways, there's a lot of area that's basically not used for anything except cars and trucks to drive over. Cover these with solar panels and now that land isn't just used for transportation, but also a lot of energy generation. On top of that, the cars underneath will be shaded from the Sun, which can be really useful in sunny climates, and would help people use less A/C in their cars, thus saving energy.
The two problems I see with this idea are people complaining about the "view" (which is why it shouldn't be done on some really nice scenic part of a road, but in the middle of an ugly part of the city it should), and the possible problems with wind-loading, especially during storms, though surely this has been dealt with before, as there's a lot of places where they cover parking lots with elevated solar panels like this. For the view problem, they don't even have to cover the whole road with panels, they could just cover some lanes, put them in the median and extending over the inner lanes some, etc.
The two problems I see with this idea are people complaining about the "view" (which is why it shouldn't be done on some really nice scenic part of a road, but in the middle of an ugly part of the city it should), and the possible problems with wind-loading, especially during storms, though surely this has been dealt with before, as there's a lot of places where they cover parking lots with elevated solar panels like this. For the view problem, they don't even have to cover the whole road with panels, they could just cover some lanes, put them in the median and extending over the inner lanes some, etc.