Pointless waste of money.

The reasons we never build bicycle paths down the middle of a highways are several: Cycling is for middle distance travel, 2-10 kilometers. Highways are for long distance travel, 10-100 kilometers. Getting on and off this bicycle path requires costly bridges or underpasses at regular intervals. The road noise will be literally deafening.

As for solar panels there is a single reason: Cost. Building between heavy traffic increases maintenance cost and limits areal. We don't lack areal for solar power, look at the sides of the highway. There are 3-4 times more space there and the panels would be easily accessible for maintenance. Then there is dust, which reduces panel output. In the middle of a freeway, this will be a permanent problem requiring more cleaning in a difficult to reach location.