A solar cycle path and/or footpath might be a really good idea; but I don't see how you can ever get past the fact that splitting the two functions is always an even better idea.
A path that doesn't generate power is never going to be more expensive to build or maintain, nor less effective at being a path or cycle way, than one that does generate power.
A solar array that doesn't act as a thoroughfare is never going to be more expensive to build or maintain, nor less efficient as a power source, than one that does also serve as a roadway.
By building an ordinary path, and then building an ordinary solar array above that path, you will always end up with a road that is no more expensive and no less useful; and a solar array that is no more expensive and no less efficient, than you could have achieved by combining the two functions in one surface.
Add to that the benefits of having protection for users of the path/cycle way/roadway from the rain and/or sun provided by the solar power structure overhead, and it is a no-brainer. The best solution is a two level structure, with solar arrays above the roadway. It will always be at least as cheap and at least as efficient - and right now, far more so on both counts.
Solar cells don't make a good road surface (when compared with tough and cheap materials), and road surfaces don't make for good solar arrays (when compared with structures specifically designed to capture the sun's rays).
