Incidentally, it seems to me that authorities are missing a trick with streetlights by not having a small solar panel on top of them (and if the light frequencies are compatible and there's no issue with reflections dazzling drivers - why not include them on the outside of the post so that they can 'recycle' light from drivers' headlights).
It may be possible to fit very small wind turbines to motorway streetlights and signage - a car whizzing past at 70 mph must be creating a wake vortex (as would larger but slower HGVs and buses), which it may be possible to harness (railways are another possibility).
Given likely future demand (and returns) for land for housing, agriculture, and other developments in renewables, the long term, commercial viability of land-based solar farms is suspect.
Technology has advanced to the extent that there are commercially available floating solar panels, and flexible rollable solar panels:Britain's first floating solar panel project installed
"Green energy developer says new floating technology is even more lucrative than solar farms on fields"http://www.telegraph.co.uk/news/earth/energy/solarpower/11110547/Britains-first-floating-solar-panel-project-installed.html
As one UK supplier describes flexible panels: "These types of panel are useful for trekking, cycling, kayakking and climbing expeditions. Rollable and flexible panels are also useful on yachts and other boats, as they can be tied over a bimini or boom."
"cover the wattage range 5W-32W and so are are suitable for 12V battery charging"