A few thoughts here:

•Shading. Cars don't occupy 100% of a highway, even when it's stopped bumper-to-bumper. The faster cars go, the larger the gaps between them. For PV, the recovery is instantaneous, and for a large road surface, the power output flat and steady in spite of car movement. The most sunlight will come in from the best angle during the noon hour, when roads experience little traffic. Morning and evening rush hour usually suck for sunlight to begin with (the sun's lower in the sky), so even bumper-to-bumper shading wouldn't be taking much away from the total.

•Dirt. The technical term for dirt on a solar panel is "dusting". It's a factor, and even in places where it rains mud on occasion, a pretty minor one. The rain eventually washes most of the gunk away. A much more significant factor would be heat buildup. PV's put out more power the colder they are. On rooftops they can convect that heat away. On road surfaces, not as much.

•Ruggedness. Having broken more solar panels than anyone I know, I can attest that a PV road surface can be done right. Cars don't jump Mario-style on surfaces but rather apply predictable compression and shear stresses that would have about as much effect on the tempered glass surfaces as your windshield wipers do on your windshield. Even a shattered solar panel still works almost as well as a new one, believe it or not. The silicon cells themselves have a diamond crystalline structure and are tough as nails in compression. If you somehow managed to bend a cell 90 degrees then it would end badly, but that's simply not happening on this road.

•Solar angle. These won't win any awards for most power output of a given surface area of solar panels, but per hectare of land, they're probably just as good as any system. All the sunlight incident on that hectare will hit a solar panel. Given how cheap solar panels are now (<$1/W), and how expensive an urban road is ($2000 a linear foot!), the solar road competes not against a solar rooftop installation (15% efficient) but against a conventional road (0% efficient).

•Economic gotchas. The biggest concern I have with this approach is the balance of system- the miles of copper wire, the mounting hardware, the inverter assemblies, and the places to store and access them outside the weather. Inverters wear out after X years, and so the question is at what point is it just more cost-effective not to replace the inverters? French electricity will probably justify it, but maybe not Pacific Northwest electricity (where cheap hydro keeps it ideal for heavy power users).

•Heat vs. Electricity- Let's face it. We run on electricity to a far greater extent than low-grade heat. Saving a few kWh from cooling and warming a building using stored heat from a roadway is great, but it's a drop in the bucket compared to what that space probably uses in electricity. You can swing the temperature in that building by literally thousands of degrees with electricity, whereas a couple dozen is the most you can hope for with heat capture. Electricity can be dispatched thousands of miles away along existing infrastructure, but that waste heat is limited to a few thousand feet at best.

•Wheel Grip/Slippage [edit]. Conventional PV's have a flat surface since that's how glass is most cheaply manufactured. If modules are small enough (apparently these aren't) then a tile/lowered grout approach will still create a rough enough surface to allow for reasonable braking distance. Glass is really hard to create coarse surface roughness in when transparency must be maintained, so I'd be concerned that minimum braking distance would be increased. Icing conditions would be particularly concerning.