Well, the first problem is the cost. I've tried to come up with a word for it. "Insane." "Ridiculous." "Comical." But really, the only one that fits is "impossible." Using their own calculations, about 68 billion panels would be required to cover America's roads, parking lots, driveways, bike paths, etc. (why do all of them? Hey, it's their idea, not mine) Even at a very lowballed $200/panel, we're kicking off at ~13.6
trillion dollars. Now, we need ~900 billion square feet of textured, tempered glass. Basic tempered glass panels can be in the $25/ft^2 range, but this will require texturing. Even if we go with $25/square foot, that's another ten trillion easy. And we haven't started wiring the stuff up into a grid, getting the bajillion LEDs, storage for this vast quantity of power, the heating elements to melt snow with, pressure sensors to detect animals, control circuitry because this whole system is supposed to be "smart." If you told me this would cost $50 trillion, I'd think you were being optimistic.
Now, a reasonable man would stop there. Fortunately, I am not a reasonable man.
Next we analyze its basic functions: being a road, and being a solar panel.
1) Is it a good road?
Tempered glass is a terrible surface to drive on. They texture it because flat glass would never have the friction we need to drive safely. They claim this textured surface has a high friction level... and they demonstrate it on video by driving a small tractor on it at minimal speeds. Tractors have giant tires with huge treads so they can claw their way across a soft field. Show me the test with an actual car slamming on the brakes at 60mph. Over time, this surface
will be worn smooth. Tempered glass just isn't that durable. This criticism was raised early, and so often that it ended up in a "rebuttal" to common criticisms of the solar roadways. They point out that tempered glass is very hard, much harder than the asphalt roads we drive on now. They compare numbers on a hardness scale whose name I can't remember.
Problem: they used the numbers for the goey tar stuff, not the actual asphalt. Asphalt also has gravel in it. Rocks are hard. Also... being very
hard is not the same thing as being very
strong. The ultimate test is easy: your car has tempered glass windows, most likely. Grab any broken-off piece of concrete or asphalt from any road you like. Put it on your car window and scrape it around. Gonna scratch the hell out of it, aren't you? Now, multiply that by five years of driving cars over this glass.
I'm also skeptical that a half-inch thick piece of tempered glass can stand up to the weight of a semi truck.
2) Is it a good solar panel?
No.
Right off the bat, we've thrown away ~31% of the annual solar generation ability, because we took the panel and placed it flat on the ground. We live in the northern hemisphere, optimal power is found by tilting your panels to the south because that's where the sun is. We've then thrown a glass cover over the panel which the solar roadway guy claims reduces solar capture by another 11%. I'll take his word for it. Then we're going to make it worse by scraping the hell out of the glass, as previously mentioned. Plus, every bit of dirt, grime, grease, garbage, skid marks, dumped coffee, and bird crap is going to make the panel worse. Solar roadway guy has a genius solution: use "self-cleaning" glass, like they have on some smartphones. I guess nobody told him that oleophobic/hydrophobic surfaces are that way due to greatly reduced surface friction. If you try to drive on a hydrophobic surface in the rain, you will die. Oh, and a full parking lot... is covered in cars. We did want to do the parking lots, after all.
Even pessimistically, we can double the efficiency of this setup by moving the panels 50 feet to one side, tilting them, and
not driving on top of our solar panels.
The secondary features of the road get worse. Like, laws of thermodynamics worse. Melting snow by using solar power happens... in the spring. Not sooner.