Been researching Solar Roads and what I've found so far is that it's a wonderful concept in
theory, and none of the small-scale trials have worked all that well. It's not as good as panels tilted at the optimum tilt, but it's still better than fossil fuels and roads provide a lot of square miles of surface area.
They've learned a lot from those failures and because it is promising folks continue to try to make it work. Same for other technologies like solar perovskite panels, good in theory but hard to turn into reality.
Some of the problems are things you might not have expected, for example, some of the hardened road surfaces also reflected more noise.[
ref]
One of the advantages to a solar roadway is putting signage in the road via LEDs to reduce above-ground signage (prone to being hit by accidents, storms, or just needing to be replaced due to updates. I've seen signage in the roads, it's nice that it isn't obtrusive and blocking the view, but it's always been in areas where you have to go slow as the angle makes it harder to read - not sure I could read it at 60 mph so that claim sounds iffy.
They also have an interest in exploring heating them to make the roads safer, and reduce snowplow costs. That will probably be very limited by latitude, at high latitudes there's probably a lot more snow and reduced power due to the greater solar angle of a winter sun. Heated Solar roads may never be practical in northern climates (freezing/melting expansion damage, plow damage, non-optimum tilt). They might work in my neighborhood though if they can stand up to flooding.
One of the things I expected was that 18 wheelers would destroy solar roads as they do conventional roads (they cause
exponentially more road wear than cars). But there are roads here where they're not allowed to go (often reinforced by turns that are too sharp/narrow for them) which might be ideal.
I also wondered about the strength. Sure, we've all seen Tesla's Transparent Aluminum, so it's technically possible...but is it economically possible?
Turns out it's nearly the
end of the road for concrete and asphalt as the costs have been rising. The search for alternative road surfaces is big business with or without solar panels underneath. These may start popping up in new park lots near you as they start to be proven as they'll be much cheaper (e.g.,
bio-bitumen). There are also a variety of glasses that they say are harder and more durable than concrete, and as they're transparent, ideal for solar roads.
ref:
"The cost to develop a glass that will hold up in the fast lane of a highway? Fifteen [million] to 25 million dollars over three to five years," Brusaw added. "The cost in mass production? About $1 per square foot."
Concrete is about $90/cuyd depending on location, so if 11" thick (
ref) that's ~$3/sq ft in comparison.