Thing is, "possible with future tech" simply doesn't matter in this case, as even if some material becomes available that makes these solar roadway panels technologically feasible, they still won't be economically viable!
This is because fundamental physics isn't going to change "in the future" and it will still be cheaper, and provide a large return on your investment, to just put normal solar panels on roofs / in fields & deserts etc!
For example, public supersonic air transport was made technologically possible in the mid 1970's (that's 40 years ago), and since then, aircraft technology has taken a quantum leap forwards, and yet, where are all the supersonic airliners today? yup, there aren't any. This is nothing to do with the absence or presence of technology,a and everything to do with practical economics!
IN 2050, putting solar panels under roads will still be a really silly idea, when for the same financial outlay, you could put probably 10x as many panels in a field next to the road, and those panels would probably return something like 3x as much energy (because they can be heliostatic and not have to be protected from vehicle traffic). That means you get 30x as much energy back for your cash (10x more panels, each being 3x more efficient), and chances are, they would be significantly cheaper to maintain etc (further improving your ROI)
So, if i said today, give me $100, and i'll give you $30, you would say. er, "no thanks" and if i ask the same question in 2050 (when we will all have flying cars, and robots and stuff?. Well, you would still answer "no thanks"!
Fundamentally, this is why pure engineering in itself is no indicator of the viability of any given project. (Betamax VCR anyone?)