The typical house install in the UK does not have 25m2 of solar panels. The average panel is rated around 300W, and is about 1m2 . IIRC, you need to apply to the DNO for permission if you want to install an array capable of more than 4.5kW peak, meaning you end up with about 14 panels or so. We have 15 on our house and because they are east/west the peak is probably around 3kW.
On a really good summers day, generation peaks around 28kWh. On really crappy winter days it can struggle to get to 1kWh.
There are other issues with sticking panels on the floor, like the effect shading has, and how the inverters can deal with it. I guess you would probably use micro-inverters so that each panel has it's own inverter, to avoid the effect where part of the roadway is in shadow so the whole thing isn't working properly, but this approach adds cost.