The idea in itself is good (no matter how much people parrot Dave) as roads catch quite a lot of sun and have no building permit & zoning issues. Low hanging fruit if you can get the durability in order. This particular project most likely produced way less than expected due to shading by trees, fence posts, etc. Elevating the solar panels a couple of meters (as suggested by some people) is not going to solve the shading from the trees as you'd have to bring the panels 30+ meters high to reach over the top of the trees. For those not familiar with the Netherlands: as we have quite a bit of wind, trees are planted along roads and cycling paths to reduce the effect of the wind in the entire country.
Dave did some videos on the effect of shading on solar panels and the effect is suprisingy large. From my own solar panel install I can also see that shade on even a tiny part of a panel, which is part of a string, has a huge impact. I have two identical strings with 6 panels each. A tree branch which casts a shade over less than 10% of one of the solar panels already cuts the power output for the string the particular panel is in, almost in half. And this is with a modern inverter. Judging from the pictures from the Solar Cycleway it looks like each concrete slab consists of the equivalent of 2 individual solar panels with all cells in series at a 90 degree angle in respect to the direction road. Ofcourse this is relatively easy to manufacture but this configuration is about as bad as it gets due to shading from fence posts and plants next to the road. If the system was constructed using smaller groups of panels with an optimiser per group and taking possible shading into account to get to an optimum group sizing versus costs, the output would be way higher.