Biggest problem with solar roads/cycleways is that you need to do the maintenance on a daily basis, to keep the way clear of mud, leaves and debris.
There is this thing called rain in this part of the world. Something like 15 rainy days / month on average.
What people also do miss is that the Netherlands are a crowded place. Hell, they drain land from the sea to have some more space (the things you do when you are surrounded by the sea in the North and West, the Germans hang around in the East and Belgium is in the South)
Australia: 2.8 inhabitants / km^2 (yes, I know, big void in the middle)
South Africa: 42.4 inhabitants / km^2 (yes, also some vast areas)
Netherlands: 406.4 inhabitants / km^2 (and also areas that are uninhabitable, because they are simply too wet)
There will be a day when the last roof is covered with solar panels. When you don't want to waste more dry land for putting up solar panels on the ground. When you want to keep space for farming, growing vegetables and flowers and weed, and not waste additional land. Then you won't look for the most efficient way to place solar panels, because you have already used all that efficient space. Then you will use less efficient means to squeeze some more energy out of already used space.
And then it is a good idea to have some reliable figures ready to estimate how bad things will be. How do you get them? You measure.