If I understand correctly what you need to do is create a copy of the site model to use as operator. For example you can drop one operator by 15cm (or whatever - that's about typical here in the US) relative to the site model and use that to intersect with the road elements, then subtract the roads with upwards extrusion from the site model. (If you want to be really fussy you can make further, deeper copies of roads and operators to model the thickness of the tarmac and roadbed materials.) The sidewalks presumably can be intersected with the site model since they would be at grade.
If you want to maintain the results without keeping the operations you have top save as library parts. While this can be very useful at times I wouldn't recommend it in this case.
