Howdy.
I find it nonsensical to restrict building on roads... Much like I find it nonsensical roads are prebuild, rather than following the most logical courses between settlements, both existing and newly arising.
From historical perspective, settlements predate roads. Roads are build to link settlements. Once they exist, other settlements arise aside these roads to cater to various needs of its travellers and serviced towns. Dealing with these settlements and roads arising irp should be the ideal LotC goes for.
From a mechanical perspective, regarding speed bonuses... This may need more staff engagement with road-maintenance. Regarding warfare, it may require better ways of dealing with lone settlements, without engaging in full blown war with a tile and its associated nation...
But, control of roads should also open up opportunities for proper smuggler rp and finding 'alternative' routes into places.
If a road is closed, it needs to be obvious what the rp reasons for it are and what alternatives there are for travellers finding themselves stuck in front of a blockade.