Roads in Adventure mode are an absolute mess, loads of trees and cliff faces that require a jet pack to get up, sometimes they're even undercut! There's also the issue of some bumps causing any vehicle to suddenly flip over or do a hard turn without any steering input. However, this is far less of a problem if you're not struggling with trees & very steep gradients as you can see when this is likely to happen.
In the small 2km and 4km maps, also 8km maps if you've not got a heavily forested map, this is fun to fix so you can use vehicles on them. However on the larger maps it's an absolute nightmare and basically makes the road system completely useless. It's quicker to get your self a battleship, in skills tree mode, and then run zigzag across the entire map than it is to follow the roads.
It seems that there are already some rules applied to the roads; If the terrain is too high they end up being at the bottom of a canyon. Could This be extended to:
1. No trees in the road, or at minimum keep the central 80% of the road clear
2. The surface gradient is no steeper than the player can run up.