I'm not 100% sure, but I think:
If you set the behaviour in the waypoint to safe or carless or if you put --this setbehaviour "CARELESS" -- in the unit's init field , he will follow the roads,even go out of his way to stay on a road. So you would only need extra waypoints at hard left or right turns.
If you set the waypoint behaviour to combat he should drive from point A to point B straight line style, roads or not. --this setbehaviour "COMBAT"--
Or maybe there is somthing in the town the AI sees as a obsticle, even if its not.
I might be way off. I am after all, a editing noob