preferRoads is config cpp value (operator or what the hell they are actually called...)
Won't do anything in scripts/inits/triggers etc...
If you put that into a init field, it just makes a global variable
preferroads and sets it
true...
(afaik)
Instead of that, try setting the car's skill all the way up...
That might not help much though because OFP AI is so crappy that they just don't know how to drive...
Randomly they actually can do that, but it seems to be quite a rare luxury
...