I've ran in to this myself. While I've been learning to dodge this npcs while driving. There are some instance of it that leave me scratching my head. For instance, I've made a few jumps that have cleared the NPCs, and some where I'm clear of their heads and get blown up, I "ran" into them some how.
As far as the time trials go. These, I currently seldom bother with. Not because of this. But because the vehicle drive like a train with no tracks. They have little to no handling, and hitting a bump with one, is the same as hitting a jump outside of them. The biggest challenge here is just getting the damn thing to turn and not bounce off the track.
As for the happy middle ground. The best way I see to handle this is simple. A runner can handle running over 3 small enemies. A roller should be able to handle, say 9, triple that of a runner. Now how to handle the arkfall problem, so that people don't use them as a weapon. Simple, make the runners/rollers agro-magnets. One shows up and it automatically taunts all near-by enemies. THis ensures a good face pounding of said vehicle. Secondly on this, since this can be abused, have subsequent uses for this reason extend the vehicle respawn timer. First time this happens, normal respawn time. Second subsequent use, double respawn timer. Third subsequent use, triple respawn timer.
Another way to go about this is to make arkfalls vehicle free zones. Much like the instance maps. The area surrounding an arkfall crystal disable vehicles. This way, they can still be used to get around in major arkfall sites, just not at the arkfalls themselves.
