I know RP servers can feel like ghost towns, especially at certain times of the day when no one's around to RP, but NPCing traffic is a no no from me.
You can't gain any RP from an NPC. There's no actual meaningful interaction to be had with a set of mindless pixels. Also the amount of bugs making NPCs crash into players and vice versa would make driving on highways dreadful.
I know there's a bigger problem here which you've mentioned and seems to be the main reason behind your suggestion.
Realistically, criminals can't just hightail it down the highway with no obstacle in their way. I'm not sure what suggestion could fix this issue, but NPCing traffic would bring even bigger problems than this one.