I am on the mission where I am suppose to turn bison into roadkill. The AI is set so hard that I can not do it. The bison run into the trees or stand in front of cliffs or lakes. Also they are very rare when I am in a vehicle but as soon as I get out of the vehicle they are everywhere.
If you are referring to the mission for the Grill Streak guy where you bring back Bison road kill I'd recommend going to the Bison area directly West/Southwest of his place. If you don't see any at first (which happened to me often) drive around that area from one end to the other and they'll eventually spawn/respawn or at least that is how they did for me. In a regular pickup it will take 3 to 4 fast collisions to bring one down so you can loot it. It takes a bit of time, and if you have the repair torch be sure to repair vehicle often.
Yeah, this is easy to do man...
Get the perk where your vehicle fixes itself, find a bison spawn point (google images and youtube) and go to town. There is an area I saw on youtube where they spawn in a flat field and it's easy to run them down. Like fuller6402 said, it will take a few passes to kill them, but it's not hard to do.