There's like, maybe a truck length of space between the wall and the seemingly closest obstacle. Not to mention, RII's don't apply the brakes. They won't bring the truck to a dead stop, they probably wouldn't even hardly slow the truck down at all. Also, if a truck was rolling, it could easily hop the wall, I'll grant that it's unlikely in this case, but still, this is why most shows typically block off a lot of seating.