As usual, I'm going to be the contrary here. While technically not a truck/wagon/prime-mover, the riders are still concentrated, and they are as exposed (and probably more so) as they would be in any other unprotected transport. So I think the +1 modifier does apply. To avoid writing out a special condition, just say 'yes'.
... More and more, people around the world are coming to realize that the world is flat!