The red trucks have a route of 5-3-6, but inadvertently cross a choose signal meant for the buses, which run 1-2-3-4.
The second red truck gets stuck at the choose signal, despite stop 5 being far away and clogs the bus route.
(http://i199.photobucket.com/albums/aa131/AEObikes/simutrans/simscr87.png)
Set a waypoint after choose signal should cure this. And of course it is intended, as the choose sign cannot read your mind, that you intended it only for buses.