--- Post updated --- A little context, I enabled the traffic in the utah map, and well..I got giant cars.
Thats a mod, the Brukell Bigran https://www.beamng.com/threads/bruckell-bigran.54990/ The traffic app just spawns random cars, and it chose to spawn that modded car in.
Oh dang, that's hilarious! By default, when the traffic system creates the spawn group, it uses type = "car" and author = "BeamNG". Looks like this will possibly have to be revised. A quick fix now would be to go into the mod folder, edit the info.json file, and change the author line to anything else.
It just won't get added to the group by default when you select "Spawn (Normal)" in the radial menu. You can still set (almost) any vehicle as traffic if you spawn them manually and then select "Start Traffic" in the radial menu, or use the Multi-Vehicle Spawner in the F10 World Editor > Tools menu to create and run a custom group of vehicles.
Is there any way to set specific vehicles to participate in traffic? "Start Traffic" seems to set all spawned vehicles on the map to participate in traffic (when I'd like to leave a few out, so I can drive them myself). Or am I just stupid?
start the traffic app and have traffic drive around. whilst they drive around you can spawn more cars that will not be affected by the ai unless you activate the traffic again... also, the pigeon trailer (I dont know which mod) seems to spawn as a car as well....so I now just spawn the cars manually