Use the adjective vehicular when you're talking about something that has to do with a car or truck. Vehicular traffic is simply the number of motorized vehicles on a certain road at the same time.
Vehicular pollution is caused by the carbon emissions of automobile motors burning oil and gas, and vehicular accidents are the ones that involve cars, trucks, vans, buses, or motorcycles. You can see the noun vehicle when you look at the word vehicular, and that's what it's all about — from the Latin root vehiculum, "means of transport or vehicle."