What is the difference between an 'auto accident' and a 'traffic accident'?
Largely semantic, like "pop" and "soda" mean the same thing.
Where I used to work in LA, they were TCs (Traffic Collisions).
Here in Honolulu, they're Auto Accidents.
They both mean the same type of incident, basically some sort of car crash. Whether that's a single vehicle, two vehicles, a pickup truck and a semi, or a minivan, or SUV rollover, or a pedestrian (or bicyclist or motorcyclist or moped rider, etc) was struck in a hit and run, or any other variation.
Just a different regional way of saying the exact same thing, at least in my experience.