luxcat, you have my sincere sympathy. My work commute is 16 miles round trip, which takes about 90 minutes a day total (on a good day), which is about as much time as I can stand in SoCal traffic. At least I don't have to get on the freeway.
When I was commuting to a class two nights a week after work, I was stopping for snacks or fast food a lot. The only way to cope with it (being a neat freak when it comes to my car) was to empty out all trash daily - preferably, as soon as I got out of the car; I'd find a trash can and just be done with it. I don't like to leave anything visible in the car because I've heard stories of people having their cars broken into because they left a fast-food bag on the floor in the back. Some people think hey, fast food = change in the car.
Anybody who leaves the house in their car has at least two opportunities to keep up with trash: when they reach their initial destination, and when they get back home. If a plastic bag helps you cope, use it! If it's just another piece of trash, lose it! :-)