When I had a regular office job, our summer hours would give us Friday afternoons off, and I'd head to Zion for the beach. To get on the road as fast as possible, I'd often grab fast food -- I'll do penance for that later.
Taco Hell is a bad choice: every one of their options likes to dribble bits: trying to drive and recover that stray tomato or bit of chicken is a scary prospect.
Arby's is only slightlly better: the sandwiches hold together, but the sauces which make it an interesting fast food choice also make it hazardous.
Hot dogs are no better: relish, onions and pickles will attempt to escape, regardless of how well you manage the waxed paper.
There pretty much isn't a sandwich without danger, although sliced deli meats are 'safer' than, say, chopped liver (but watch the mustard!)
I've mostly given up on that practice (partly because I'm working out of my house where lunch food is available), and at worst will eat the sandwich stationary and save the drink and/or fried things (frites, onion rings, etc.) for the drive.