honestly, im with heather - quit looking at all the reasons you cant, and look at the reasons you can. i mean, isnt it WORTH your health to pack a lunch if you know that you cant trust yourself with a drive through (trust me - im talking to myself just as much here). ice packs, insulated thermoses, there are a ton of ways to keep food hot and cold.
also, if you have the time to go to subway, can you use that time to go home if you forgot your premade lunch?
the other thing - my friend does this a lot - she makes a big batch of soup or stew or whatever and preportions it for the week, then its ready to go. in the time that you waited for a subway sandwich (which i do not care for at all) you have something you know is totally nutritious