E.B.
I got two extra large ziplock bags (bigger than the gallon size) for my son.
In one, I put immediate emergency supplies, like a flashlight, gloves and hat, extra batteries for the flashlight, a reflecting triangle, magnetic combo flashlight/blinker, some wrenches and duct tape and electrical tape, jack knife, etc. - the kind of stuff you would need quick access to in the event of a breakdown or mechanical problem.
In the second bag, I put things that could come in handy in case he ever got stuck in a blizzard or found himself having to pull over for a long time (if there was a traffic emergency, or he ran out of gas in a remote area, for example. In that bag I put a space blanket, hand warmers, energy bars, a couple of bottles of water, notebook and pen, laminated card with his ID on it and phone numbers, some quarters and dollar bills, a first aid kit, some basic tools, etc. That bag wouldn't need to be grabbed as quickly as the first.