Your Low-Carb Emergency Kit
This can be as simple as a bag of nuts or something more elaborate. It should contain nonperishable low-carb foods such as:
Protein bars (but watch for suspicious ingredients such as maltitol, and watch the carb count on the bars)
Nuts, seeds, or low-carb trail mix
Jerky
Snack sausages
Cheese & salami travel pac (Trader Joe's)
Protein mix to put into coffee or diet soda
Tuna in foil packs or small cans (but probably not on public transportation!)
Soy nuts
Sturdy vegetables like celery will be fine without refrigeration for a day or so
Small wax-coated cheeses such as "Mini-Bel".
Flax seed meal for extra nutrition and fiber. Besides adding it to yogurt, cottage cheese, etc, you can add hot water to it to make a cereal at the hotel continental breakfast. Sometimes there's even peanut butter to add to it. (Also sweetener and a pinch of salt are good additions.)
After recommending to avoid nuts and Atkins bars, those are the very things that might work best for the car. But Quest Bars are better quality ingredients than Atkins (GNC and Trader Joe's has them).
Some of these items can also be had in gas station convenience stores...even low carb snack pacs now.
TIP: You can always scrap all the toppings off a pizza, and often pizza places have salads/antipasta to make it a meal.
Edit: half hour later, there is a new post on DietDoctor on Low CarbTravel
it is getting a bit spooky how often similar posts show up for me
http://www.dietdoctor.com/low-carb/travel. Good Lists!
Btw, cheese crisps...check out Costco Whisps.