Ah, the dreaded road trip. The bane of any dieter's existence. The cure? Plan ahead!!
If you know you are going to be driving for umpteen hours, bring your own snacks. Don't rely on gas station stops. You will end up getting the Twix or something equally not good for your. Pack your own dehydrated fruit. Or nuts. Pre-portion them out like I showed in Portion Control Made Easy. That way you are not eating the whole bag of trail mix. (Has more calories than you might think. Good for you though)
Plan ahead what you are going to eat if you stop at fast food along the way. Or use a smart phone app that tracks calories. That way you can make a good informed choice if you have to eat at those golden arches.