As I've said before, I like the whole "label" concept that Blogger uses. I use it quite a bit. Today I mentioned Sauce in a post, and I didn't want to label it "Road Food" so created a new label for "Restaurant" and then went back and relabeled a few of my past entries.
So what's the difference? They are both places to eat, and a place to eat is a restaurant. Well to me, road food is a bit more like a diner. Some place regional, not too fancy. Not sure burgers and fries, but maybe Mexican as well - as different locations have their own "road food". Whereas a place listed as a Restaurant might not be so "colorful" or "local". Not a place you might just pull in off the road and grab a bite.
So that's the difference - perhaps not always obvious, but some distinction in my mind. :)