Any alcoholic beverage consumed inside a moving automobile, but the term is used almost exclusively for beer (or perhaps malt liquor). The beverage is consumed by driver, passenger, or both. This activity is illegal in many states due to open container laws, but is still widely practiced and enjoyed!
"Since your sorry ass is two hours late, we won't have time to tailgate before the concert. Let's pick up some road sodas at the next liquor store."