If legal in your locale, a "trunk rifle" for possible emergency use can't be too bad of an idea. After all, two zillion rednecks with pick-'em-ups and thutty-thuttys can't be wrong.
Depending on where you live, however, a loaded long-gun may not be legal for you to keep in the car. (In a lot of Southern states, that's not an 'anti-gun' thing, it's an 'anti-road-hunting' thing.
) A friend used to use a Steyr-Mannlicher Stutzen
for his trunk rifle, along with a bandolier of ammo on en-bloc clips. His reasoning? It was fast to load (pull back the bolt, toss in a clip, and start shootin'.) It was cheap, and thus would be no great loss should it get stolen. Lastly, he could hit out to 200yds with it just fine. When has any good law-abiding citizen had to shoot out to 200yds? Probably never, but wouldn't you hate to be the first one and wind up having to flag down some passing pickup and ask Billy Bob if you could borrow his Marlin, "just for a minute?"