I can only really think of Highway Hunter (dos shareware) as a quintessential shmup

Depending on how loose you are with the definition of shmup, Traffic Department 2192 might also qualify (dos shareware)

I can think of several other top down car games where you shoot at stuff, but it is usually in the context of a race (IE: death rally). So I think those are more racers than shmups.
It would be interesting to see a modern take on the idea - something bullet hell/dodonpachi like, that takes place on a highway

Can anyone think of any others?