Insurance while racing street car?

Back in '70's when I took my parents '71 Road Runner to Fremont Raceway, Dad calls his insurance agent to report it. Don't know what happened, but I'm sure if the rate/coverage was increased it was not valid on the drag strip. Dad (a "cowboy") was only worried about his own liability, he could care less about my interest/hobby in bracket racing.

At that same time, I wrote a letter to a car magazine (I believe it was called "Super Stock Magazine") asking about insurance while racing. The magazine published my letter and answered it by saying tracks carry their own insurance. I never found out what, if any, insurance Fremont Raceway carried and neither do I know (to this day) what it may have covered at that time.

The answer to this question should be common knowledge, but it isn't.