You sign at the gate saying you can't sue the track.
As for insurance... you can't exactly get an insurance adjuster or police report for your "accident" at the track. You wreck it there, you've got a pile of scrap and you get to call your insurance to cancel your policy. There is talk that people have driven/trailered their car out to nowhere and dumping it, then reporting it as an accident/making a claim, but I don't know of anyone who actually did it.
I know of a lot of people who have crashed a lot of cars at these things... It sucks. I remember when my dads first mustang shell was put in the wall... not fun.