I'm actually an Insurance Broker and wanted to weigh in on this just so you have some insight from the industry...
Anytime you are behind the wheel of a motorized vehicle (or Bars if you are an ATV or moto guy guy like me) you are exposed to liability the same way you are when you drive your car out of your driveway at home.
That being said, contrary to a popular miss conception that I have heard over the years, your Auto insurance doesn't cover you on something that you own that isn't part of the contract for insurance you signed with them. Now there are exceptions like someone else's car may have coverage on it which entitles you to coverage but this is typically the state minimum as a "Permissive driver" (This means your name isn't expressly written on the policy) but that doesn't put you in a good spot when you are looking state minimums of 15K/30K and 5K.
The other miss conception is that OHV's are covered under your home owner's insurance policy. Every policy in the state of California excludes these vehicles from the policy. As a matter of fact they also exclude boats with motors over 50 horsepower!
You can insure buggies, quads, dirt bikes, etc with any Broker although many of the carriers won't touch them at all and others will only insure these items if they have your auto insurance as well (By the way, if you are insuring everything through one company they are more apt to give you a substantial discount on your auto and home too!) so if you are dealing with a "Single line" insurer you may have to look elsewhere.
Most policies will exclude any vehicle during "Sanctioned racing events" but that doesn't mean you are excluded when testing or out trail riding or spectating during an event in a race car that is not competing.
Feel free to ask me anything you have concerns about via inbox or on this thread if you want.
I'd be more than willing to go over your needs and give you real numbers if you want to take a few minutes to talk. I write business in California but can help you out in AZ, NV or Oregon as well.