Most states require that drivers maintain continuous insurance on all registered cars. Letting a policy lapse — or canceling it yourself — will lead to a warning notice from the state’s DMV that will let you know how to proceed and, should you ignore it for 30 days, your vehicle registration will be suspended.
As a rule, if you want to cancel an insurance policy for a car you are no longer going to drive — temporarily or for an indefinite period of time — you should notify the DMV of your intent before the policy lapses or is canceled and turn over your license plates. This may be the case for broken cars or vehicles that you are going to keep in your garage.
Should you, however, want to drive that car every once in a while, like a family van you only take out on vacations or a sports car you drive on sunny days, you should look into limited use auto policies.