In theory, rental car insurance is a worthy hedge against an unforeseen event that, through an accident or even no fault of your own, could saddle you with tons of debt. For the vast majority of us, however, the reality is that it's superfluous at best and a total waste of $30 a day or more.
For most everyone in the U.S., your regular auto insurance policy almost always includes some form of collision insurance, which protects you even with rentals (and would save you up to $19 on many rentals). Likewise, many credit cards include car rental coverage against theft or damage to the vehicle (but not liability), so long as you use the card to pay for the rental, saving you another $7 to $14 a day. And some credit cards offer their own coverage, such as a flat-rate rental policy that offers substantial liability coverage and is triggered automatically any time you rent a car. So before heading out on your next trip, do as the Insurance Information Institute recommends and confirm in writing what your auto-insurance credit cards offer (and specifically exclude), so that you can limit or even eliminate unnecessary coverage.