Sometimes offered separately, both of these do essentially the same thing. They protect you from the physical and economic harm that others do to you, and your passengers. It is often mistakenly believed (and declined on this basis!) that this coverage protects people who drive around without insurance, (the “uninsured”) and “dangit I won’t pay one penny for them!” If the other driver is at fault in an accident, and has no or very little BI insurance, UMBI coverage is the one you will want to be carrying.
Some states have made these coverages optional, giving you the right to decline them if you choose to do so. Because of the very high numbers of uninsured drivers in many states, this coverage is not the cheapest, and it is very expensive in some areas. Be sure to ask about it, because it may not be, relative to your budget. The expense, and whether or not you have health insurance, or carpool regularly, will be the factors you should consider in making the decision to include or decline this coverage.
How to Stack UI
This is yet another important reason to carry high limits of BI coverage, at least 100k/ 300k or more, because the more BI coverage you carry on other people, the more UMBI you can carry on yourself, and your passengers. You cannot insure yourself for more than you are willing to insure others. Additionally, UMBI will also replace your lost wages if you cannot work for a period of time due to your injuries.
In states with mandatory PIP coverage, though, this coverage would only benefit you after the limit of your personal injury plan had been reached. Florida, for example, requires all drivers to carry 10k in PIP coverage. If you are injured, it does not matter who is at fault or whether they have insurance, until the limit of the available PIP coverage is reached. Only then would the determination of fault and other liability coverages be looked at to provide additional coverage for you. This usually means your family as well. Keep this in mind: The limit of UMBI you select, if any, will also apply to all of the passengers in your car, up the maximum stated amount.