Liability insurance is built on an assumption: that the driver who causes an accident has coverage that pays the people they injured. When that assumption fails, uninsured and underinsured motorist coverage is what fills the gap — and it is coverage on your own policy, not theirs.
The two coverages, and why they are different
Uninsured motorist (UM) responds when the at-fault driver has no liability insurance at all. In many states it also applies to hit-and-run, where the at-fault driver is never identified.
Underinsured motorist (UIM) responds when the at-fault driver has insurance, but their limits are not enough to cover the loss.
Both are commonly split further into bodily injury and property damage components, and states differ in whether both are offered.
This is coverage you buy on your own policy to protect yourself against other people's lack of coverage. That is counter-intuitive, and it is the reason many drivers decline it — they assume the other driver's insurance is the other driver's problem.
How UIM actually calculates — and why states differ
This is the part that is genuinely confusing, and it varies by state in a way that changes the number materially.
Suppose you carry $100,000 in UIM. The at-fault driver carries $25,000. Your damages are assessed at $80,000.
Under a reduction approach, your UIM limit is reduced by what the other driver's insurer pays: $100,000 minus $25,000 leaves $75,000 available from your UIM, so you recover $25,000 plus $75,000 — enough to cover the $80,000.
Under an excess or add-on approach, your UIM sits on top: you recover $25,000 from them plus up to $100,000 from yours.
Some states also apply a trigger requirement: UIM only becomes available if the at-fault driver's limits are lower than your own UIM limits. Under that rule, if you carry $25,000 in UIM and they carry $25,000 in liability, your UIM may never activate no matter how large the loss.
Which approach applies to you is a matter of your state's law and your policy language. It is one of the clearest examples of why generic insurance advice is unreliable.
Where it is required, offered, or optional
States fall into roughly three groups. Some require UM, UIM, or both. Others require insurers to offer it, with a written rejection needed to decline — and in several of those states, a rejection that does not meet the statutory form requirements can be ineffective, meaning the coverage may be deemed to exist. A smaller group leaves it entirely optional.
If you declined UM or UIM years ago and have since had a claim, whether the rejection was validly executed is a question worth asking.
What it typically covers
- Medical expenses for you and your passengers.
- Lost wages from injuries.
- Pain and suffering, in states that allow it under UM.
- Vehicle damage, where UM property damage is available — though a separate deductible often applies.
Coverage generally extends to you as a pedestrian or cyclist struck by an uninsured driver, and to household family members. The specifics are in your policy's definitions of "insured."
Hit-and-run
Most states treat an unidentified driver as uninsured for UM purposes, but usually with conditions: prompt reporting to police, and in some states physical contact between the vehicles. A near-miss that causes you to swerve into a barrier — a "phantom vehicle" claim — is treated differently depending on the state and may require independent corroboration.
The step people miss: consent to settle
This one costs people their UIM claims, and it is buried in the policy.
Most policies require you to notify your own insurer and obtain consent before accepting a settlement from the at-fault driver's insurer. The reason is subrogation: your insurer may have the right to pursue the at-fault driver, and settling with that driver can extinguish it.
Accepting a quick settlement and signing a release without notifying your own carrier can, under many policies, void the UIM claim entirely.
If an at-fault driver's insurer offers you their policy limits, that offer is a signal that your damages may exceed their coverage — which is precisely the situation UIM exists for. Notify your own insurer in writing before you sign anything.
How claiming against your own insurer works
A UM or UIM claim is made against your own carrier, which puts your insurer in the position of evaluating and potentially disputing your damages. That is an unusual relationship and it is worth understanding in advance.
Practically, it means the same documentation discipline applies as with any other claim: medical records, wage documentation, the police report, and photographs. Many policies also require you to submit to an examination under oath or an independent medical examination, and refusing can be treated as a breach of a policy condition.
Many UM and UIM policies also route disputes to arbitration rather than court. Check whether yours does, and on what terms.
What we are not saying
We are not telling you to buy UM or UIM, or how much. We do not sell insurance, we receive nothing if you buy any, and the right answer depends on your assets, your health coverage, and your state's rules.
What we are saying is that this coverage is on your own policy, that the calculation method varies enough by state to change the outcome substantially, and that the consent-to-settle requirement is a real trap that costs people claims they otherwise had.
Where to verify this yourself
- Your declarations page — whether you carry UM and UIM, and at what limits.
- Your policy — the consent to settle requirement, the definition of "insured," and any arbitration provision.
- Your state Department of Insurance — whether your state uses a reduction, excess, or trigger approach, and the rules on rejecting the coverage.
This is general education, not advice. Insurance law and claim rules vary by state and change over time. Nothing here is legal, financial, or insurance advice for your situation, and reading it does not create any professional relationship. For your specific case, consult a licensed professional in your state or contact your state Department of Insurance.