Roughly one in seven drivers on U.S. roads carries no insurance at all, according to the Insurance Information Institute. UM and UIM coverage exist for the day one of them hits you. Here is how the variants work, what stacking means, and why the rules change at every state line.
The liability system has an obvious weak point: it only works if the at-fault driver actually carries insurance. Uninsured motorist coverage is the patch for that hole. When a driver with no insurance injures you, UM coverage steps into the shoes of the liability policy that should have existed, paying for your medical bills, lost wages, and related losses up to your UM limits. The problem it addresses is not rare. The Insurance Information Institute (III) estimates that roughly one in seven drivers nationwide is uninsured, and in some states the share is substantially higher. That means on a typical multi-lane road, the odds that a nearby vehicle carries no insurance are far from negligible. UM coverage is first-party coverage, a claim you make against your own insurer, but it is handled somewhat like a liability claim: the insurer evaluates the uninsured driver's fault and your damages, and disagreements are typically resolved through arbitration provisions in the policy rather than a lawsuit against a stranger with no coverage and often no collectible assets. That last point is the practical heart of it: a legal right to sue an uninsured driver is frequently worth little, while a UM claim is backed by an insurance company.
Uninsured motorist coverage splits into two parts, mirroring the structure of liability insurance. UM bodily injury, UM-BI, covers injuries to you and your passengers caused by an uninsured at-fault driver: medical treatment, rehabilitation, lost income, and pain and suffering, up to the UM-BI limits, which are written in the same per-person and per-accident notation as liability, such as 25/50. UM property damage, UM-PD, covers damage to your vehicle caused by an uninsured driver. UM-PD is the less universal of the two: it is not offered in every state, it often carries its own deductible, and in some states it includes caps or restrictions that UM-BI does not. Where UM-PD is unavailable or limited, collision coverage is the fallback that repairs your car after a crash with an uninsured driver, which is one of the quiet arguments for carrying collision. Some states treat UM-PD as unnecessary for drivers who already have collision, and some insurers will not sell UM-PD alongside collision at all. Because the menu of what is offered, required, or excluded differs meaningfully from state to state, this is an area where confirming your own state's rules matters more than general principles.
Underinsured motorist coverage, UIM, addresses the driver who bought insurance but not enough of it. Picture a serious crash caused by a driver carrying a state-minimum policy. Their insurer pays out the full bodily injury limit, and your losses still exceed it. UIM coverage pays the difference, up to your UIM limits. The two coverages are siblings and are often sold together as UM/UIM, but the trigger differs: UM requires the at-fault driver to have no applicable coverage, while UIM requires their coverage to exist but fall short. How the shortfall is calculated varies by state in a way that genuinely changes outcomes. In some states, your UIM limit is reduced by whatever the at-fault driver's insurer paid, an approach called offset or difference-in-limits. In others, your UIM sits fully on top of the at-fault driver's payment, called excess or add-on coverage. Identical limits can therefore deliver very different recoveries depending on the state. UIM claims also usually require you to exhaust the at-fault driver's limits first and to notify your insurer before accepting a settlement, procedural steps that can affect your rights. A licensed insurance professional in your state can explain which model applies where you live.
Stacking is the practice of combining UM or UIM limits across multiple vehicles or policies to increase the coverage available for a single accident. If you insure two cars, each with UM-BI limits of 50/100, a stacked policy could make up to 100/200 available when one of you is injured by an uninsured driver, because the limits on both vehicles add together. Stacking can happen within one policy covering several vehicles, or across separate policies in the same household. Whether stacking is permitted at all is a matter of state law: some states allow it, some prohibit it, and some allow insurers to offer stacked and non-stacked versions at different prices, with a signed waiver required to elect non-stacked coverage. Where stacking exists, the anti-stacking language in policies is heavily litigated, and courts in different states have reached different conclusions about when it is enforceable. For a multi-vehicle household in a stacking state, the choice between stacked and non-stacked coverage is a real decision point with meaningful consequences after a serious crash. It is also easy to get wrong on a form, which makes it exactly the kind of question worth putting to a licensed insurance professional who knows your state's rules.
It depends entirely on your state, and the variation is wide. A substantial group of states makes UM coverage mandatory, sometimes UIM as well, often at limits matching your liability coverage or the state minimum. Another large group requires insurers to offer UM/UIM but lets you reject it, usually in writing on a state-prescribed form; if the insurer cannot produce a valid signed rejection, some states treat the coverage as included by operation of law. A smaller set of states leaves it fully optional. Details that vary by state include whether UM-PD is offered at all, whether UIM is bundled with UM or elected separately, minimum and maximum available limits, and whether your UM limits are allowed to exceed your liability limits. Because the rejection forms have legal consequences, it is worth understanding what you are signing when a policy application asks you to accept or decline. Declining UM/UIM reduces the premium and removes a layer of protection against a risk you cannot control, since you have no influence over whether the driver who hits you bought insurance. That is the honest tradeoff, and the right answer depends on your circumstances rather than on a universal rule.
Often yes, but with state-specific conditions that matter. Most states treat a hit-and-run driver as uninsured for purposes of UM bodily injury coverage, since an unidentified driver's insurance, if any, cannot be claimed against. Some states go further and apply UM to miss-and-run crashes, where a phantom vehicle forces you off the road without contact; others require physical contact between vehicles as an anti-fraud safeguard, and a few require corroborating evidence such as an independent witness. UM property damage is stricter still: where UM-PD exists, hit-and-run vehicle damage may be excluded, subject to a mandatory deductible, or require the at-fault vehicle to be identified. Prompt reporting requirements are common, both to police and to your insurer, and missing those windows can jeopardize the claim. If you are struck by a driver who flees, the practical playbook is consistent everywhere: get medical attention, report to police immediately, document the scene, and notify your insurer quickly. Whether the resulting claim runs through UM-BI, UM-PD, collision, or some combination depends on your policy and your state, which is a useful thing to understand before it happens rather than after.
UM/UIM coverage typically protects a wider circle than people realize. The named insured and resident family members are usually covered not only while in the insured car, but also while riding in someone else's vehicle, and often while walking or cycling, if an uninsured driver injures them. Passengers in your insured vehicle are generally covered while occupying it, regardless of whether they live with you. That pedestrian-and-passenger reach is one of the more underappreciated features of UM coverage: it functions partly as personal protection against uninsured drivers, not merely as car insurance. As always, definitions matter and vary. Who counts as a family member or resident relative, how the policy treats household members with their own policies, whether coverage follows you into vehicles you own but did not insure on that policy, and how priority works when multiple policies could respond, between your policy and the policy on the car you occupied, are all governed by policy language and state law. Exclusions commonly target vehicles owned by you but not listed on the policy. If your household has multiple drivers, vehicles, or policies, mapping out how UM/UIM applies across them is a worthwhile exercise with a licensed insurance professional.
| Variant | What it covers | Key things to know |
|---|---|---|
| UM-BI (uninsured motorist bodily injury) | Injuries to you and your passengers caused by a driver with no insurance | Written in per-person/per-accident notation like 25/50; mandatory in some states, rejectable in writing in others |
| UM-PD (uninsured motorist property damage) | Damage to your vehicle caused by an uninsured driver | Not offered in every state; often carries a deductible; hit-and-run claims may be restricted |
| UIM-BI (underinsured motorist bodily injury) | Your injury losses beyond the at-fault driver's insufficient limits | States use offset or excess calculation models, which changes what identical limits actually pay |
| UIM-PD (underinsured motorist property damage) | Vehicle damage beyond the at-fault driver's property damage limits | Availability varies by state; collision coverage often fills the same role |
| Stacked UM/UIM | Combines limits across multiple vehicles or policies in the same household | Allowed in some states, prohibited in others; electing non-stacked coverage typically requires a signed waiver |
| Hit-and-run treatment | Unidentified drivers treated as uninsured for UM-BI in most states | Some states require physical contact or corroboration; prompt police and insurer reporting is commonly required |
Common enough to plan around. The Insurance Information Institute (III) estimates that about one in seven drivers nationwide carries no auto insurance, drawing on Insurance Research Council studies. The rate varies dramatically by state, from a small share of drivers in the states with the fewest uninsured to well over a fifth in the states with the most. Underinsured drivers, those carrying only low minimum limits, add to the picture. You cannot control what the driver who hits you bought, which is the entire premise of UM/UIM coverage.
It depends on which parts you carry and what your state offers. UM bodily injury covers injuries to you and your passengers. UM property damage, where available, covers your vehicle, often with a deductible and sometimes with caps or hit-and-run restrictions. Not every state offers UM-PD, and some insurers do not sell it to drivers who carry collision coverage, since collision already repairs your car regardless of the other driver's insurance status. Checking your declarations page shows which pieces you actually have.
In many states, no: insurers commonly cap UM/UIM limits at the level of your bodily injury liability limits, on the theory that you should not buy more protection from others than you provide to them. A few states handle it differently, and insurer practices vary within what state law allows. One practical consequence is that raising your liability limits often unlocks the ability to raise UM/UIM to match. If higher UM/UIM protection is your goal, the two decisions usually travel together.
Health insurance and UIM overlap less than they appear to. Health insurance pays medical bills, subject to its deductibles, copays, and network rules, but it does not cover lost wages, ongoing loss of earning capacity, or pain and suffering, all of which UIM can. Health insurers also frequently assert reimbursement rights against injury settlements, which can claw back part of a recovery. Neither coverage makes the other pointless; they address different slices of the same loss. How they fit together in your situation is a fair question for a licensed insurance professional.
Your UM claim proceeds with your own insurer either way, and being struck by an identified uninsured driver does not change the coverage analysis. After paying your claim, your insurer may pursue the uninsured driver directly for reimbursement through subrogation. You can also sue an uninsured driver personally, but recoveries are often limited by what the driver can actually pay, which is precisely why UM coverage exists. Many states additionally impose penalties on uninsured drivers, including license and registration consequences, though those penalties do not compensate you.
No. Stacking depends first on your state, which may allow it, prohibit it, or require insurers to offer both stacked and non-stacked options. Where both are offered, your policy documents and any signed waiver determine which you have, and electing non-stacked coverage usually requires a written rejection. Even in stacking states, policy language attempts to limit stacking in ways courts sometimes enforce and sometimes do not. If you insure several vehicles, asking your insurer directly whether your UM/UIM limits stack, and getting the answer in writing, removes the guesswork.