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You're driving below the speed limit and obeying the trafficsignals when another driver plows into your car.

You might assume the other guy's insurance company should payyour medical and car repair bills — but that's not the case inevery instance.

“Unfortunately it's not as simple as that,” says Insure.comconsumer analyst Penny Gusner. “The little details of the accidentreally do matter.”

For the other driver to be liable for the accident, and forhis liabilityinsurance to kick in, there must be evidence that he wasnegligent.

“Liability insurance is only going to pay if the injured partycan prove the other driver is at fault in court or out of court,”says attorney Benjamin Zimmermann, a partner with Sugarman &Sugarman P.C. in Boston. “If you can't prove negligence, you can'twin the case, and if you can't win the case, insurance companiesknow that and won't pay. One of the keys to a successful claim isto establish the other driver's fault early and thoroughly.”

The rules also vary by state, notes David Reischer, a New Yorkattorney and co-founder of LegalAdvice.com.

In states with no-fault auto insurance systems, your owninsurance generally pays for your medical bills, regardless of whowas at fault, and you're restricted in when you can sue otherdrivers for injuries. However, in most no-fault states an at-faultdriver may still be liable for property damage. It's a good idea tounderstand how the rules work where you live before you have anaccident because insurance laws vary widely by state.

If you don't have collision coverage, which would pay forrepairs to your car in an accident, you have to rely on theat-fault driver's insurance to pay for repairs.Here are fivescenarios when the other driver's insurance company may refuse topay out — even if you think it should.

1. The other driver has a sudden medicalemergency.

“A sudden incapacitating medical event is a defense that is morecommon than people might think,” Zimmermann says.

If a driver is suddenly incapacitated by a medical emergency — aheart attack or stroke, for instance — he may not be liable if hedidn't have sufficient warning before losing control of thecar.

However, a driver could still be found negligent if he shouldn'thave been driving with the medical condition or neglected to takecare of his condition.

Attorney Thomas Simeone of Simeone & Miller LLP inWashington, D.C., says he represented a client whose car was hit bya vehicle driven by a pregnant woman who passed out behind thewheel. Simeone won the case because he was able to determine incourt that the woman had enough time to pull to the side of theroad safely after she started feeling flushed and before shefainted. Her insurance company then had to pay his client'sclaim.

2. You're hit by a fire truck racing to anemergency.

Anytime you file a claim against a government, “you have to jumpthrough additional hoops,” Simeone says.

Local and state jurisdictions have varying rules and timelinesfor filing claims against them, and the standard for proving anemergency vehicle driver was liable is much higher than thestandard for other drivers.

Generally if the siren and flashing lights were on, then you'dhave to show gross negligence on the driver's part, not justnegligence, Simeone says.

“A lot of lawyers don't take those cases,” he adds.

3. The other driver hits you because of an accident witha hit-and-run driver.

Depending on the state, you may be able to make a claim underyour own uninsured motorist coverage, Reischer says. Uninsuredmotorist coverage covers your injuries if an uninsured driver, orin some states a driver who flees the scene, causes anaccident.

Or imagine this scenario: A mattress falls off a pickup truck,causing another driver to crash into you, and the pickup drivermanages to escape without anyone getting the license number. Youruninsured motorist insurance could come into play in this exampleas well, Reischer says.

Keep in mind that your word about a vanishing vehicle is notsufficient. You'll need evidence, such as eyewitness testimony,vehicle damage and a police report to back up the claim.

4. A thief driving a stolen vehicle hitsyou.

Generally an auto insurance policy covers you and other licenseddrivers in the household who are listed on the policy and anyoneyou give occasional permission to use the car.

“When a thief takes a car, there's no permission or consent,”Simeone says.

So the car owner's insurance wouldn't pay. The thief'sinsurance, if he had a policy, probably wouldn't pay either becauseinsurance often doesn't apply to criminal or intentional acts,Zimmermann says.

“You can sue the thief, but good luck,” Zimmermann says. “Evenif you win, the chances of recovering any money damages areslim.”

In some states, the car owner might be found at least partiallyliable if he did something negligent that led to the theft, such asleaving the keys in the car with the car running, Reischersays.

Or your own uninsured motorist insurance, which covers yourinjuries if an uninsured motorist causes an accident, might kickin.

“Uninsured motorist coverage is something people don't haveenough of,” Zimmermann says. “They'll buy $100,000 of liabilitycoverage and only $25,000 of uninsured motorist insurance. Whenthey do that, they're protecting other people more thanthemselves.”

5. A driver hits a deer, loses control of the car andcrashes into you.

“Sometimes you can have an accident in the true sense of theword — where no one is at fault,” Zimmermann says.

This could be the case if a deer appeared out of nowhere andleapt in front a vehicle, causing it to crash into someoneelse.

But Simeone says the other driver could be at least partiallyliable if he did something careless, such as drove too fast.

Finally, keep in mind that liability isn't always an either-orproposition. In states with comparative negligence laws, liabilityis calculated on a percentage basis. One party might be 30 percentliable, and the other party 70 percent.

The insurance company's decision isn't final. Depending on thestate and the claim, you can take the case to court.

“You can sue anyone,” Zimmermann says. “The question is can youwin.”