When I started in the Insurance industry over 20 years ago, nearly everybody had health insurance that was "coordinated" or "primary" with their auto insurance. What that means is that your insurance from work (BCBS, Aetna, United Healthcare, etc..) would pay for your injuries in an automobile accident and your auto insurance would pay for what some additional coverages that weren't covered by your employers insurance. As the economy tightened in Michigan, most employers took steps to reduce the cost of their medical insurance. Most people saw their deductibles increase, some people saw coverages get reduced and nearly everyone's coverage no longer "coordinated" with their auto insurance. Like most items in a trickle down economy, the expenses are passed down while the profits are not. When the economy recovered, few if any employers returned this coverage. Now that the auto insurance companies are burdened with the cost of these medical claims, they have become very diligent in making sure insureds are paying a different rate when their medical insurance does not "coordinate". Some of the Auto Insurance companies that I work with do not have much of a change in price for "primary" vs "coordinated" coverage. Most of the companies end up having a pretty significant savings when you have "coordinated" medical insurance.
This lack of "coordinating" is just one of the pieces of the puzzle that causes the medical portion of auto insurance in Michigan to be so high. The biggest issue in my opinion is the medical facilities ability to charge whatever they want to for auto related injuries. If you go to the hospital for a non auto related injury they have to use a fee schedule and charge accordingly. Insurance fraud is another piece. There are people filing medical claims for injuries that did not occur from their vehicles and their are people that are faking injuries that are too difficult to detect.
A possible piece of the puzzle is the unlimited coverage. No other state has this coverage, however it is there to prevent innocent people from going bankrupt from an auto accident. I have insured multiple people that had medical claims over $1,000,000 from car accidents that would have gone bankrupt if they did not have the coverage. To me this is the most important piece of the puzzle to keep in place. No one should go bankrupt from something they did not cause.