Motorcyclists in Michigan can now ride without a helmet after Gov. Rick Snyder (R) signed legislation Friday repealing a 35-year-old safety requirement.

By backing the bill, Governor Snyder made Michigan the 31st state to provide that option. Motorcyclists who want to ride without a helmet must be 21 and have passed a motorcycle safety course within the past two years. The new requirement also is a boon for the insurance industry: motorcyclists must carry at least an additional $20,000 in medical insurance.

Snyder framed his decision as one of individual liberty: “While many motorcyclists will continue to wear helmets, those who choose not to deserve the latitude to make their own informed judgment,” he said in a statement released Friday.

Safety advocates bemoaned the decision, saying that safety studies show helmet laws lower health-care medical costs and increase public safety.

A March 2012 study by the University of Michigan Transportation Research Institute in Ann Arbor found that, had the Michigan law been repealed in 2009, the average cost per crash involving a motorcyclist would have increased 48 percent, from $213,770 to $317,031.

Data from the National Highway Traffic Safety Administration in 2008 showed that motorcyclists who do not use helmets are three times more likely to suffer a traumatic brain injury in a crash than those who are wearing helmets.

Tom Constand, a spokesperson for the Brain Injury Association of Michigan in Brighton, said the repeal is “unconscionable.”

However, motorcycle advocates in the state say the change is necessary to boost tourism and that personal safety should be a matter of choice for each rider.

Jim Rhoades, the legislative director of the Michigan chapter of American Bikers Aiming Toward Education, said in a statement released Friday that road safety “lies in rider education, car driver awareness, and license endorsement.”

Mr. Rhoades said Michigan lost “millions of dollars” each year due to the helmet requirement. Also supporting the repeal was the Michigan Licensed Beverage Association, a trade association representing bars, restaurants, and other liquor vendors in the state.

“Every year we watch our customers ride into neighboring states and very few motorcycles ride” because of the previous law, executive director Scott Ellis said in a statement.

The federal Highway Safety Act of 1966 required all states to have a motorcycle-helmet law on their books so they could qualify for highway construction funds and other federal safety programs.

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Today, 19 states and the District of Columbia have laws mandating helmet use for all riders, but most states' laws vary regarding the age of riders and if helmets are required for all low-power cycles such as scooters and mopeds. Illinois, Iowa, and New Hampshire are the only three states that have no motorcycle helmet laws, according to the Insurance Institute for Highway Safety in Arlington, Va.

Two bills to repeal the state helmet law were vetoed by former Gov. Jennifer Granholm (D).