Nassau and Suffolk had about one-fifth of all state fatalities in car crashes in one category: backseat passengers 16 or older who had not buckled up, the AAA said Tuesday.

Suffolk had the worst record in the state, with 88 such deaths from 1995 to 2014. Nassau ranked third, with 70 fatalities, just one fewer than in Queens.

In all of New York State, 886 back-seat passengers in this category were killed.

Unlike 28 other states and the District of Columbia, New York does not require anyone older than 16 who is riding in the backseat to belt up, according to the nonprofit’s survey.

“What is particularly shocking to me is that we were the first state with any seat belt law,” said Alec Slatky, policy analyst, AAA Northeast chapter.

Despite heated opposition, then-Gov. Mario Cuomo in 1984 enacted the nation’s first seat belt requirement; only in 2000 was it expanded to include children aged 10 to 16 who sit in the backseats.

Noting deaths jump once teenagers no longer must belt in, Slatky said the AAA supports bills the legislature is considering requiring anyone 16 or older to wear seat belts if they ride in back.

Back-seat passengers from ages 16 to 24 “had by far the lowest rate of belt usage and accounted for more than half of the fatalities,” the survey said.

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“This is a major problem … If you look at just Long Island, it’s about 8 adults a year killed in the back seat of a car while not wearing a seat belt,” Slatky said.

Though people sitting in the back might feel they are at less risk of being ejected than those in the front, they are twice as likely to kill front seat passengers — becoming a “bullet” in the AAA’s parlance — than if they were wearing seat belts, it found.

Unbelted back-seat passengers are three times more likely to be killed and eight times more likely to be seriously injured than if they were buckled in.

Pondering why back-seat passengers, especially young adults, are not buckling up, Slatky said:

“I think part of it’s people think they are safer in the back seat; part of it is just bravado.”

And for young adults riding in cars driven by their peers, “the social norms in such a situation may discourage restraint,” the survey said.

These kinds of fatalities rise with the number of people who live in an area and how much driving they do, the survey found.

All of New York City’s five counties had 190 deaths — about twice the number in Suffolk.