New roundabouts, more guardrails and sidewalks, and better crosswalks would sharply reduce deaths and injuries from car crashes around the country, including on Long Island, a new AAA report says.

Spending $146 billion on safety would — over 20 years — save 63,700 lives and prevent more than 350,000 serious injuries in the United States, according to the AAA report released Tuesday.

Nassau and Suffolk counties, like most of the East Coast, have much to gain because so many roads were designed decades before the advent of high-speed vehicles.

Both the Northern and Southern State parkways, for example, would be safer if overpasses were wrapped in guard rails, said Robert Sinclair Jr., AAA New York spokesman.

Flattening the depression between the Long Island Expressway’s eastbound and westbound lanes in Woodbury might have prevented an horrific 2016 crash, he said, in which six people were killed after a speeding Subaru Outback traveling east careened across the median, went airborne and struck two cars in the westbound lanes.

The United States ranked 32nd — just ahead of Mexico and Chile — among countries when 2015 traffic deaths were measured per 100,000 people, the report said.

“That’s the impetus for this report — road safety is poor,” Sinclair said.

U.S. traffic-related fatalities, after falling for years, rose 7 percent in 2015, and a “similar increase” looked likely in 2016, the report said.

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The AAA’s national report “scaled up” a previous study by U.S. Road Assessment Program that devised safety upgrades for 12,000 miles of roads.

Fixing four-way intersections, especially by turning them into traffic circles, was the most effective upgrade, which would cut deaths and serious injuries by nearly one-third, the report said.

Cars that collide in roundabouts strike each other at “glancing angles” instead of much more forceful broadside or T-bone crashes, explained Sinclair.

And four-way intersections have 32 places where vehicles can collide and 24 points for pedestrians, he said.

In contrast, roundabouts only have eight such “conflict points” for both vehicles and pedestrians.

Though the report also recommended clearing away “roadside objects,” including trees, that might not be possible on the land along the Northern and Southern State, which are considered parkland.

But rumble strips could be added to these and other Long Island roads, Sinclair said. Those upgrades would compress accidents by 9 percent; paving and widening shoulders would achieve a 3 percent reduction, the report said.

It also recommended building sidewalks and improving signals for pedestrians.

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That would achieve the same 20 percent reduction in deaths and serious injuries as adding barriers and clearing away nearby hazards, the report said.

Sinclair cited the benefits of deploying several strategies.

“A lot of those things would be working in tandem.”