The data and the proposed rules came as hundreds of thousands of motorcyclists began heading to the District for the annual Rolling Thunder event Sunday. (Michael S. Williamson/The Washington Post)

Around midnight one evening last June, Forrest Williams struck a curb on his Harley-Davidson motorcycle and catapulted to his death.

On a Saturday afternoon about a month later, Timothy J. Humphrey was on his Harley- ­Davidson Sportster when a Toyota sedan bolted through an intersection into his path. The pale blue outline police drew where his body landed lingered at the crossroad for months.

Both deaths happened in Anne Arundel County. Both riders were wearing substandard motorcycle helmets, police said. And both accidents took on new relevance Wednesday.

New data indicate that despite the deaths of Williams, 54, and Humphrey, 30, the number of fatal motorcycle accidents nationwide decreased in 2014, for the second consecutive year.

On the same day those statistics were released by the Governors Highway Safety Association (GHSA), the federal government took steps to combat use of poorly constructed motorcycle helmets that don’t meet U.S. Department of Transportation standards.

The data and the proposed regulation from the National Highway Traffic Safety Administration (NHTSA) came as hundreds of thousands of motorcyclists began heading to the District for the annual Rolling Thunder event Sunday.

As overall roadway deaths have generally declined in recent years, deaths among pedestrians, bicyclists and motorcyclists have remained stubbornly high. In all three instances, the victims don’t benefit from advances such as air bags and collision avoidance systems installed in cars.

The GHSA recently pointed out in another safety report that while overall roadway deaths dropped 28 percent over 10 years, motorcycle fatalities climbed by 26 percent.

“The number of motorcyclist deaths on our roadways is still unacceptable,” said Kendell Poole, GHSA chairman and director of the Tennessee Office of Highway Safety. “While we support technology advances such as anti-lock brake systems and traction control, state laws and behavioral changes are critical to saving more motorcyclist lives.”

The GHSA on Wednesday projected that 4,584 motorcyclists died in crashes last year. Definitive final totals for 2014 are being compiled. That number is about 1.8 percent less than the 4,668 recorded in 2013. There have only been three decreases since 1997.

“By far, helmets are the single most effective way to prevent serious injury and death in the event of a motorcycle crash,” Poole said.

Only 19 states and the District require motorcyclists to wear helmets. They are mandatory in Virginia and Maryland, but 31 other states have repealed their helmet requirements.

NHTSA on Wednesday proposed changes that would further define which helmets meet federal requirements and which do not. The agency wants to help police identify “novelty” helmets that do not meet the minimum performance standards.

A study done in hospital emergency rooms and cited by NHTSA found that 56 percent of riders wearing a novelty helmet suffered serious head injuries, compared to 19 percent of riders who were wearing a DOT-certified helmet.

“Wearing a helmet that meets DOT standards can literally mean the difference between life and death,” NHTSA Administrator Mark Rosekind said. “Our proposal ensures that when motorcyclists put on a helmet it offers that life-saving protection.”

The proposal establishes preliminary screening criteria to help law enforcement agencies quickly identify helmets that don’t meet the minimum performance requirements. The screening involves examining the thickness of the inner liner and outer shell, and of the liner’s ability to absorb crash energy.

GHSA supported the federal action.

“This will help eliminate potential confusion among consumers and make it easier for law enforcement officers to enforce state helmet-use requirements,” said Jonathan Adkins, the GHSA executive director.

All 50 states and the District provided GHSA with preliminary motorcyclist fatality counts for the first nine months of 2014. Compared with the first nine months of 2013, motorcyclist fatalities decreased in 27 states, increased in 19 states and remained the same in four states plus the District. In Maryland, the number rose by two, to 57 deaths. In Virginia, it was up by nine to 73 deaths.