When New Jersey State Trooper Sean Cullen was fatally struck while at a motor vehicle crash this week, he was in one of the most vulnerable positions for a state police officer: On a busy expressway, without even the protection of his cruiser.

In or out of their vehicles, New Jersey's highways have been deadly for its state police force.

The department has seen 70 line-of-duty deaths since 1923, the earliest death recorded, according to the Officer Down Memorial Page, which tracks law enforcement deaths nationwide. Nearly three-quarters of those deaths - 52 - have been in traffic incidents.

"If anything bad is going to happen, it's traffic-related," said Trooper Lawrence Peele, a State Police spokesman.

Cullen, 31, who was hit while standing outside his vehicle investigating a wreck on I-295 in West Deptford on Monday night, was the third officer killed in the last 10 months. Troopers Eli McCarson and Anthony Raspa died in crashes.

Nationwide, traffic incidents have been the leading cause of police fatalities in 15 of the last 20 years, according to the National Law Enforcement Officers Memorial Fund.

Last year, 52 of the 124 officers who died were killed in car or motorcycles crashes, or were struck outside their vehicles, the fund's data shows. Gunfire accounted for the deaths of 42 officers.

Between 2005 and 2014, 616 officers nationwide died in traffic incidents, outpacing the 539 slain in shootings, according to the fund.

"A lot of law enforcement officers spend a lot of time in a vehicle," said Steve Groeninger, a spokesman for the fund. "So when you do that, you're going to be increasing your risk of being involved in some type of crash, especially folks who do a lot of traffic enforcement."

Traffic deaths can take a particularly high toll on state police forces, which spend much of their time patrolling interstates.

For the Pennsylvania State Police, traffic incidents accounted for 53 of its 99 officer deaths, according to the memorial page figures. In comparison, 23 percent of deaths in the Philadelphia Police Department's history are traffic-related, while two of Camden's nine fallen officers died in crashes.

"We really are conducting business in dangerous situations," said Sgt. Jeff Flynn, another New Jersey State Police spokesman. Troopers are often responding to accidents in inclement weather or are standing on the shoulder as they investigate a crash. (Stranded motorists are advised to stay in their vehicles, an option not available to troopers.)

A 2011 study by a division of the International Association of Chiefs of Police made recommendations for curbing traffic deaths that included focusing on seat belt use, distracted driving, reflective markings, and "move over" laws.

New Jersey's law, requiring drivers to change lanes or reduce speed when approaching stopped emergency vehicles, has been in effect since 2009.

The 22-year-old woman who struck Cullen, whose name has not been released, has been cooperating with investigators, police said. Officials said it was too early to determine whether she would face any charges, though none had been filed as of Friday.

Flynn said traffic safety is stressed at the state police academy, from which Cullen graduated in August 2014.

"We train troopers not to feel too safe on the roads," he said. "When you're out there, you're unprotected and you're vulnerable."

Public viewings for Cullen will take place from 2 to 7 p.m. Sunday and 8 to 11:30 a.m. Monday at St. Charles Borromeo Church on Branch Pike in Cinnaminson. A Funeral Mass will follow Monday's viewing.