Could heightened tensions over race and policing have an impact on highway safety? The head of a national traffic safety organization and some law enforcement officers think so.

Since the August 2014 shooting of a young black man by a white police officer in Ferguson, Mo., some officials have suggested that the killing and its violent aftermath have made police more reluctant to carry out their basic duties, in what they’re calling the Ferguson effect.

Some, including the head of the FBI, have argued that violent crime has surged in some cities because police are on the defensive following a series of high-profile police shootings after Ferguson – an assertion disputed by criminologists and others, who say there’s no evidence to prove that the Ferguson effect exists.

But Jonathan Adkins, executive director of the Governors Highway Safety Association, and some law enforcement officials believe the Ferguson effect might be a factor in the apparent slowdown in traffic enforcement by police in some areas and an increase in traffic fatalities nationwide. Though they acknowledge there is no evidence to support their theory.

“[I]f you’re getting ready to make that stop at 11 o clock at night [or] at midnight — are you going to pull somebody over for what many people see as a small violation, i.e., speeding or seat belt use?” Adkins said.  “And we’re hearing this in a lot of different places across the country. They’re saying, ‘You know what? It’s just not worth it to pull that guy or gal over for a seat belt violation.  I may end up on YouTube. My own safety be at risk. I’m only going to pull someone over if they’re doing something serious, not a traffic violation.’ ”

At its annual meeting in Seattle recently, the Governor’s Highway Safety Association held a panel with law enforcement officers discussing policing in an “era of heightened sensitivity” and a “downturn in traffic enforcement.”

“It’s a very sensitive topic, to say the least,” Adkins said in a followup email.

The so-called Ferguson effect also received renewed attention in June when a National Institute of Justice researcher —  who had initially been skeptical of the theory — reexamined the thesis and said it could a plausible explanation for a large and nearly unprecedented increase in violent crime in some cities. But he also suggested that more data would be needed, particularly on arrests, traffic stops and other self-initiated police activity.

A recent study by Johns Hopkins University researchers — which was the subject, along with another study, in an op-ed in The New York Times this week examining pro and con arguments for the Ferguson effect — found that arrests in Baltimore fell by 19 percent from August 2014 through April 2015 and suggested a possible link between the Missouri police shooting and arrest patterns for less-serious crimes. Particularly noteworthy was a 33 percent drop in arrests for driving violations, the study found.

What’s more, arrests for minor offenses fell even more following the death of Freddie Gray, a Baltimore man who was critically injured while in police custody — and crime rates rose, according to the March 2016 study.

Adkins and others who believe a Ferguson effect has had an impact on traffic safety caution, however, that their views rely on anecdotal evidence and traffic data from a few states, including Missouri. They also say other possible reasons for a pullback on traffic enforcement and a rise in fatalities include a wave of retirements among baby boomers and even differences in attitudes toward work among the younger officers replacing them.

Washington State Highway Patrol Chief John R. Baptiste said he’s heard other law enforcement officials talk about the possibility that the spotlight on race and policing has caused some officers to pull back. But he hasn’t seen evidence of that in his agency.

Baptiste said that for Washington’s highway patrol, the slowdown in traffic enforcement is more about a shortage of officers. With the good economy and baby boomers’ retiring, his agency of nearly 2,500 officers is understaffed by about 200 officers. That has translated into lower numbers of traffic summonses, Baptiste said.

“We’re seeing a tremendous decline in DUI arrests and that sort of thing because we just don’t have a number of boots on the ground,” Baptiste said. “When agencies are short and being impacted by large budget cuts, unfortunately, one of the first things to go for many agencies, particularly smaller agencies, is traffic units.”

But Baptiste also sees a possible Ferguson effect in a larger, macroeconomic way: the tense climate between civilians and police has made it harder to recruit new troopers, especially at a time when the economy has created other job opportunities.

“Law enforcement is not one of the most popular things right now in the minds of a lot of our citizens,” Baptiste said. “So, therefore, that has a tendency for folks to ask themselves, ‘Do I want to go work for less money and also subject myself to being unappreciated in a profession?’ So that’s causing a lot of folks to pull back.”

Traffic deaths rose nearly 8 percent in 2015, compared with the previous year. Federal traffic safety experts say much of the increase can be attributed to a stronger economy and cheap fuel.  Others also point to distracted driving as a factor, especially since texting has become as popular as talking on a mobile phone or more so. Still others cite a slowdown in enforcement, but numbers, especially on a national scale, are spotty.

In Missouri, total traffic stops declined about 6 percent from 1,681,382 in 2014 to 1,579,488 in 2015. In its annual report on traffic stops, the Missouri Attorney General’s office also reported that although traffic stops of African Americans were still disproportionately high compared to the size of the state’s black population, the over-representation of blacks declined slightly in that same period of time.

In North Carolina, total traffic stops declined nearly 8 percent in the January-August 2016 period compared with the same period in 2015, from 436,836 to 402,490. Speeding stops went up by 2.6 percent in those comparative periods, but there were more significant declines in DUI arrests (21.3 percent) and seat belt violations (18.3 percent).

Yet Adkins said traffic stops are a critical tool in keeping highways safe.

“We still want the driver to be pulled over because we know one of the most effective things to change driver behavior is to give someone a ticket,” Adkins said. “It’s far more effective than a public education campaign, that fear of enforcement.”

About 42 percent of the face-to-face contacts with police came from traffic stops in 2011, according to the Bureau of Justice Statistics’ Police-Public Contact Survey, which is conducted about every three years. About half of those traffic stops resulted in a traffic ticket; about 3 percent of the stops led to a search.  Speeding was the reason most often cited for a stop.

The head of the National Safety Council, a top official at Mothers Against Drunk Driving, and other safety advocates have also said there’s a widespread perception that some states have pulled back from proactive enforcement of traffic laws, including sobriety checkpoints.

“I think you hear some of the Ferguson effect. You hear that law enforcement is frustrated,” J.T. Griffin, MADD’s chief government affairs officer, said in a recent interview. But Griffin also said he hasn’t seen data to back up that perception. “Again, I think it’s speculation.”

What’s more, although traffic stops represent the most common reason for contact with police, the federal government does not require the regular collection of data on them, Griffin said. Such data are available only on a state-by-state basis.

“There’s not good data on it, and it’s something MADD has been trying to get for years and years,” Griffin said. “[I]t’s state by state data. There are no federal requirements to collect that data.”

Still, even before there was talk of the Ferguson effect, safety advocates urged law enforcement to do more to stop the deadliest behaviors on the road. Only a fraction of impaired drivers are arrested, MADD says. The group says data show drivers drive 80 times drunk before they’re caught. They argue that police must be proactive to bring the number of fatalities down.

Whatever the cause, Adkins said the slowdown is real, and its consequence has been more fatalities on the road.

“It’s always been tough to be a cop,” he said.  “But it’s really tough right now.”

Read more of Tripping: