Policing in America today is a rib dinner paid for by a stranger, and a protester kicking a dent into your patrol car door. It’s warning a young man speeding down a country road to beware of errant deer, and searching through trash cans for a gun on the streets of a big city.

But it’s also watching a video in your Facebook feed when another officer shoots a black man — a young man stopped for a traffic violation; a man selling CDs. And it’s facing the protests that follow, which are prompting introspection and even more of an attitude of us-versus-them.

About 477,000 sworn officers serve in the roughly 12,000 police departments in the United States. The demands, challenges, resources and cultures of each police force vary. But there are also commonalities.

With the exception of some cities still awash in violence, crime has dropped, and the job has changed. And after the fatal ambushes of police officers in Dallas and Baton Rouge, Louisiana — as well as years of intensifying protests about the deaths of black men, women and children at the hands of the police — officers everywhere are under pressure to change still more.

On the streets of every town and city, each shift is a search for safety. Here’s a look at one such shift, compiled through ride-alongs last week with officers in departments across the United States.

Bullets and Beginnings

Officer Michael Virgilio’s shift has just begun. He is already on high alert.

He slowly eases his vehicle out into the alley of a precinct in Seattle, scanning left and right as the precinct’s metal garage door trundles down behind him.

The brick alley has recessed alcoves that could conceal someone lying in wait, and the new safety protocol in his department says to take no chances, make no assumptions and avoid being predictable.

“One of the most vulnerable times in our day is shift change,” Virgilio, 31, says as he checks the route. “It’s a matter of avoiding certain patterns. Anybody who is trying to organize some sort of attack on police officers is going to do some sort of surveillance. They’re going to figure out when our shift changes are, they are going to figure out what doors we use to get in and out of the precinct. And it doesn’t take long to gather that type of intelligence.”

Lt. Scott Finn begins all his shifts in Prince George’s County, Maryland, a densely packed suburb just outside Washington, by passing through a set of doors scarred by two bullet holes, each a round shock of silver, embedded in mundane tan paint.

A few months before the ambushes in Dallas and Baton Rouge, a man with a semi-automatic handgun opened fire outside the police station in Landover as officers were leaving roll call. With two of his brothers filming, police said, the man shot randomly at cars and an ambulance, while officers inside the station house returned fire.

A 28-year-old off-duty police detective, Jacai Colson — who, like the gunman, was black and dressed in street clothes — happened to pull up to the front of the station, on his way into work. A fellow officer mistook him for the assailant and shot him dead.

“It was just the fog of war,” Finn says. “Everything had to align perfectly to have something like that happen.”

The lieutenant pushes through the doors and walks to his unmarked patrol car, where he slips on his bulletproof vest, pressing the Velcro on its sides to make sure it is tightly secured. Another shift begins.

Coffee and Criticism

In many places, officers have been showered with gifts of food, Starbucks, cold drinks.

“Over the past five days, I’ve had more people offer to buy me a coffee than I can remember,” says Sgt. Thomas Glynn, 46, during a patrol through Cambridge, Massachusetts.

But officers around the country are also confronting tension, and it’s been building for years. Glynn’s city is where, in 2009, Henry Louis Gates Jr., a Harvard professor who is black, was arrested for trying to break into his own home. And in 2013, the fugitive Boston Marathon bombers murdered an MIT police officer.

Near Harvard Square, Glynn pulls up to a bus at a parking meter that has expired. Another officer had given the driver a ticket and told him to move the bus, but while backing up, the bus driver hit the officer’s motorcycle.

The driver, whose first language was not English, grew agitated and shouted, “And you wonder why police officers kill people!”

Glynn says: “That was not what he meant to say. What he meant to say was, ‘And you wonder why people kill police officers!’”

Dallas and Baton Rouge had made their way to the land of Harvard. The man’s outburst, Glynn says, “had everything to do with what’s happening now.”

Finn points to his patrol car’s door. “Like the dent?” he asks.

Just days earlier, he says, someone had kicked his car and yelled “Black Lives Matter” before taking off on a motorcycle.

It was, he says, part of a pattern of increased tension that he has noticed in Prince George’s County ever since police officers shot and killed Alton B. Sterling in Baton Rouge and Philando Castile in Falcon Heights, Minnesota, on back-to-back days this month.

A World of Hurt

Deputy Constable Steve W. Faulkner, 39, joins two other deputies at a taco truck and food trailer parked side by side in a strip-mall parking lot in Precinct 1 in Houston. They are talking about guns, life, the job.

A few days ago, he says, after the attack in Dallas, he and other deputies responded to a call from a woman who had concerns about social-media messages her teenage daughter had been receiving.

The woman and her daughter were black. The deputies were white. The woman and her daughter were not pleased with the deputies after they suggested removing an app from her daughter’s phone as a possible solution.

“The younger girl, she’s probably about 13, she just piped up and said, ‘You don’t like black people. If this would have been a white girl, this would have been handled totally different,’” Faulkner recalls.

“That,” he says, “is a direct result of everything going on right now.”

Officers John Buchanan and Robert Bramble are cruising through Bedford-Stuyvesant, one of Brooklyn’s rapidly changing neighborhoods.

Buchanan, 24, is white. He grew up in suburban Suffolk County, on Long Island, where he still lives.

Bramble, 24, is black. He grew up in Brooklyn, in East Flatbush. He still lives in the city, about 15 minutes from the station house.

It is hot, and as they drive along the streets of the 79th Precinct they notice a now-familiar sight: “It’s SOS,” Bramble says.

On the sidewalk, a small contingent from the group Save Our Streets is protesting — a woman with a bullhorn condemning violence and other people wearing T-shirts that say: “Stop shooting. Start living.”

Bramble says the protests are now a part of life in Brooklyn. “There are people in the law enforcement world that are hurt right now; there’s civilians out there reading and seeing these things, and experiencing them, that hurt right now,” he says. “It affects everybody.”

Buchanan shifts the discussion to the importance of neighborhood policing.

“We can use that to try and change some people’s perspectives, or at least be an example for what policing in a community should be,” he says. Still, he thinks anyone who is not an officer cannot understand the pressures and risks of the job.

“You can’t have a perspective on this job, in my opinion, unless you do it,” he says. “Unless you’re in policing, doing it every day as a career, you can’t know what it’s like.”

© 2016 The New York Times Company