“In a democratic society, people have a say in how they are policed, and people are saying that they are not satisfied with how things are going,” said Sean Whent, the police chief in Oakland, Calif. The city has a troubled history of police abuse and misconduct, but some policy changes and a new approach to training have led to sharp declines in the use of force, Chief Whent added.

Like the 21-foot rule, many current police practices were adopted when officers faced violent street gangs. Crime rates soared, as did the number of officers killed. Today, crime is at historic lows and most cities are safer than they have been in generations, for residents and officers alike. This should be a moment of high confidence in the police, said Chuck Wexler, executive director of the Police Executive Research Forum, a law enforcement policy group. Instead, he told the New York Times, policing is in crisis.

“People aren’t buying our brand. If it was a product, we’d take it out of the marketplace and re-engineer it,” Mr. Wexler said. “We’ve lost the confidence of the American people.”

Mr. Wexler’s group will meet with hundreds of police leaders in Washington this week to call for a new era of training, one that replaces truisms such as the 21-foot rule with lessons on defusing tense situations and avoiding violent confrontations. While the Justice Department and chiefs of some major police departments are supportive, the effort has not been widely embraced, at least so far. Some police unions and others have expressed skepticism, saying officers are being unfairly criticized.