Mirroring a national trend, killings in the Bay Area’s biggest cities ticked up in 2016 for the second year in a row, rising 14 percent and prompting police to seek ways to shore up long-term gains that have made the region safer than it was in past decades.

The region’s 15 biggest cities recorded 277 killings last year, up from 242 in 2015 and 222 in 2014, according to a Chronicle analysis.

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Robert Weisberg, a criminal justice expert at Stanford Law School, said police staffing levels and increased police presence do tend to cut violence.

“The major effect is not stopping a crime in progress,” he said, “but deterring people from engaging in the kind of encounters on the street that have a certain probability of resulting in violence. People do respond to and communicate about police presence.”

In 2014, violent crime in the state dropped to the lowest level since 1976. And a 2015 study by the Stanford Justice Advocacy Project, which helped draft Prop. 47, found that fewer than 5 percent of state prisoners released early under the measure were convicted of a new crime and sent back to prison over the course of a year — compared with 42 percent for all inmates.