Mr. de Blasio estimated that giving most people summonses for smoking in public would eliminate about 10,000 arrests per year — more than half the total number — which is clearly an improvement over the status quo. This would also move the city a step closer to abolishing a class of arrests that no longer even exists in some other states — including those where marijuana has been legalized.

The new policy, however, is less than ideal. For starters, summons court judges often issue warrants when people forget their court dates, which can lead to those people being fingerprinted and dragged through the criminal justice system. There is something patently unfair about letting a petty offense morph into a criminal record that dogs a person for the rest of his or her life.

New York City released its new plan a day after the New York State Health Department announced that a study commissioned by Gov. Andrew Cuomo would recommend that the state allow adults to use marijuana legally. This comes as something of a surprise, given that Mr. Cuomo last year described marijuana as “a gateway drug.”

Legalization would require approval of the State Legislature. Under ordinary circumstances, the conservative State Senate would most likely reject the idea out of hand. But the legalization of recreational marijuana that just passed in Canada — with which New York State shares hundreds of miles of a border — combined with legalization campaigns in neighboring states could force the normally hidebound Legislature to legalize the drug, if only to make sure New York gets a piece of an emerging industry.

Meanwhile, critics both inside and outside law enforcement are rightly attacking the part of New York City’s new marijuana plan that will continue to allow for the arrest of people on probation or parole who are struggling to re-enter civic life. On Tuesday, the Manhattan district attorney, Cyrus Vance Jr., said through a spokesman that such a system could easily intensify the already glaring racial inequities that come with marijuana enforcement.