On the evening of March 4, a man riding the 3 Train in Manhattan ­allegedly punched an 8-year-old autistic boy. The man reportedly was homeless and mentally ill; his attack was unprovoked. The ­attacker ran away after the incident and was arrested the following day. News outlets picked up the story, and reporters spoke to commuters who were ­understandably upset.

But were the commuters surprised? Every New Yorker who rides the subway has witnessed, or been victimized by, disorderly behavior from the homeless mentally ill, and the problem is only getting worse. In my district in Queens, there has been a surge of complaints about the behavior of the mentally ill and homeless population on the M train.

“Last week, this homeless guy was threatening to stab people in the eye, because he said we were CIA operatives,” one of my constituents told me on Facebook. Another constituent spoke of a man who was following a young woman on the M train, ­exposing himself to her.

Mayor de Blasio and his wife, Chirlane, however, urge us not to believe our lying eyes. They insist that the city’s Thrive NYC initiative — which has already spent more than $850 million and is slated to receive an additional $1 billion over the next four years — has significantly improved the state of mental health.

But when I raised concerns about the program’s ­efficacy at a recent City Council hearing, the Thrive leadership had trouble quantifying how many mentally ill residents had benefited.

It’s clear to me that the subways aren’t seeing the benefits of Thrive, and we need to come up with more creative, aggressive solutions to get these individuals off the street and into the hands of people who can help them. Rather than a large-scale social campaign, we must make significant policy changes and be willing to expand the criteria for involuntary treatment.

Yet another constituent, a registered nurse, told me that the mentally ill are most often brought to her hospital by police or emergency services. Once they have been treated, however, the patients are immediately released and back on the streets. My constituent asked: Why can’t we keep these ­patients committed in psychiatric institutions so they can receive the treatment they need?

The trouble is that New York sets too high a bar for involuntarily committing patients who pose a danger to themselves or others. Which is why the Treatment Advocacy Center gives the Empire State a “C” grade for its laws regarding involuntary commitment to treatment centers. Twenty-nine states receive a higher score.

In fact, New York doesn’t have any language in its laws recognizing that a person is a danger to himself if he can’t provide for his own basic survival needs. New York law also doesn’t recognize the harm that is done to a mentally ill person’s brain when he isn’t being treated, referred to as “psychiatric deterioration.”

The longer a person goes without treatment, the lower his chances for recovery, per the Treatment Advocacy Center.

The group’s study also shows that, between 2010 and 2016, the number of public psychiatric beds in New York decreased by 1,741. New York received another demerit because every inpatient commitment requires a doctor’s approval rather than leaving that decision to the family or adult responsible for a mentally ill person.

Thrive has spread its large budget far and wide, to focus mainly on offering more resources for those seeking help. But while that effort means well, it isn’t nearly focused enough. The seriously mentally ill end up falling through its too-wide net, precisely because these folks are least likely to seek help.

As the Treatment Advocacy Center emphasizes, one of the main symptoms of severe mental illness is a lack of insight into what your illness is. So if someone with a severe mental illness can’t even recognize that he is mentally ill, why would he ever voluntarily seek help? And if New York’s laws on involuntary commitment are too vague, how can we expect to get these individuals the help they need?

The state and city legislatures must address these gaps in mandatory mental-illness treatment if we are going to prevent people from harming others or themselves. If we remain reactionary, waiting for the next violent incident to ­occur, we will never be able to reach all of the people who need help. Taking them off the streets and into professional care is for their own good and the good of the public.

Robert Holden is a member of the City Council, representing the 30th District.