The writing is on the wall.

Graffiti, the scourge that former Police Commissioner Bill Bratton once described as the “first sign of urban decay,” has exploded in New York City in the last five years.

Complaints to the 311 hotline have soared during Mayor de Blasio’s leadership from 13,560 in 2015 to 21,006 last year, a 54 percent increase.

And complaints through mid-June have already hit 11,367 — up 69 percent compared to the same period in 2018.

At the same time, arrests for graffiti have plummeted. The NYPD logged 3,586 graffiti busts in 2013, but by 2015 the number dropped to 2,583 and then to 1,849 in 2017, a 49 percent slide over five years.

The NYPD could not provide numbers for 2018 and 2019.

The simultaneous surge in vandalism and plunge in arrests points to a breakdown in “broken windows” policing — where authorities fight visible signs of crime, antisocial behavior and civil disorder to discourage further crime and disorder, observers told The Post.

The city has already decriminalized quality-of-life of offenses like public boozing and urinating.

“We’re having a nervous breakdown in the city at this point,” said Eugene O’Donnell, a former cop and prosecutor who teaches at John Jay College of Criminal Justice.

“There’s a need to reform criminal justice, for sure. What the city now is facing is some very profound questions about whether anybody can be held accountable.”

O’Donnell said he suspects the number of 311 complaints represents just a fraction of the vandalism taking place because residents feel like nothing will be done so they don’t report the crime.

He blamed de Blasio for not encouraging public safety.

“The best you can say about him is he’s a bystander,” O’Donnell said. “He just stands by and allows this stuff to rage on.”

In 1995, when then-Mayor Rudy Giuliani battled the spray-paint scourge by creating the Anti-Graffiti Task Force, he said, “Where graffiti flourishes, communities suffer. Graffiti intimidates residents. It encourages street gangs. It discourages tourists, lowers property values and invites other kinds of crime. Graffiti painted New York City into a corner, but we don’t have to stay there.”

Former Mayor Ed Koch also raged about graffiti’s blight on the landscape. He once described graffiti, along with pickpocketing and shoplifting, as “all in the same area of destroying our lifestyle and making it difficult to enjoy life.”

He famously suggested that the MTA put wild wolves in its train yards to scare off taggers.

The MTA declared the subways graffiti-free in 1989, after instituting a policy to immediately pull any tagged train out of service for cleaning so that vandals would never see the fruits of their labor.

According to a Post analysis of 311 data, Brooklyn leads the city with graffiti complaints at 36,365, followed by Manhattan, with 15,606, The Bronx, with 12,936, Queens, with 11,758 and Staten Island, with 996.

Manhattan’s Lower East Side, with 2,988 complaints, and Sunset Park, Brooklyn, with 2,556, were the city’s hardest-hit neighborhoods.

The rails have also suffered a return to the bad old days. The number of major subway graffiti incidents skyrocketed 70 percent from 262 in 2016 to 443 in 2018.

The MTA blamed the bump on international rings of vandals coming to town to score hits on a what is considered a big prize — a New York City subway car.

The number of major incidents dropped to 146 through June 20 of this year compared to 250 for the same period last year, which the MTA attributed to several busts of vandals in 2018.

One of the alleged taggers, Francisco Antonio Jimenez San Nicolas, was arrested in December after the NYPD worked with police from his native Spain.

He told the NYPD, “All the Europeans come to New York because that’s where graffiti was born.”

Timothy Kephart, who runs the Graffiti Tracker web site to help police find vandals, said ignoring graffiti only allows it to grow.

“The kids who are doing graffiti vandalism, they feel like they can get away with it,” Kephart said. “Ultimately the taxpayer is the one that suffers because they have to pay more for the abatement costs.”