The Post performed a public service last week by revealing that the Department of Transportation under Commissioner Polly Trottenberg wasn’t going to remove graffiti or tags from its property for a full five years.

Pleading poverty, she said it wasn’t in her budget.

Unfortunately, graffiti vandals and taggers have recognized that since last summer. That’s when it became obvious that if you left your tag on the utility box, stop sign or street sign, it would no longer be scrubbed, painted over or removed. Vandals who hadn’t used felt-tip markers or Sharpies in years renewed their personal addiction and dusted off their favorite tools.

In hipster heaven Bushwick, Brooklyn, there’s lots of rubbernecking on the subway. Ride the M train toward Myrtle and Wycoff and you’ll see the taggers and vandals not only painting the town in broad daylight but videoing and hashtagging their work on Instagram. And why not? They’re proud of their art, and the city seems to be as well.

From the rooftops to the fronts and sides of buildings to street furniture and everything the DOT controls, graffiti groups like WTO (We’re Taking Over) are metastasizing, banging out tags and spray-can bombing all night.

Tagging is an ego trip. Each time a graffiti tagger passes his work he gets that pride rush. Social media has only magnified this: If others “like” it, retweet it and blast it out, his high is that much higher.

But let’s not kid ourselves about what we’re seeing. The cancer of street vandalism has returned with a vengeance. The question is, are there enough New Yorkers who won’t let our city return to the bad old days?

In the early 1970s, tags with the names TAKI 183 and TRACY 168 started appearing throughout Washington Heights and The Bronx. You knew who they were and where they lived — the number after their name was the street they lived on.

And yet, nothing was done. So the graffiti vandals began to bomb subway trains in the layups and train yards.

Mayor John Lindsay declared the first war on graffiti in 1972. But then he declared victory, pulled up stakes and abandoned the city to run in the Democratic presidential primary. The graffiti vandals struck back with a vengeance.

By 1980, graffiti was everywhere. The city used guard dogs, razor wire and fences to keep these self-proclaimed artists from their landscapes. And as they bombed away with spray paint, liberals and the art world embraced and lionized their creations.

It got so bad that in 1983, the dullards at the MTA painted hundreds of subway cars white — practically begging the graffiti vandals to bomb and tag away. From rooftops to signs to store grates to subways and trucks, they did just that. And turned parts of the city into one big stain.

But in 1985, MTA President David Gunn kicked off the Clean Train Movement. Trains were taken out of service at the end of their runs and scrubbed of any graffiti and tags. The MTA led the way in taking back the city’s “visual space.” Other agencies and the private sector followed.

Thirty years later, the de Blasio administration is taking us back to those ugly times.

Anyone who has traveled to major cities in Europe — Berlin, Milan, Paris, Brussels — has seen those historic landscapes get bombed with graffiti. It’s a blight that rarely disappears.

You’ll go back and see graffiti still there from your last trip. Even private shops and businesses ignore longtime graffiti defacing their front facades. Many of them believe that we exported this blight there from New York City while we were effectively stomping it out here at home. But now we’ve begun the descent back into the belly of the graffiti beast.

Mr. Mayor: Remove the tags and graffiti. Chase down the vandals. Arrest them, fine them and shame them by forcing them to paint over their own tags.

Don’t let the bad old days of a graffiti-covered city return, because no matter how colorful it looks, it ain’t pretty.

Curtis Sliwa, founder and CEO of the Guardian Angels, can be heard on the “Curtis & Kuby” show on 77 WABC radio.