Last week, Mayor de Blasio and Sen. Chuck Schu­mer rode the subway around and called on Washington to give us more cash for trains, bridges and roads. On this issue there should be “no debate,” the mayor said. It’s “investing in our country’s future.”

But it’s not so easy. Federal meddling is making sure we get fewer streets — and bike lanes and pedestrian plazas — for our buck.

You would think making Times Square into a pedestrian plaza for Elmo, creating a bus lane in Brooklyn or rebuilding a cobblestone street after 9/11 would be straightforward.

But President Obama’s Department of Labor has made it into an insanely complicated exercise of economic and demographic engineering.

The issue is “flaggers” — people who stand near construction sites and make sure that cars don’t crash into pedestrians or each other when a traffic lane or sidewalk is closed for roadwork.

This work is directing traffic, not construction.

So with the written approval of city agencies and the city comptroller, the construction contractors working for the city have, for decades, paid the workers traffic-control rates — now $11.25

to $22 an hour, similar to what NYPD’s traffic agents get.

Though the work is non-union, the city requires that contractors hire full-time workers.

That means $30,000 a year or more for people without much higher education or vocational training.

Enter the federal government — whose labor investigators visited a Lower Manhattan job site two years ago and determined that the flaggers aren’t traffic-control agents but construction laborers.

It’s a nice promotion: Federal rules mandate that “laborers” in New York City earn $39.85 an hour, plus another $34.88 in benefits, for a total of $75 an hour.

Unless New York City started paying $150,000 and up for each flagger, the feds would take away our road money.

Even for rich New York City, nearly quadrupling the cost of a job is a big deal (remember, the feds don’t pay the full cost).

On just five recent or current jobs — including Times Square, the Brooklyn bus lane and rebuilding work downtown — the feds uncovered $2.6 million in wages owed.

And de Blasio wants more bike lanes, bus lanes and pedestrian plazas. Plus, we’re always fixing stuff.

These jobs need flaggers. Six-figure flaggers would add tens of millions of dollars — maybe more — to the city’s $8.9 billion, 10-year capital plan for transportation.

And it gets worse: The city borrows for these projects. Thirty years from now, you’ll still be paying for the flagger who stood out in Times Square yesterday.

The costs aren’t only direct.

In trying unsuccessfully to fight this ruling, an official in the city’s construction-design department noted that it could have cascading effects. He pointed out that workers who are doing “heavy manual labor” (the traffic agents aren’t even allowed to set up cones and barriers) would resent being paid the same wage as a person directing traffic, causing a “trade-union dispute.”

He also noted that the NYPD’s traffic agents would have a “valid wage grievance.”

The feds’ response? To threaten more action.

“We have approached this matter as a [labor-law] violation,” the feds wrote. But “since the involved workers were primarily female/minorities . . . there is a potential . . . civil rights violation.”

Nice capital plan there — would be a shame if something happened to it.

Ironically, the ruling may mean less minority hiring.

Already, city transportation officials are upset that their contractors keep missing their targets for employing minorities.

But a Times Square plaza contractor had to dump a minority partner because of the feds’ flagger ruling.

In defending the contractor’s action, another design-department official told city transportation officials that the contractor “has a collective-bargaining agreement with the unions which prohibits them from having non-union workers . . . performing union trades.”

If directing traffic is a laborer job, then it’s a union-trade job, which means the people doing it must be union members, so the non-union minority contractor had to go.

And when you make directing traffic a union job and a lucrative enough profession, the job will go to people with better connections.

In the long run, the contractors won’t care. The city pays.

It sends the wrong signal, though: When government doesn’t care about any cost efficiency, why should construction companies?

If the mayor cares about building and fixing stuff, he’ll call for Schumer and other New York pols to work with Obama on fixing federal construction-wage laws.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.