The Port Authority — the New York and New Jersey government agency that runs the airports — is thinking about charging people $4 for their annoying habit of actually using the airports. More specifically, to come to or leave them: The PA would levy the fee on people coming and going by taxi or hired car.

But the fee fails three economic tests.

The idea is that charging people $4 would cut down on traffic. The fee would also pay for a more efficient way to manage hired-car lines.

The PA’s primary goal here is to change people’s behavior through pricing. If something is free — say, being dropped off at the airport — and people are using “too much” of it, just charge them money to do so and they’ll choose a different way of getting to their plane.

But what works economically in a perfect world — or even in a things-are-working-kind-of-OK world — doesn’t work in this case.

It’s fine to nudge people to make better choices when they actually have better choices. But there is no good way to get to La Guardia or Newark by public transit, so there’s no alternative to the current system.

To get to La Guardia, if you’re starting from Midtown, you’ve got to take a crowded subway to Queens.

When you get to Queens, you’ve got to exit the subway, cross a busy street and hop on a bus.

The state-run MTA recently improved the bus, painting it a distinctive shade of blue and installing machines so that people can pay before they board, eliminating lines.

That’s good — but you still often spend 10 minutes waiting for the bus to show up.

This is a great option — if it’s warm and sunny, and if you are one of the 46 percent of people starting your trip in Midtown, with the best subway connections, or one of the 20 percent of people going on a business trip. (The latter means you don’t have much luggage and no kids with you.)

But if you’ve got luggage or small children, you’ve got to schlep down two stories to get the subway, crowd your stuff onto an already packed train, schlep back up and haul your stuff again onto a public roadway and onto a crowded bus. (You can also catch the bus from Manhattan above 100th street, but the same principles apply.)

The situation at Newark is worse. Newark has an AirTrain that more closely resembles a crazy-fun-ride at a carnival set up in a corrupt town whose mayor took a lot of bribes from the carnival barker. Your reward, after taking the AirTrain, is to watch Amtrak trains speed by while waiting for a New Jersey Transit train that doesn’t come. And yes, more stairs.

JFK is better — it has a real AirTrain. But you still can’t get to the train without taking the LIRR or the subway. Because of this inconvenience, only 7.1 million out of JFK’s 56.8 million passengers took the AirTrain to the airport in 2015.

The proposed airport fee isn’t a good fit for a second economic principle, either: that people should pay directly for the resources that they use. It’s OK to toll a bridge even if there’s no other way to get somewhere, because the bridge’s users should pay for its upkeep.

But people who go to the airport are already paying for the airport, through their airline ticket. New York’s three airports threw off $638 million in profits in 2015.

Taxpayers and people who don’t use the airport aren’t subsidizing them.

There is a third economic problem with this idea: the notion that people are overusing hired cars because they’re too cheap.

It already costs $40 to take a cab from Midtown to La Guardia.

Uber can be cheaper — sometimes. But that’s because Uber subsidizes passenger fares. Uber may subsidize the new fee, too — gaining an advantage over cabbies who can’t afford to do that.

The Port Authority should build first-world rail connections to our airports — where customers have a one-seat ride from some convenient location, with ample room for luggage and children, and reliable elevator service.

The Port Authority is taking steps to build a rail link to La Guardia — but it will be a version of the JFK link. Better, but not equivalent to what London, Paris and Tokyo have. And it’s not fair to charge a fee now when the finished product is still years away.

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

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