Investigators probing Sunday’s horrific LaGuardia Airport crash say “multiple failures” — not a single air traffic control error — are to blame for the tragic deaths of two Air Canada pilots and scores of passenger injuries.

But perhaps the biggest such failure is one that National Transportation Safety Board experts won’t examine.

The Air Canada jet plowed into a fire truck heading to assist a disabled United Airlines plane that was out on the tarmac because no open gate was available for it at the terminal.

Any transportation system that is overloaded clearly has fundamental problems.

Flying has become fragile: The US air travel network, once resilient, now operates permanently on the edge of failure.

A routine thunderstorm, a late inbound aircraft or a staffing hiccup can seize up an entire web of connections — and these delays no longer stay local, but cascade across the continent.

Why the frailty? Pilots aren’t less skilled, nor are airlines greedier than before; the weather hasn’t become less forecastable.

The problem is structural: North America stopped building airports, demand kept rising, and there’s no longer any slack in the system.

The 50 largest US airports are 82 years old on average, and only three major commercial airports have been built in the past half-century.

Building new nuclear reactors is famously difficult, but as Brian Potter of the Institute for Progress has noted, the United States has built more of them in the past 25 years (two) than major commercial airports (zero).

Meanwhile, US passenger traffic has ballooned from about 666 million passengers a year in 2000 to nearly 1 billion today, a 50% increase absorbed by infrastructure that has barely changed.

Most days, the consequences are banal.

A storm in Atlanta delays your departure; by the time you land, your gate is occupied and you miss your connection.

What should have been a minor glitch becomes a six-hour airport campout or even an overnight stay.

This isn’t bad luck but the predictable consequence of a system running at maximum capacity; when something unexpected occurs, there’s no room to absorb the shock.

Airlines have tried to adapt by padding schedules, adding 20 minutes on average to flight times, because the system needs built-in slack to function.

The system is so fragile that in November, the Federal Aviation Administration ordered a 10% reduction in flights at 40 major airports.

The government shutdown had worsened chronic air traffic control staffing shortages, and rather than risk cascading failure during the busiest travel period of the year, the agency cut flights altogether.

And the pressure is rising.

The FAA projects the number of US annual air passengers to rise from today’s 1 billion to roughly 1.4 billion by 2040.

Yet capacity is not expanding where it matters most: The world’s 100 busiest airports added only a handful of new runways in the past decade.

Our airports are expected to carry more weight on the same frame.

The answer is obvious: We must build more of them.

That shouldn’t be a radical idea. For most of the 20th century, North America responded to rising demand in any area by expanding capacity.

We didn’t treat congestion as a permanent condition to be managed, but as a signal to build.

In other sectors, like housing, this logic is returning.

Air travel should be no different.

Mobility determines where people can work and which cities can grow.

When it becomes unreliable, the costs are borne quietly — missed opportunities, lost productivity, narrower horizons — but they are real.

More runways, more terminals and more geographic distribution of capacity would reintroduce slack into the system.

Deciding to build is only the beginning of the solution: We need to figure out where to build, how to pay for it, and especially how to navigate the permitting morass that has made new airport construction nearly impossible.

These are formidable problems, but not impossible ones.

New York is an obvious place to start.

The metro area’s three airports have been under federal slot controls for decades, open acknowledgment that the status quo is insufficient.

The last time a fourth New York airport was seriously proposed was 1965, when Gov. Nelson Rockefeller put forward a former military airfield at Calverton on Long Island.

That conversation needs to be reopened.

For half a century, the United States has pretended that airports built before the Carter administration could handle exponentially growing passenger demand.

They can’t.

Sunday night at LaGuardia, overcrowded terminals put emergency responders on an active runway, and two pilots died.

A system with margin can absorb ordinary disruptions, but this one has none.

We need to build airports again.

Andrew Miller, a fellow at the Roots of Progress Institute, writes “Changing Lanes,” a weekly Substack newsletter on innovative mobility.