Over the past 40 years or so, a remarkable improvement in American society has gone mostly overlooked: fires are way, way less common than they used to be.

The number of fires responded to by municipal fire departments in 2013 — about 1.2 million — is roughly a third of the number from 1977, when the National Fire Protection Association started keeping track: 3.3 million. (Note that these numbers don't include wildfires, which aren't dealt with by local fire departments and keep getting worse.)

Given that the US population has increased by about about 44 percent during that same time (going from 220 million to 316 million), this is a pretty amazing trend. And it turns out that it can be attributed to a few pretty mundane factors: stricter fire codes, fireproof building materials, cars that catch on fire less often, and installation of protective devices like smoke alarms.

At the same time, this dramatic decline has led to some sharp criticism for fire departments, focusing on the increasing number of firefighters over time, coupled with the fact that they now spend little time fighting fires. Instead, they mainly act as medical responders — which is why you still see fire engines careening down the streets so often.

How fire rates declined so dramatically

Natalie Simpson, a professor at SUNY Buffalo who studies the history of emergency response, traces the dramatic decline in fires to the influential 1973 report America Burning. The report laid out a number of recommendations for preventing fires — rather than just putting them out — and noted the need for less flammable materials and increased public knowledge of fire prevention. It also led to the establishment of several federal fire research and response training organizations.

In the following years, these changes — along with a few others — led to a big drop in all three types of fires: vehicle, outdoor, and indoor.

Car manufacturers can be largely credited for the big drop in vehicle fires. "This decline is mainly due to improved vehicle quality," says Christina Holcroft, an analyst at the National Fire Protection Association. "Manufacturers are paying more attention to fire risk, so cars just don't burn as often."

There's also been a big decline in outdoor fires in towns and cities, which mostly has to do with increased municipal restrictions on open burning of leaves and trash. (Again, this doesn't include wildfires in less developed areas, which are responded to by the US Forest Service and other agencies instead of fire departments, and aren't part of this data set.)

Finally, stricter fire codes, better enforcement of them, fireproof building materials, and improved public knowledge of basic fire prevention techniques have all cut down on the number of structure fires. "People are using and checking smoke alarms at a greater rate, and there are more sprinklers being installed," Holcroft says. Lower rates of smoking — especially indoors — and less frequent use of flammable materials in clothes and other goods has also helped.

So what are firefighters doing?

This dramatic decline in fires has put firefighters in a curious position. "Firefighters face what I've called the 'March of Dimes' problem. After polio was cured, the March of Dimes looked around and said 'what do we do now?,'" says Alex Tabarrok, an economist at George Mason University who's previously criticized fire departments for taking on other roles. "Firefighters have been facing the same problem."

As Tabarrok points out, the number of career (that is, paid) firefighters has increased as fires have declined. During this same time, the number of volunteer firefighters has held steady, so we now have more firefighters to fight fewer fires.

In response, firefighters have taken on a new role as medical responders.

Beginning in the late 1970s — right when fire rates were starting to decline — the use of paramedics and other emergency responders as a whole was becoming more common.

"It really came out of the Vietnam War," Simpson says. "Medics were long known within the military, but not within civilian life, but then we started seeing a lot of data on the benefit of immediate treatment of patients with traumatic wounds."

Hospitals began sending trained paramedics in ambulances (which had historically been used solely as vehicles designated to transport people to hospitals) to provide emergency medical care on the spot. And fire departments began responding to emergency calls by sending fire engines crewed by similarly-trained firefighters.

As a result, firefighters now respond to many times more medical calls per year than actual fires. When you hear a fire engine's siren or see one speeding down the street, it's probably responding to a medical call, in addition to an ambulance that's heading to the same spot. Interestingly, there's actually a better chance it's responding to a false alarm than an actual fire.

Is it a problem that firefighters mostly don't fight fires?

Some people — including Tabarrok and other libertarian economists — think that dispatching huge trucks loaded with firefighters to respond to every medical call made to 911 is not an ideal situation.

"Typically, they'll get there a minute or two before an ambulance does, and in some cases they start treatment," Tabarrok says. "In my view, it's mostly redundant. Other people will say that getting there earlier can save a life, but most of the time it has no bearing whatsoever."

He points to a study conducted in Toronto that found having firefighters on the scene improved health outcomes in about one to two percent of all medical calls — essentially, instances of cardiac arrest in which they happened to arrive first. The study also found that, on average, firefighters respond to a call no more quickly than paramedics and had much less medical training.

some think that dispatching huge trucks to respond to every medical call is not ideal

The reason firefighters' numbers continue to grow, he says, has more to do with their strong unions than their usefulness. This ends up being expensive for taxpayers: in Boston, the fire department consumed 7.5 percent of the city's total budget. And even if we do want to use firefighters as extra paramedics, there are cheaper, more efficient ways of doing so. "Why are we sending these huge trucks careening through traffic just to give someone oxygen?" Tabarrok asks. "I say, 'send the guy on a motorcycle.'"

But this sort of criticism isn't universal. Natalie Simpson, the emergency response researcher, says that because of the nature of the demands we put on fire departments, we can't really shrink their ranks, and there are problems with putting them in different vehicles too.

"If you say, 'there's very few fires, so we don't need as many firefighters or fire engines,' a fire is still eventually going to break out," Simpson says. "And without the same response resources, you're going to have the same number of very few fires, but some of them are going to become catastrophic." In other words, we need a baseline number of firefighters and engines for any given area to make sure that the few fires that do occur can be put out quickly.

Moreover, she says, even putting them in designated vehicles (typically called fly-cars) rather than fire engines can cause problems. A small department, for instance, might only have enough people on duty to staff a single engine, so if a fire call comes in while they're out with the car, they won't be able to respond with the engine as quickly.

Read more: Boston Globe's excellent article on this issue, Plenty of firefighters, but where are the fires?