For most of the automobile’s first century of existence, it became safer.

In the 1920s, the death toll from vehicle crashes was so high that gruesome photos of accidents were a staple of newspaper coverage. By 2010 — thanks to better design of roads and vehicles, the addition of seatbelts and greater awareness of drunken driving, among other things — the death rate from crashes had fallen almost 90 percent from its 1920s level.

But the progress ended about a decade ago, or at least it did in the United States. Even as vehicle deaths have continued falling in most countries, they have risen in this country.

Here’s a stark way of thinking about the problem: If the U.S. had made as much progress reducing vehicle crashes as other high-income countries had over the past two decades, about 25,000 fewer Americans would die every year.

My colleagues Emily Badger, Ben Blatt and Josh Katz have published a story this morning that tries to solve one part of the mystery of this country’s outlier status. Emily, Ben and Josh focus on a specific part of the problem: Pedestrian deaths have surged at night.

THE SMARTPHONE

Many of the potential explanations for the trend don’t seem to fit. Cars in this country are large, but they have become only slightly larger since the early 2000s. Drunken driving has not become more common, and roads have not become more dangerous.

But there has been one major change in driver behavior: the use of smartphones.

“Smartphones have become ubiquitous with remarkable speed, overlapping closely with the timeline of rising pedestrian deaths,” Emily, Ben and Josh write. “Apple’s iPhone was introduced in 2007. Within a few years, one-third of American adults said they owned a smartphone.”

Smartphones have also become ubiquitous in other countries, of course. But American drivers seem to be addicted to their phones in ways that drivers elsewhere are not. Surveys suggest Americans spend more time on their phones while driving than people do in other countries. In part, this phenomenon may reflect this country’s culture, which emphasizes professional success and immediate gratification.

It also partly reflects vehicle technology. Nearly all cars in the U.S. are automatic transmission, freeing drivers’ hands (or so they may think) to use phones. In Europe, almost 75 percent of cars still have gears that a driver must change manually.

“​​The adoption of smartphones for the past 15 years — where we are today, being addicted on social media and other apps — absolutely contributes to the increase in fatalities on our roads,” Matt Fiorentino, a vice president at Cambridge Mobile Telematics, which tracks dangerous driving for carmakers, insurers and regulators, told Emily.

POT AND SIDEWALKS, TOO

Smartphones aren’t the only likely cause of the trend, Emily, Ben and Josh write. The spread of legal marijuana may also play a role, as may the rise in opioid addiction. In one recent federal study, half of the drivers involved in serious accidents tested positive for at least one active drug.

The continued growth of the population in the Sun Belt, where roads often lack sidewalks, crosswalks and bike lanes, may also be a factor, as may the recent increase in homelessness. People living on the streets are especially vulnerable to being hit by a car.

Some of these problems are difficult to solve. For others, however, there are promising solutions that state and local governments have simply chosen not to try. Building safe sidewalks, as Europe has done, is relatively cheap. Using traffic cameras to identify drivers who are texting — and imposing significant fines on them — would not be difficult, either.

Instead, the U.S. has chosen to accept a vehicle death rate that is almost three times higher than that of Canada, Australia or France, more than four times higher than that of Germany or Japan and more than five times higher than that of Scandinavia, Switzerland or Britain.

Among the recent victims of America’s uniquely high vehicle-death toll: A woman died after being hit by a vehicle while crossing a street in east Las Vegas on Friday and then being hit by a second car while she was on the ground. A person in Redmond, Wash., died on Wednesday night after being struck by a driver in a gray Nissan Pathfinder who then fled the scene. Another pedestrian died in a hit-and-run accident in Colorado Springs on Friday.

And on Thursday night, mourners gathered at a ShopRite parking lot in Stamford, Conn., to remember Marie Jean-Charles, a 74-year-old cashier who had worked at the supermarket for 25 years. She was killed by a speeding driver while she was crossing the street to go to work.