Cars kill people. More than 37,000 in 2017—what would statistically be considered a good year. Big tech has a solution: Have the cars drive themselves, free of distractions, drunkenness, and other human foibles. Flood the roads with autonomous vehicles and watch collision deaths plummet.

Too bad this lovely narrative has a major plot hole: Blunt-force trauma isn’t the only way cars kill. Each year, hundreds of thousands of people die prematurely from breathing exhaust-poisoned air. Even more insidious is how the mere act of sitting in a car for an hour or so each day drastically increases peoples’ risks of developing life-altering—sometimes life-ending—conditions like obesity and heart disease.

A widespread shift to vehicles that drive themselves, some experts say, could weaken humanity’s already slipping stance in a two-front war against pollution and sedentary behavior. They worry that once the stress of traffic and overt dangers of car travel are gone, people will spend more time in their rolling Barcaloungers. And just because cars drive themselves doesn’t mean they’ll all be battery-powered or that they’ll somehow inspire their occupants to get active.

The only way to ensure the chauffeured masses don’t reach their final destinations far too soon is to address the automobile era’s original sins. Sins that aren’t rooted in who’s working the pedals.

Fear and Loafing

If people were rational, top phobias like snakes, sharks, and spiders would be replaced by things like sitting too much, skipping workouts, and living close to freeways. Together, sedentary behavior and exposure to pollution are two of the most common risk factors for heart disease, which accounts for one in four deaths in the US each year. They also put people at risk of developing many types of cancers, lower respiratory diseases, diabetes, stroke, and other lifestyle diseases. (If you’re morbidly curious, car accidents mop up about 1 percent of the nation’s annual death toll, below marginally more prolific killers like suicide, Alzheimer’s, and the flu/pneumonia.)

Now, pollution and sedentary behavior are nebulous things, each with a variety of causal factors. But want to hazard a guess at what is one of the biggest contributors to people breathing bad air and not getting enough exercise? That’s right: spiders.

LEARN MORE The WIRED Guide to Self-Driving Cars

Seriously, though, it’s cars. Numerous studies have linked traffic to air pollution, in the form of lung-spackling poisons like particulate matter, volatile organic compounds, carbon monoxide, and nitrogen oxide. Not even homebodies are safe, as vehicular belches can waft indoors. Motor vehicle pollution is so bad that Environmental Protection Agency warnings about it survived former administrator Scott Pruitt’s information purge.

At the same time, car commuters tend to live more sedentary lives and, as a group, experience elevated rates of obesity, diabetes, and heart disease. “Although people think these risk factors are individual behavior choices, the reality is that our choices are shaped by our environments,” says Karen Lee, a physician and associate professor of preventive medicine at the University of Alberta in Canada. Over the course of the 20th century, vehicle ownership made it possible for people to work in cities without having to live in them. Personal chariots begat the personal fiefdom, which begat urban cores surrounded by ever-thickening rinds of suburbia.

So, for many people, walking is no longer an option. Sprawl makes public transportation untenable: You’d need too many buses, or too many miles of rail, to service all that acreage. The ironic twist of the personal freedom that cars enabled is they required everyone make the same choice: Get a car, or get bent.

“Why do we care about safety?” says Peter Norton, a transportation historian at the University of Virginia. “Because we care about human health.” Norton warns that the autonomous future will result in less walking, more sprawl, and—without dedication to electric propulsion—a huge spike in pollution. “When your car drives itself and you can spend the commute doing whatever you want,” he says, “who cares if a trip that used to take 20 minutes now takes an hour?"