School buses stop to drop students at Merion Elementary School. (Photo: William Thomas Cain/Getty Images)

When John Wargo's daughter was 12 years old, he started sending her off to school as any loving father would: He strapped her up to an air-quality monitor, walked her to the school bus, and told her to measure the toxic diesel emissions she was bound to inhale.

Her father, an environmental scientist and professor at Yale University, hoped to learn about the diesel emissions her bus pumped out, and how long she and her classmates were being exposed to carcinogens. It turned out to be quite a lot. Over the next several months, he affixed monitors onto 14 other students, and published a report of his findings, Children's Exposure to Diesel Exhaust on School Buses, in 2002. Kids who rode the school bus were breathing in five to 15 times more particulates than they would have been otherwise.

Sixteen years later, Wargo's daughter is far out of college, but not much else has changed: The school buses that haul the nation's kids to and from school closely resemble the same diesel-chugging beasts you remember from the 1990s, '80s, or '70s. According to estimates from the Diesel Technology Forum, a non-profit advocacy group, 95 percent of school buses in the United States burn diesel fuel. If each ride takes an average of 30 minutes, that's 180 hours spent on buses a year, which translates into a collective three billion hours, nationwide, spent breathing in varying levels of diesel exhaust. And, as Wargo's report notes, "there is no known safe level of exposure to diesel exhaust for children, especially those with respiratory illness."

That's just one metric by which the school bus has been slow to evolve. Compared to most other passenger vehicles, the iconic yellow bus remains a crude and truckish conveyance; most lack air conditioning, seat belts, and other features that riders in just about everything else on the road have long enjoyed. And there are a lot of them in the U.S.: The inventory of vehicles in the national school bus fleet is bigger than that of commercial buses, trains, and air travel services combined, made up of around 500,000 buses deployed mornings and afternoons nationwide. This vast network has a captive ridership of 26 million.

Offering students free bus transportation to school is a vitally important service, especially for lower-income households. But, for school districts, it's a costly and resource-intensive chore. Thanks to development patterns and an increasing preference for "school choice" in cities, bus routes in some urban and suburban areas can be labyrinthine. As with city buses and trucks, there's a serious shortage of drivers, and safety regulations are a growing concern: A deadly crash in New Jersey in May has re-ignited the national debate over whether seat belt use should be mandatory in school buses. Recently, the National Transportation Safety Board recommended lap and shoulder belts be installed in all U.S. school buses. (They are currently not required in vehicles over 10,000 pounds.)

Most other vehicles on the roads, including the school bus' municipal counterparts, have evolved dramatically over the past few years, growing sleeker, safer, more efficient, and more electric. So how has the school bus—carrying what is arguably the country's most precious cargo—evaded revolution?

There has been at least significant technological progress in the school bus industry, especially in powertrains. The Environmental Protection Agency implemented new cleaner diesel standards in 2006, and about 40 percent of the current bus fleet is equipped with newer "clean diesel" technology, which "virtually eliminates nitrogen oxides and particulates," says Allen Schaeffer, executive director of the Diesel Technology Forum. (In other words, the buses still guzzle fossil fuel, but release fewer emissions and adhere to EPA standards while doing so.) Only 1 percent use natural gas, 2 percent use gasoline—and less than 1 percent are electric.

In citywide public transit systems, green buses are farther along: 385,000 electric commercial and public buses currently exist worldwide. Shenzhen, China, just rolled out an all-electric, 12,000-bus strong fleet; New York and Los Angeles have both promised to fully transition their own fleets to electric in the next few years; Proterra, the largest manufacturer of electric buses in the U.S., has partnerships with 70 transit agencies across the U.S. who plan to add electric models too. Their motivations are clear. Diesel buses contribute significantly to urban air pollution. Breathing in diesel fumes has been linked to asthma and cancer; and while lower sulfur diesels can cut emissions by more than 85 percent, even limited exposure to these particulates can be harmful.

And kids are particularly vulnerable to those incidences of asthma, allergies, and infections: Their airways are not fully developed, and therefore narrower and easily inflamed. In 2012, the Centers for Disease Control and Prevention reported that about one in 10 children had asthma, and a staggering near-50 percent increase in asthma rates was recorded among black children from 2001 through 2009.

Wargo's school-bus emissions research was conducted in Connecticut, where that year 6,100 buses carried nearly 387,000 students to school every morning and afternoon. But his findings had broad implications for the national school bus industry. Students were exposed to the highest level of emissions as they entered and exited the bus, when the vehicle sat idling, Wargo found. "They tended to diminish during the school day. And then, at the end of the day, there was another burst to their exposure," said Wargo in an interview with ABC News at the time. "It was a surprise to me."

Diesel became the fuel of choice for school buses for practical reasons: Full-size buses are built on the bones of large commercial trucks, which typically rely on diesel power. (Though, as diesel prices rise, some school districts are moving back to to gas-powered buses.) There's also a safety issue that makes diesel preferable to gasoline. "In an accident, a school bus powered by diesel fuel is less likely to catch fire than a school bus powered by gasoline," Schaeffer said. "Plus, school districts don't have a lot of excess capital lying around, so their investments need to be made on technologies that last a long time." That tends to mean at least 10 years, he said.

Blue Bird, a major bus manufacturing corp, unveiled two electric school bus models in 2017, using a $4.4 million grant from the U.S. Department of Energy; and IC Bus and Thomas Built Buses piloted their own concept electric models (the ChargE and the "Jouley," respectively).

But even with a growing menu of school bus electrification options emerging, primarily over the last three years, districts have been slow to hop aboard. The cost of trading up may be the biggest barrier to broader adoption. It's hard for transportation officials to justify extra spending on a new fleet, or spending less on upkeep of their existing fleets to invest in new models, said Marie Bédard, a manager at Lion Electric, currently the most established distributor of electric school buses in North America. Lion Electric has deployed 150 vehicles in four states (Minnesota, Massachusetts, New York, and California) so far. While its school buses cost up to three times as much as typical school buses, long-term costs can depreciate significantly when you account for the money saved on upkeep and fuel. "Diesel school buses are the cheapest the day you buy them, and ours get cheaper as they grow older," Bédard said.

Estimates from the public bus sector indicate that lifetime costs of a diesel bus are $1.4 million, versus an electric's $1 million. "It costs one-sixth to one-eighth as much to operate [an electric school bus] as a diesel bus," Jim Castelaz, the chief executive officer of Motiv Power Systems, which has outfitted California schools with electric school buses, told SFGate.

For districts to surmount those cost barriers to entry, state grants and partnerships with electric companies provide a boost. California school districts have adopted 90 percent of the entire share of electric school buses in the country, because there's a lot of money there for electrification: This year, the state earmarked $180 million on vouchers for hybrid and zero-emission trucks and buses. Utility regulators have also allocated $2.2 million to investigate the idea of using electric school buses as "big batteries that can send energy back to the state's electric grid when needed," SFGate reported. And recently, California's electric utility company announced it would invest $768 million on electric vehicle charging infrastructure, installing chargers for heavy-duty vehicles like school buses at 1,570 sites.

Elsewhere, the Massachusetts Department of Energy provided grants for the Concord School District to pilot and charge a bus this year, and NYSERDA, the New York State Energy Research and Development Authority, just started providing electric school bus vouchers through the NY Truck VIP program. But outside of these states, school districts "need a little bit more incentive," Bédard said. "There are really no steps right now to push them toward that direction, other than some states [that] are looking at subsidies and government funding and whatnot."

Transportation officials in the California districts piloting grant-funded electric school bus programs are seeing positive results. The Rescue Union School District's director of transportation told SchoolbusFleet.com that he expects saving 50 percent on the district's fuel costs, while the Twin Rivers Unified School District reported lowering costs by 82 percent, based on data from January and February alone. Both districts use charging stations when electricity is cheapest, and Twin Rivers also partnered with the local utility company to cover some extra costs.

There's one other big potential benefit to going electric, Bédard said. "When kids get to school, they are a bit more relaxed," she said. "They don't have to yell over the engines."

The other big challenge when it comes to cutting the carbon footprint of the big yellow box goes beyond switching powertrains: Schools also need to figure how to rein in their sprawling routes.

Efficient routing is a big challenge for transit planners in city systems, too, but school buses can be uniquely difficult. In districts where they're deployed, they're designed to serve every student in the district within a certain radius. In some cities, they are asked to arrive at special education students' front doors. And after the passage of the McKinney Veno Homelessness act, all public schools are legally mandated to send homeless students to their schools of origin, even if they're living outside of district lines, in shelters or on the street.

Lately, the distances school buses have to travel are stretching longer as charter schools and other, often distant options have gained popularity, especially in big urban districts. "Fewer and fewer families are attending neighborhood schools," said Kristin Blagg, who researches school choice at the Urban Institute. Historically, school buses tended to run neighborhood-based routes close to schools. But as kids disperse, systems have become less efficient, and costs have gone up. "The other side of that is there is a lower quality of service for families—the school transportation system is not meeting the needs for families, and not meeting the needs of schools," Blagg said.

Routing efficiency isn't just about saving fuel: Transportation gaps caused by strained budgets can also limit the education options of low-income students to schools close by. "When lack of transportation means less dispersion, it is a source of both economic and racial isolation, leaving diverse cities with segregated schools," wrote Gail Cornwall, an education columnist at Forbes.

There may be tech-based solutions to the routing puzzle. The Boston Public School District has one of the most stretched-out bus systems in the country, thanks to its expansive school choice policy and requirements to serve a high number of students with special education needs and homeless students from a pinwheel of far-reaching neighborhoods. Every day, BPS buses travel to 268 locations to pick up and drop off students, which is more than double the number of schools within BPS itself. That's led to a snarl of routes and the highest per capita transportation costs of any urban school district in the country.

There had to be a more efficient way to route the buses—"a way to take some buses off the road, to ease traffic congestion on the streets, and to save money for the school district," said John Hanlon, chief operating manager of the Boston Public School System's bus fleet. In olden days, he said, "people were literally putting push-pins on maps and using string to draw bus routes." Today, a team of eight planners works 3,000 man-hours every summer assembling the routes together. So BPS decided to ask someone to fix it. Last March, it hosted its first school-led transportation hack-a-thon, and invited teams across the country from Oklahoma to Ohio to Chicago.

The best answer came from a team of Massachusetts Institute of Technology researchers led by mathematician Dmitris Bertsimas. They selected seven to 10 optimal sets of bus trips for every school. Then, they built an algorithm that would allow them to consolidate bus stops, strategically link trips together, and choose routes that would minimize the travel time to every school across the city.

The algorithm helped them eliminate 50 bus routes at the start of the 2017–18 school year, saving the district roughly $5 million upfront. "If each piece in a jigsaw puzzle is a bus, we have a jigsaw puzzle we were doing upside down and has 650 pieces, and sometimes the pieces didn't fit together," Hanlon said. "[Now], we've gotten to a point where we could actually do the jigsaw puzzle face up, with only 600 pieces."

And each day, they shrink their carbon footprint by 20,000 pounds of emissions. Scale that up over an entire 180-day school term, and this year they saved almost four million pounds.

A Boston-style algorithm could easily be retooled to serve other school districts (except those with fixed-route systems, like New York)—just plug in the stops and routes, and let the algorithm do its thing.

But fully modernizing and cleaning up the nation's entire school bus fleet remains a much taller order. "Some new manufacturers of electric buses are painting some really incredible pictures for what's possible there," said Diesel Technology Forum's Allan Schaeffer, "but that has to square with the reality of what this is about, which is moving kids reliably and safely to school each and every day. It boils down to a lot of operational savvy, high efficiency and service, and keeping costs low to keep routes expanded. It makes it very difficult to make these broader investments in these different kinds of technologies."

This story originally appeared on CityLab, an editorial partner site. Subscribe to CityLab's newsletters and follow CityLab on Facebook and Twitter.