The North American school bus has experienced few changes over the years.

They're still purpose-built under federal regulations to be clearly distinguishable from other transit buses; they're still clad in a specifically formulated shade of yellow and feature the same impossibly high-backed seats, exterior warning lights and safety devices; they're still largely seatbelt- and AC-free; they're still subject to same painful pre-adolescent social hierarchy that dictates who sits in front and who sits in back.

And while some improvements in school bus fuel efficiency have been made, the same type of exhaust-spewing behemoths — or "rolling cancer machines," as Motherboard dubs them — that ruled the roads in the 1970s, '80s and '90s are still making the rounds today. And there's little chance for radical change unless the roughly 500,000 school buses in the United States, which collectively consume a whopping $3.2 billion in diesel fuel annually per the American School Bus Council, start plugging in.

So why has the American school bus been held back as its peers — cars, trucks and even public transit systems — have advanced? How did something that provides such a vital service fall so far behind in adopting electric vehicle technology?

CityLab's Sarah Holder recently pondered that very question in a fascinating deep-dive that explores the slow start school buses have had in ditching dirty diesel and embracing clean, emissions-free technology.

As Holder points out, there's a couple key reasons why 95 percent of American school buses — a fleet, mind you, that's 2.5 times larger than all other forms of mass transportation combined — continue to burn diesel.

The most obvious is that big-boned school buses are built like trucks, and trucks, for the most part, are built to run on diesel.

As Allen Schaeffer, executive director of the Diesel Technology Forum, tells CityLab, the protection of precious cargo also helps explain school buses' enduring love affair with diesel: in the event of an accident, diesel-powered school buses are less likely to catch fire than gasoline-powered counterparts, which are also, to their further disadvantage, less efficient. "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," Shaeffer says.

Of the American school buses that aren't diesel-powered, CityLab reports that 2 percent use gasoline while 1 percent run on clean-burning natural gas. Electric batteries power less than 2 percent of the national school bus fleet. But that last figure — in some states, at least — is starting to grow.

School bus in Brooklyn Come rain or shine (or snow): School buses, which overwhelmingly depend on diesel fuel, are the single largest form of public transportation in the United States. (Photo: Spencer Platt/Getty Images)

Today's buses are cleaner, but few are emissions-free

A decent number of school buses have indeed cleaned up their acts ... but still use diesel.

These buses, constituting about 40 percent of the half-million school buses in operation, are outfitted with low-sulfur "clean" diesel technology that meets 2007 EPA emission standards and claims to dramatically limit the amount of sooty, carcinogen-filled fumes spewed from bus tailpipes. (These emission levels are roughly comparable to the emissions of a natural gas-powered bus.)

Still, a majority of America's aging school buses run on diesel — regular, reliable health-compromising, not-so-clean diesel. Some engines might be wildly less polluting than in the past; the noxious exhaust that's linked to lung cancer and a variety of respiratory diseases, diseases to which children are particularly vulnerable, is still present. And because an estimated 25 million pint-sized bus riders — more than half of schoolchildren — are regularly exposed to short and concentrated bursts of this exhaust, there's legitimate reason to worry even if there are stringent emission controls in place that didn't exist a decade ago.

According to a 2002 report published by Connecticut-based environmental scientist John Wargo in which he outfitted his own bus-riding daughter and 14 of her fellow students with air quality monitors, kids who take the bus are exposed to five to 15 times more particulates than kids who traveled to school in another manner. Again, things have improved over the last 16 years but, as Wargo notes in his report, "there is no known safe level of exposure to diesel exhaust for children, especially those with respiratory illness."

Tailpipe and bus Diesel exhaust has been linked to a range of health concerns including lung cancer and exacerbation of asthma symptoms. The nitrogen oxides produced by diesel engines are also a major component of smog. (Photo: David McNew/Getty Images)

The kid-carrying 'workhorses of America's transportation system.'

The not-so-insignificant issue of diesel exhaust aide, yellow school buses play an invaluable role in rural and urban communities alike. Despite its faults (and some recently renewed concerns over safety), this free service is particularly vital for students coming from low-income and working households where other forms of transport to-and-from school aren't financially or logistically viable. There's also environmental perks. For every school bus making the rounds, 36 cars are eliminated from the road per American School Bus Council estimates. This saves a massive amount of fuel (an estimated $6 billion worth in 2010) and halts a just-as-sizable amount of transport-related emissions from polluting the air.

But what if yellow school buses across the nation, including those running on clean diesel, were replaced with electric buses?

A recent report led by the U.S. PIRG Education Fund titled "Electric Buses: Clean Transportation for Healthier Neighborhoods and Cleaner Air," notes that transitioning from diesel to electric school buses could curb an estimated 5.3 million tons of polluting greenhouse gas emissions each year — about the same as removing 1 million cars from the road. If all diesel-burning transit buses including municipal bus systems went electric, an additional 2 million tons or more of greenhouse gas emissions would be avoided.

Describing buses as the "workhorses of America's transportation system," the report notes that the primary reason school districts aren't enthusiastically making the leap from diesel to electric is, not surprisingly, the upfront cost.

Exhaust-free electric school buses typically cost more than twice the amount as models with diesel-burning internal combustion engines. However, the cost of electric buses are on the decline while potential savings — nearly $2,000 a year on slashed fuel expenditures and $4,400 in reduced maintenance costs not to mention the priceless value of improved local air quality and healthier students — are demanding the attention of school districts across the country.

Reads the report: "Dramatic declines in battery costs and improvements in performance, including expanded driving range, have made electric buses a viable alternative to diesel-powered and other fossil fuel buses."

Buoyed by government aid, no other state has applied this optimistic outlook to school buses quite like California has.

California is spending billions to boost electric car sales and build out its charging infrastructure. A growing number of school districts are also adding plug-in buses to their fleets. California is spending billions to boost electric car sales and build out its charging infrastructure. A growing number of school districts are also adding plug-in buses to their fleets. (Photo: torbakhopper/flickr)

California leads the charge

Several California school districts have already fully or partially made the switch to electric, representing that teeny-tiny (for now) fraction of American school buses that have decided to ditch diesel.

These districts are getting much-needed assistance from local and state governmental agencies to help cover the prohibitive up-front costs for electric buses, which the San Francisco Chronicle notes can run from $225,000 to $340,000 versus roughly $100,00 for a new diesel model. These districts hope to recoup the money through the aforementioned reduction in annual fuel and maintenance costs.

"We want to make sure the (environmental) footprint we leave out there is as minimal as possible," Terry Guzman, director of transportation for the Napa Valley Unified School District, which has converted two diesel buses in its fleet to electric, tells the Chronicle. "And with the kids, their respiratory systems aren't fully formed yet. Diesel's something we want to move away from."

East of Napa County in suburban Sacramento, the Twin Rivers Unified School District has introduced a slew of new electric buses into its fleet. "It really fits for school districts, with the way we operate," Timothy Shannon, the district's transportation director, tells the Chronicle. The kids are excited about riding them, because they're electric and they're new."

One of the boons of student-carrying electric buses is that there's ample time during school hours to recharge them — after all, school buses spend most of the day sitting idle between their morning pick-ups and afternoon drop-offs. However, some new-model electric school buses don't quite yet possess the far-reach range needed for field trips and other off-route excursions.

"They don't go far enough for us to use them on athletics, after they've run a full day," Mark Plumb, transportation manager for the Torrance Unified School District in Los Angeles County, explains to the Chronicle. "They wouldn't have the range to take a team out to someplace in L.A. and bring them back."

Plumb's district currently has two buses that have been converted from diesel to electric by the same company — Escondido, California-based TransPower — that helped Napa Unified School District make the switch. Joshua Goldman, TransPower's vice president of business development, anticipates that the cost of plug-in school buses will drop to the same level as conventional school buses sometime between 2025 and 2030 as battery costs continue to drop and the production of plug-in buses ramps up.

In May, leading electric bus maker, Quebec-based Lion Electric, announced that it had, in partnership with clean transport solutions provider First Priority GreenFleet, completed the largest deployment of emissions-free electric buses in North American history to be made by a single original equipment manufacturer. The mighty eLion buses — numbering 40 in total — were rolled-out in 15 California school districts over a 12-month span.

Those 40 buses were funded in part by the Rural School Bus Pilot Project, a $25 million state program run by the North Coast Unified Air Quality Management District that aims to phase-out aging diesel school buses that predate EPA diesel emission standards in rural stretches of the state, particularly disadvantaged ones. The project itself is funded by California Climate Investments, an initiative that exists largely to help drive down the daunting upfront costs of electric vehicles. In total, California Climate Investments is picking up the tab for 150 green school buses across the Golden State. This includes as many as 60 electric and alternative fuel buses to have their cost covered by the Rural School Bus Pilot Project. Industry website School Bus Fleet reports that Ukiah Unified School District and Rescue Union School District are just two of the 15 districts that now have eLion buses in their fleets.

Outside of California, a small number of other school districts, including ones in Minnesota and Massachusetts, are also experimenting with adding electric buses to their existing fleets.

A bus in California A potential money-maker: Some school districts could financially benefit from plug-in buses by allowing them to be used as giant backup batteries for the electric grid during the summer months. (Photo: David McNew/Getty Images)

Illinois dreams electric

Thousands of miles away from rural northern California, the Illinois Environmental Protection Agency is ramping up for a significant investment in electric school buses.

Using the entire $10.8 million sum rewarded to the state by Volkswagen as part of the scandal-plagued automaker's $14.7 billion "Dieselgate emissions settlement, the Illinois EPA recently pitched a plan to purchase up to three dozen plug-in buses and distribute them to school districts throughout the state.

As reported by Energy News Network, this would be the largest dedicated investment in electric school buses using VW settlement funds in the nation and could potentially prevent 2.2 tons of harmful nitrous oxide, a potent greenhouse gas, from entering the atmosphere each year. There has, however, been pushback to the proposal from proponents of natural gas- and propane-powered bus fleets, who claim that these diesel alternatives provide the most bang for the buck as far as the reduction of smog-forming NOx emissions are concerned.

Another added benefit of electric school buses — and this is something California is actively exploring — is that, when not in use, they can serve as backup batteries for the electric grid. In theory, school districts in California, Illinois and beyond could someday be paid by state electric utilities to use school buses as mobile energy storage units while schools are on break for the summer.

"They can be used as a grid service in times of peaking, especially in the summer when school is out and everyone turns on their air conditioning," Aloysius Makalinao of the Natural Resource Defense Council tells Energy News Network.

Makalinao states this is one reason why Illinois is better off spending the funds on electric buses over natural gas buses, despite the higher upfront costs. "... in the long term, electric school buses — especially with their grid resource capability — is better overall," he says.

Still, other school districts aren't quite ready to take the plunge.

Francine Furby, director of transportation services for Fairfax County Public Schools in Virginia, tells the Washington Post that her district wants nothing more than to ferry its students in sleek, clean, pollution-free buses. But the sprawling district, which has one of the country's largest school bus fleets with 1,630 vehicles, is holding off due to high costs (the county has not received grants or aid including VW settlement funds, which it applied for) and technology that it has deemed as being not-quite-there.

"I think probably in time, once we do more of the bus vendors producing this type of vehicle and it's kind of tested through other school districts, it might be something we'll entertain," Furby tells the Post. "But right now, it's just too new."

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.