Houston hasn't always been in the vanguard when it comes to weaning its transit fleet away from diesel, but an upcoming initiative could begin connecting its bus system to the cutting edge.

Metropolitan Transit Authority will put an all-electric bus into service on Dec. 4, giving the agency its first close look at a new type of transit vehicle.

Many cities are far ahead of Houston in making the transition from diesel buses to cleaner fuels. Almost two-thirds of Metro's fleet still burn diesel.

"Electric vehicles is what everyone is focused on," said Doran Barnes, CEO of Foothill Transit in Southern California and chairman of the American Public Transit Association.

Moving away from diesel - even the "clean" diesel Metro officials lauded when they bought many of the current buses a decade ago - is emerging as an important strategy toward improving air quality.

In places like Houston, which already is under heavy pressure to reduce emissions by federal regulators, large vehicle fleets like Metro's buses and trucking firms are some of the most effective targets for cleaner air options.

"The way you innovate is you look at technology and see how it can improve your community," said Metropolitan Transit Authority CEO Tom Lambert.

The electric bus debuting in Houston will operate completely on battery power and has a range of about 40 miles on a single charge. Built by Proterra, based in the San Francisco area, the bus will be tested for 90 days.

Metro will run the bus through a number of stress tests to analyze its worthiness for operating in Houston. Lambert said the tests include putting the bus in a special garage meant to dry paint at the transit agency's maintenance facility.

"We want to know exactly how this system performs in real-day service," Lambert said, joking that Houston experiences extreme heat. "We're going to take it from 110 degrees and see if it can cool down to 70 degrees in 30 minutes."

To riders, the electric bus is similar to any other bus with low floors, fare vending machines and wheelchair accessibility.

The big difference is in emissions. Aside from the pollution related to generating the electricity itself, the bus has zero emissions. The Proterra bus gets the equivalent of 22 miles per gallon, or roughly five times the fuel efficiency of a diesel bus.

It will operate on the 63 Fondren route, and will be noticeable with special decals on the exterior. The bus line, which averages 5,000 boardings on a typical work day, is ideal for testing the electric vehicle because it's about 20 miles round-trip and has a minimal layover at the Missouri City Park and Ride lot.

The time at the park and ride lot is crucial because those 10 or 15 minutes are what make operating the bus all day possible. The bus can charge its batteries while the driver is taking a short break. According to Proterra, the bus can accomplish a 90 percent charge in less than 10 minutes.

Technology in electric buses, however, is evolving quickly, said Terence Fontaine, Metro's executive vice president. By the time Metro is ready to buy an electric bus, the vehicles should have a range of more than 200 miles before needing a charge.

Waiting, however, has put Metro behind many of its peers, something Fontaine said the upcoming test is meant to address, especially as air quality in the Houston area puts pressure on transportation entities like Metro to clean up their act.Here in the heart of the petroleum industry, diesel remains dominant years after other transit providers converted their entire bus fleet to compressed natural gas.

"It is something that goes on in Texas and our agencies generally," said Adrian Shelley, executive director of Air Alliance Houston, of the diesel-heavy fleet.

Fewer than 60 of Metro's more than 1,200 buses operate on compressed natural gas, and only a third are diesel-electric hybrids that while cleaner than previous models still fall far short of the emission improvements related to alternative fuels. Nearly 60 percent of the agency's buses run strictly on diesel.

Metro, in touting the purchase of 50 natural gas buses and opening of a new natural gas fueling station in January, said the buses produce 20 percent less carbon dioxide and 70 percent less carbon monoxide than diesel models.

Shelley acknowledged "there has been a missed opportunity" for Metro to aggressively transition from diesel and improve the region's air quality. Moving forward, however, he said an electric bus could allow Metro to skip natural gas altogether and jump to electric.

"Most of us see (compressed natural gas) and propane as bridge fuels," Shelley said.

Houston transit officials have had a two-decade love-hate relationship with switching fuels. Metro was the first transit agency in the nation to switch to liquefied natural gas in 1992. Three years later about one-fifth of Metro's buses and vans ran on natural gas - then the effort ran out of steam. Maintenance issues and low gasoline prices pushed officials back to diesel. Revised federal policy that allowed low-sulfur diesel and reluctance to experiment kept previous Metro administrations from adapting - something the current regime has only started to change.

"We're interested," Lambert said in September, when asked about the agency's slow development of alterative fuels. "But there is a benefit in not being the first one out there trying something new."

Meanwhile other transit agencies have traded in most if not all of their dirty diesel buses for compressed natural gas.

Los Angeles County Metropolitan Transportation Authority, one of the largest bus agencies in the U.S., phased out its last diesel bus in 2011. It was the culmination of a nearly 20-year effort to buy only clean buses and eliminate diesel use, though officials acknowledged it came with an additional financial cost. Supporters, however, pointed to improved air quality lowering health costs and dramatically improving the quality of life of many Los Angeles communities.

Nearly 590 of Dallas Area Rapid Transit's 650 buses are powered by compressed natural gas.

The remaining diesel buses operate DART's express bus system, transit agency spokesman Morgan Lyons said.

"They'll be gone at the end of the year," Lyons said.

DART also is exploring electric buses for its D-Link downtown circulators. Dallas officials won a $7.6 million grant from the Federal Transit Administration to buy seven Proterra buses, which Lyons said are expected to be delivered in the summer.

Each of the Proterra buses costs about $800,000, or $200,000 more than a normal diesel bus. Over the lifetime of the bus, however, Proterra estimates the bus costs $400,000 less to operate based on fuel costs compared to electric power.

Depending on the outcome of the test, Metro officials said they might also be in the market for electric buses. Securing new buses, however, is a laborious process, with agreements to buy buses coming months prior to that first bus being delivered. The agency buys about 100 vehicles per year between its conventional bus fleet, a growing number of large 60-foot buses used on major routes, commuter buses and smaller vehicles.

Under current scenarios, the earliest an electric bus could begin operating in Houston is around 2018. Once ordered, Proterra vice president Matt Horton said it takes the company about a year to fulfill the order.