How green are trains, public transportation, and hybrid cars? It depends.

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Most of us assume that some things are givens when it comes to environment-friendly transportation choices. Among those assumptions: Taking the subway is better than driving an SUV, riding a train tops hopping on a plane, and a hybrid car is much preferred over a conventional gasoline-powered vehicle.

But that's not always true. Recent research [PDF] points to just the opposite, sometimes.

Environmental engineers Mikhail Chester and Arpad Horvath of the University of California found that instead of taking a train into the city from suburbia, there are times when  "people would be better off traveling through town in a gas-guzzling, high emission SUV," reports Red Orbit. Ouch!

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But it gets even worse: Taking the commuter train across Boston results in higher greenhouse gas emissions than traveling the same distance in a jumbo jet, says New Scientist.

How could that possibly be?

Part of it comes down to how the power that fuels the transportation is generated. Boston's electric commuter trains use electricity that comes mostly from burning fossil fuels.

What makes this study different from some in the past: Instead of comparing the climate effect of modes of transport according to their emissions, the researchers looked at emissions caused by building and maintaining various types of transportation,the infrastructure associated with them, and the generation of the fuel required for each.

To represent short, medium, and long-haul travel, they measured light rail systems in San Francisco and Boston, three types of gas-powered vehicles  (2005 models of Toyota Camry, Chevrolet Trailblazer SUV, and Ford F-150 truck), and Embraer 145, Boeing 737, and Boeing 747 airplanes.

Then they figured out how many passengers each would carry over its lifetime, how many miles it would travel, and the life expectancy of its infrastructure – tracks, roads, and airports.

According to New Scientist:

"Including these additional sources of pollution more than doubles the greenhouse gas emissions of train travel. The emissions generated by car travel increase by nearly one third when manufacturing and infrastructure are taken into account. In comparison to cars on roads and trains on tracks, air travel requires little infrastructure. As a result, full life-cycle emissions are between 10 and 20 per cent higher than 'tailpipe' emissions.

Occupancy also matters when it comes to measuring greenness – almost-empty buses at off-peak hours were less efficient than even SUVs and pickup trucks.

This way of measuring the environmental impact of transportation should be taken into account when planning new  initiatives, say the researchers.

"We should avoid building rail systems that are disconnected from major population areas and require car trips and parking to access," Chester advises.

Electric cars were mentioned by Chester and Horvath (their green score can depend on how the electricity is generated), but hybrid cars didn't make the report. Still, they were in the news this week: Hybrid cars are great for air quality, they save money at the pump, and they make the owner feel good, but, in the words of the Houston Chronicle, "They’re terrible for roads."

Do hybrids cause huge potholes? Do they emit some kind of chemical that destroys blacktop?

Well, no. "Fuel-efficient cars are draining government coffers of money needed to repair roads and build new ones," the Chronicle says.

In other words, because hybrids use less gas, they generate fewer tax dollars to pay for transportation infrastructure.

Many think that the fairest way to tax car use is not on gas purchased but on the number of miles driven. Hybrids and electric cars could certainly hasten that.

Of course, none of this is to deny the long list of benefits of buying a hybrid vehicle (or even taking the train or bus). But it does point out the complexity of many environmental issues.

ADDED LATER: Click here for a discussion about the latest in hybrid buses.

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