Choosing the greenest form of transport is like solving a complicated mathematics problem: the number of passengers, the type of fuel, the length of trip and the time of travel are all variables in the equation of calculating a traveller’s carbon footprint.

But there are a few simple rules that can help when trying to decide on the mode of transportation with the lowest greenhouse gas emissions.

Take the bus
Coach travel has increased in recent years with several low-cost carriers hitting the US market. The good news, besides cheap prices? Bus travel is a great choice for low carbon emissions per passenger, whether you’re making a three-hour jaunt from Washington, DC, to Philadelphia, or a 1,000-mile slog halfway across the country. A 2008 peer-reviewed analysis from the Union of Concerned Scientists put motorcoach at the top of the list for greenest travel options, for both solo travellers and groups. According to the report, “a couple boarding a motorcoach will cut their carbon [footprint] nearly in half, compared with driving even a hybrid car. And if they take the motorcoach rather than flying, they will cut their emissions by 55 to 75%, depending on the distance they travel.” The free wi-fi usually on offer is nice, too. The only time the bus doesn’t win? On a local commute when a subway or train line is an option. City buses, which don’t take the most direct route, have “a lot of stop and go”, said Laura Draucker, sustainability manager at the World Resources Institute (WRI), an environmental think tank.

Hit the train station
Trains are another low-emission option, especially if you’re travelling by electricity-operated trains like those running up and down Amtrak’s northeast corridor or high-speed trains around the world, such as France’s TGV. For trips of 500 miles or less for one or two travellers, train was one of the top options on the Union of Concerned Scientists report (see the handy Vacation Traveler Carbon Guide). Trains continue to be better than planes when you take into account taking a subway to and from the train station, rather than hailing a cab at the airport. “Think about your whole trip in a holistic way,” Draucker advised.

Board a jet plane
Road-weary travellers rejoice: the plane beats the train for long trips. “If it’s more than 500 miles, likely in the US, you wouldn’t take the train anyway,” said Draucker, who adds that in Europe the long-haul train-versus-plane dilemma is “probably a wash”, since the trains in Europe are so reliable. A first-class airline ticket, however, doubles your carbon footprint, since the seats take up twice the space as the smaller chairs in economy.

Nonstop flights also save emissions. Those takeoffs and landings – and taxiing around the runways – pump out a lot of carbon. Draucker said that a layover in Dallas on a Washington, DC, to San Francisco route adds 10% more emissions. Even the route of a flight makes a big difference. A Charlottesville, Virginia, to New York City flight produces fewer than 200 pounds of CO2 emissions, but stopping in Washington, DC, along that same route  bumps that up to more than 200 pounds . Flying through Atlanta instead (a common hub for connections along the US East Coast) causes emissions to skyrocket to about 600 pounds . (See our report on Greenopia’s rating of eco-friendly airlines for more on how carriers stack up on the environmental front.)

Load up the car
That traditional summer family road trip isn’t a bad choice at all. In fact, a typical car or even a typical SUV is a better carbon choice for four passengers than the train or plane, whether you’re stuck in the car for 100 miles, 500 miles or – pack some DVDs for the kids – 1,000 miles or more. Avoiding congestion helps, too – just as a plane sitting on a runway eats up fuel, so do cars stuck in traffic. And, of course, a hybrid or fuel-efficient car emits less carbon dioxide than an SUV. But solo travellers are better off leaving the car at home: even flying economy solo beats driving, whether it’s a short or long trip.

Draucker said that “ideally, what we want to see is more transformative changes” – such as airlines using biofuels and high-speed rails being built with better infrastructure. Researchers at the University of California, Berkeley, for instance, found that factoring in infrastructure construction plus vehicle and fuel production increased total emissions for various modes of transportation, particularly train. But in the meantime, both business travellers and vacationers need to get from point A to point B. “We realize every little bit helps,” Draucker said.

Lori Robertson writes the Ethical Traveller column for BBC Travel. You can send ethical dilemmas to