Travelers tend to think of the world as something worth preserving, which forces them to confront a problem: Travel can actually be pretty damaging to the environment. A lot of forms of travel have high carbon emissions, and a lot of tourist activities do significant damage to the sites being visited.

There are plenty of things you can do, of course. There’s the famous “Take only photos, leave only footprints,” mantra, there’s ecotourism, and there’s political involvement. But on a more personal level, how should you travel if you want to travel with the lowest possible carbon emissions?

The obvious answer is to travel by your own power. This could mean walking, biking, kayaking, paddleboating, skateboarding, scootering, or pretty much any other form of travel that doesn’t involve an engine. You could sail, or you could put together a skiff like Huck Finn and only visit places that are downriver. In a lot of cases, like international travel, these aren’t practical. Here’s how to travel with the lowest carbon emissions possible while still using an engine.

How to get there greener

Back in 2008, the Union of Concerned Scientists (UCS) put together a guide titled Getting There Greener. It basically took apart each mode of travel and calculated its total carbon emissions over certain distances. The answer isn’t as cut-and-dry as you might suspect — there are three main factors you need to consider when you’re calculating total carbon emissions for your trip.

The first is the distance you’re traveling, as some options become more efficient and more reasonable over longer distances. For example, planes tend to be big carbon emitters. But if you’re traveling a distance of a thousand miles, the plane is going to be running for about two hours while a car could be running for 15 to 20.

The second thing you have to consider is how many people are traveling with you. If you’re traveling in a car and you have two people instead of one, you’ve already cut your collective carbon emissions in half. If you’re traveling on a plane, you’re splitting those emissions with anywhere from a few dozen to a few hundred other people. But if you’re in first class, you’re taking up more space on that plane — space that could seat another passenger.

The worst modes of travel

Unsurprisingly, the worst mode of travel is first-class airplane travel. This is because of the high emissions of the plane and because of the space you’re taking up. That said, if you’re traveling on your own and are traveling more than 500 miles, the worst way to travel is by SUV. SUVs are huge polluters, but this doesn’t mean that in some situations they aren’t a viable option — if you’ve got a family of four or more, an SUV comes in the middle of the pack for efficiency.

But flying first class is always a mistake, and is never recommended by the UCS. If you can, book your trip on an all-economy flight, and fly direct whenever possible. If you have to make a connection, still try and travel in as straight a line as possible.

For one person going long distances, even the average car is a big polluter. Back during World War II, US propaganda attempted to convince Americans to carpool to save on fuel. The famous tagline was “WHEN YOU DRIVE ALONE, YOU DRIVE WITH HITLER!” A little heavy-handed, yeah, but now we could just as easily say, “WHEN YOU DRIVE ALONE, YOU DRIVE WITH MASS EXTINCTION!”

In short: When it comes to cars, carpool whenever you can.

The best modes of travel

It turns out there’s a single answer to this in literally every scenario: If you can’t bike where you’re going, take a motor coach. Every time. This is especially good news for budget travelers, because in the absence of a solid public transportation system in America, we’ve seen an influx of budget bus companies like Megabus and BoltBus. These are not only some of the cheapest modes of travel, but they’re universally the best. And, hey, free wifi!

The reason is that, while buses use a lot of gas, you’re usually splitting it with a couple dozen people, and that dilutes the emissions more than any other form of travel. So take the Megabus if you can.

If buses aren’t your thing, the next best option is usually to take the train. Trains have way more in the way of carbon emissions than motor coaches, but they also split them among hundreds of passengers. The Northeast United States is the best place to take trains because there’s more of them here, and many run on electricity rather than diesel.

If you’re traveling with a family of four, though, it turns out the second-best mode of travel is actually taking a road trip in a typical car, and if there’s just one or two of you traveling, and you’re going a long distance, the second-best way is to fly economy.

A few more tips

The UCS did a full chart of travel modes by rank. They also provided some useful tips as well. If you’re traveling by car, for example, try to travel when there’s little or no traffic, as traffic increases your emissions. And obviously, if you travel with a hybrid, an electric car, or at least a car with very high fuel efficiency, you’ll be doing a lot to lower that carbon footprint.

Do your research before traveling. You can still see the world and keep your carbon footprint to a minimum.