In the United States we often don’t think twice about hopping on a plane to get from one place to another? But, what if there was a way to get from Point A to Point B that is faster, more convenient, and pollutes less? Would you do it?

Maybe.

After all, cost is often the overriding factor when comparing travel options.

Well, let me first explain the Four Hour Rule and then we’ll compare some trips side by side.

The Four Hour Rule is simple. All other things being equal, a four hour train, bus, or car ride will always be preferable to a plane trip of any length.

Here’s why. A one hour plane trip is never one hour. By the time you factor in the time it takes to get to the airport (two hours early, BTW), flight time, deplaning time (and waiting for baggage if you checked your luggage), and getting from the airport to your final destination, you’ve invested far more than the four hours it would’ve taken on a bus, train, or car.

Let’s look at a real life example. Say you’re on vacation in Boston and are going to New York. If you’re staying in the historic section of Boston, getting an Uber to Logan will cost about $15 and take about 20 minutes. And remember, you need to arrive at the airport two hours in advance to allow for check-in and security screening (an operation that seems to take longer and longer all the time, even with TSA PreCheck). The lowest cost for the flight is about $75 and takes an hour and fifteen minutes. Deplaning and getting out of the terminal takes at least a half hour (more if you checked bags) and the ride from La Guardia or JFK is anywhere from 30 minutes to an hour and costs between $45 and $75 for an Uber to Manhattan.

All in, the one hour and fifteen minute flight from Boston to New York will take at least five hours and cost a minimum of $135.

Intuitively, it might seem that in our hypothetical Boston to New York trip, driving would be the least expensive option. But, there are a few things to consider.

The driving distance from Boston to New York is a little over 200 miles and takes about four hours (depending on traffic). So, if you car gets 20 miles to the gallon and gas costs $2.50, you’ll spend about $25 in fuel to get from Beantown to NYC. But, tolls will cost an additional $10 (assuming you have a toll tag, more without). But, keep in mind that once in New York, you’ll need a place to park, always a pricey proposition in the big city, not to mention the hassle of having to deal with New York traffic.

Meanwhile, ridesharing from Beacon Hill to Amtrack station at Back Bay takes about ten minutes and costs about $10. The Amtrack to Penn Station takes about four hours (a faster train taking 3 1/2 hours is more expensive) and costs as little as $59. Once in New York, Penn Station is walking distance to the Empire State Building and lots of other attractions. But, if you wanted to take an Uber to your hotel in Manhattan, it would probably take fifteen minutes and cost about $20, depending on where you are staying. All told, the train option would take about 4 1/2 hours and cost about $90.

So, the train in this example is not only faster and more convenient than flying, but it also costs less. And, the carbon emission from taking the train is a fraction of what it would be by flying.

But, this is just one route, Boston to NYC. What about others?

This model works best on train trips five hours or shorter. For example, Portland to Seattle takes 3 1/2 hour on Amtrak and costs about $35 while flying between these two cities takes 50 minutes and costs about $75.

Longer trips usually only make sense when you can take an overnight train. That way you’re sleeping during the extra time the train takes as compared to flying. As a bonus, you’re also saving the money you would be spending on a hotel for the night.

Internationally, I use the same formula. Flying from Paris to Amsterdam take about 75 minutes and costs about $55. The train takes 3 1/2 hours and costs about $105. But, once you factor in the time to get to the airport and arriving early for security, etc. the train comes out ahead once again. Plus, with the airports being further from the center of the city than the train stations, an Uber or taxi costs less as well. And, in Europe, an extensive infrastructure of high-speed rail allows visitors to move further in less time.