It's code for "don't travel, especially long distance"... because most people would simply not be willing to make many trips if the trips took as long as the non-flight option would require.

It's about 60 mpg per passenger to fly domestically and 90 mpg per passenger to fly internationally.

If you have a family of 4, you can think of it as the equivalent of a 15 mpg vehicle for domestic flight and 22 mpg vehicle for international flight. So somewhere in the range of a full-size pickup truck.

But -- when you fly, you go very far. If you go on vacation to Hawaii from San Francisco once a year with your family, that's the equivalent of driving a Ford F-150 for 5000 miles. If you visit India or China that's 15,000 Ford F-150 miles! In a single trip, more than what most people drive in an entire year!

So you can make a big difference just preferring local vacations instead of remote ones.