Fast travel

Because the area of empty desert between earlier Fallout games would be too much to render, and the raw amount of time spent walking would be boring, fast travel is a function provided on the world map in Fallout, Fallout 2, Fallout 3, Fallout Tactics, and Fallout: New Vegas.

Fallout, Fallout 2, and Fallout Tactics
When stepping on the exit grid in any map, you'll be kicked out to the world map. If you want to get anywhere at all, you'll be fast traveling.

Moving across the vast desert takes an amount of time based on the terrain; mountains cause slower travel, while deserts are easier to get through.

Driving in a vehicle such as a Chryslus Motors Highwayman or a Brotherhood Hummer will increase your fast travel speed, but only as long as there's gas in the tank in Fallout 2.

About once a day, the game will roll a random encounter, though the probabilities are derived from your Outdoorsman skill.

Fallout 3
Fast travel in Fallout 3 is significantly more simple. Simply select any discovered location marker from the world map, and you can fast travel to it. Any fast travel, be it from Raven Rock to Rivet City, or from Vault 101 to Megaton, will take one game hour to complete.

You cannot fast travel to locations that haven't been formally discovered, and you cannot fast travel while inside, while in the air (such as in the middle of a fall), or while in the vicinity of or fighting enemies.

Fallout: New Vegas
Fast travel in Fallout: New Vegas is identical to that of Fallout 3 except in the time it takes to get to your destination. If you travel to a close destination, time seems to pass in increments of 10, while farther destinations in increments of hours. It should be noted that holding a two handed weapon does not affect the time it takes to travel, but sneaking while traveling will effectively doubles the time it takes. You can fast travel to locations already discovered but you cannot fast travel to locations that have not yet been discovered. You can also fast travel underwater which can be handy if you don't have the Rebreather and are about to run out of oxygen.