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, and Fallout Tactics.

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 travelling.

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, or while fighting enemies.

Fallout New Vegas
Identical to Fallout 3. You can fast travel to locations already discovered and you cannot fast travel to those locations not discovered.