Fallout, Fallout 2, and Fallout Tactics
When stepping on the exit grid in any map, you will enter the world map. If you want to get anywhere at all, you'll be fast traveling.
Moving across the vast desert takes a varied amount of time based on the terrain; mountains cause slower travel, while deserts are easier to get through.
About once a day, the game will roll a random encounter, though the probabilities are derived from your Outdoorsman skill.
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), while over-encumbered 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. The player may also fast travel while over-encumbered using the Long Haul perk. If you travel to a close destination, time seems to pass in increments of 10 minutes, 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.