"Mountains" are the slowest, and "Ocean" sectors are impossible to travel across. "Desert" and "Ruins" sectors are the fastest to travel across. Travel speed across the world map is determined by the sectors it's comprised of. In Fallout Tactics, player's location would be marked by radiation trefoil, and no tracing is available. In Fallout and Fallout 2, player's location would be marked by a red cross icon, and his path would be traced by line strokes (the stroke length hints at player's speed - the longer they are, the more time it took the player to travel through that particular sector). To move the player across the map, one has to click at any area he's not currently at. When stationary, the player's location is marked by an icon (inverse pyramid), clicking on which would take him to the location he's currently at, which does not have to be a specially designated location - the player can make stops in the desert or the mountains to relieve himself of unnecessary equipment, change or reload the weapons he's holding, or administer drugs to himself or one of his party members. When player leaves a location with intent to travel to another one (typically located far away, due to Fallout world's extremely low population density), he is presented with the world map. The world map's primary function is to enable navigation for player's travels. In Fallout, Fallout 2 and Fallout Tactics, the world map is bird's eye view map of the landscape of the games, divided into a matrix of square-shaped, equally sized sectors.
Fallout, Fallout 2, Fallout Tactics Introduction