With each level of this perk, your travel time on the world map and the drain on the Highwayman's battery is reduced by 25%. This is the in-game time that is lowered, and not the actual time to travel between towns. Your actual speed on the map will not decrease.
The Fallout 2 manual erroneously states that the Outdoorsman requirement is 60%.
Behind the scenes
The image for this perk is a reference to the DC Comics superhero Flash.