The total time in hours is based on a rough estimate of the average terrain type between destinations. This should not be viewed as absolute. I used a terrain type modifier of 0.75. The DMG doesn't have an entry for Desert with road, but I averaged the terrain modifier between Highway (1) and Trackless(0.5). The results shown for travel time are given for a humanoid with base speed of 30 feet.
This is a companion discussion topic for the original entry at http://athas.org/tools/distance