By the SRD:
30 ft, road through plains, moor, or forest: 24 miles
20 ft, road through plains, moor, or forest: 16 miles
30 ft, road through hills, jungle, mountains, swamp, or frozen tundra: 18 miles
20 ft, road through hills, jungle, mountains, swamp, or frozen tundra: 12 miles
30 ft, road through sandy desert: 12 miles
30 ft, road through sandy desert: 8 miles
30 ft, highway through forest, moor, plains, sandy desert, hills, jungle, swamp, or frozen tundra: 24 miles
20 ft, highway through forest, moor, plains, sandy desert, hills, jungle, swamp, or frozen tundra: 16 miles
30 ft, highway through mountains: 18 miles
20 ft, highway through mountains: 12 miles
A highway is a straight, major, paved road. A road is typically a dirt track.
Edit: Beaten by Camarath!