the logic behind MapQuest.com

heres one that i've been trying to figure out for awhile. Mapquest.com, how in the heck can they program knowing the roads to take to get from point A to point B.... their database must be intense will all the different maps it has but I just can't figure out how to map which highway to be on, where to turn, which direction to turn... it's just crazy. Anybody have an idea of how they do this?

I just posted it in the this forum because its a web development thing.. I'm just concerned with their logic.