How MapQuest works

Ever wonder how MapQuest, or the OnStar navigation system, directs you from one point to another? The New Yorker's April 24 issue has an in-depth article about it. Here's an interesting synopsis of the system:

Generally, MapQuest and OnStar choose a road based on their calculations of which will get you there fastest. The criterion is time, a function both of speed and of distance. They do not, as some people suspect, simply pick the shortest route; otherwise, you might spend all your time on side streets, stuck at traffic lights or goat crossings. The algorithms consider the length of a road segment and the expected speed of the road and calculate the time it will take you to pass along it. Every road segment has a “costing,” a sum of the features that can slow a driver down. Turns, merges, exits, toll plazas, stoplights, speed zones: they all carry a cost. (Navteq has five “functional classes” of road, ranked according to connectivity and speed. An interstate highway is a one; a local street is a five.) These systems do not yet take into consideration traffic, construction, weather, time of day, or one’s tendency, on certain roads, to go faster than the speed limit.

Nor, I might add, do they suggest the most serendipitous or breathtakingly scenic routes.