Route Finding in Street Maps by Computers and People

R. J. Elliott, M. E. Lesk

We wrote a computer program which gives driving directions in northern New Jersey. Its data base combines a street map and a tele- phone book, so requests like "give directions from the Lackawanna Diner to the nearest dry cleaner" (properly specified) can be answered. This problem involves both human factors and algorithmic problems. From the human factors standpoint, what kind of route is best: shortest distance, most concise directions, fewest turns, or some combination of these? And from the algorithmic standard, what is a good shortest-path algorithm: breadth-first search, depth-first search, pre-storing important routes, divide and conquer, or keep a hierarchy of maps with progressively fewer streets? We implemented breadth-first and depth-first search both single-ended and double-ended. Double-ended search was faster in 14 of 16 examples and produced shorter or equal length routes in 13 of 16. Depth-first search was always faster than breadth-first, and produced shorter routes half the time. We also asked eight subjects for directions on 4 maps. The 32 tries at 4 problems produced 22 different routes. People’s strategies typically include finding main roads, and applying divide-and-conquer as well as depth-first search. But it is difficult to characterize the experimental subjects, since different problems caused them to try different search algorithms.


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