For instance, I've been playing with the Tiger/Line dataset for Pennsylvania. There are 9160094 line segments (not all of these are actually streets). So having an index to quickly find the 50-100 segments around my current location is very important, otherwise I have to check every line to see if it happens to intersect with the currently displayed area.

That sounds like a typical programming competition task... the kind where I read the winners notes hoping to learn something, but they're all over my head