Quad Tree
K-D Tree
BKD Tree
Extra
You have two numbers, latitude and longitude, and what you actually care about is the straight-line distance between points. A single B-tree only knows how to sort on one of those numbers at a time. Index on latitude and you get a horizontal strip of the earth. Index on longitude and you get a vertical strip. Even a composite index on both columns can't save you, because a B-tree sorts by the first column and only breaks ties with the second, so the database still effectively sorts on a single dimension, and the strip it pulls back still holds millions of rows, none of them ranked by distance from the rider. So it falls back to the brute-force version, computing the real distance from each driver to the rider one row at a time and discarding everyone past 2 kilometers.