1. A method comprising: receiving first data collected by a first sensor while a vehicle is driving on a roadway, the first data being collected within a first sensor range of the first sensor; using the received first data from the first sensor to generate a map of the vehicle's environment within the first sensor range; estimating, by one or more processors, the vehicle's geographic location and heading by comparing the generated map to a stored map identifying the geographic locations of objects including reference objects having particular retroreflective, brightness, or intensity characteristics; receiving second data collected by a second sensor while the vehicle is driving on the roadway, the second data being collected within a second sensor range of the second sensor, wherein the second sensor range extends beyond the first sensor range; identifying a second data reference object having a particular retroreflective, brightness, or intensity characteristic from the second data collected by the second sensor, wherein the second data reference object is identified from a portion of the second data that was collected outside of the first sensor range; correlating the identified second data reference object to a reference object of the stored map based on the particular retroreflective, brightness, or intensity characteristic, the reference object of the stored map having a known geographic location; and using the known geographic location of the reference object of the stored map to refine the estimated heading of the vehicle.