1. A method comprising: receiving probe data from radar sensors of a plurality of vehicles at a road segment, the probe data comprising an identification of static objects in proximity to the respective vehicles at the road segment and geographic locations of the static objects; determining, using a processor, a first road boundary and a second, opposite road boundary at the road segment from the identified static objects received from the plurality of vehicles; computing a width of the road segment from the identified first and second road boundaries; estimating a number of lanes at the road segment by dividing the computed width of the road segment by a predetermined average lane width; and updating a map database with the estimated number of lanes at the road segment.