  In this paper, we combine the most complete record of daily mobility, based
on large-scale mobile phone data, with detailed Geographic Information System
(GIS) data, uncovering previously hidden patterns in urban road usage. We find
that the major usage of each road segment can be traced to its own -
surprisingly few - driver sources. Based on this finding we propose a network
of road usage by defining a bipartite network framework, demonstrating that in
contrast to traditional approaches, which define road importance solely by
topological measures, the role of a road segment depends on both: its
betweeness and its degree in the road usage network. Moreover, our ability to
pinpoint the few driver sources contributing to the major traffic flow allows
us to create a strategy that achieves a significant reduction of the travel
time across the entire road system, compared to a benchmark approach.
