The city is not an undifferentiated terrain and travel does not occur along straight-line paths at constant velocities. Variations in travel velocities at different locations bend the minimum time paths away from regions of high congestion. This paper discusses a transformation of the urban plane into a time surface on which distance corresponds to travel time, and describes the construction of minimum paths and isochrones for various velocity fields. This view of the urban transportation system allows us to discover some of the important features which are often hidden in a network description of the system.