1. A computer-implemented method for travel time prediction, the method comprising: receiving, by one or more computing devices, a route request associated with a user; determining, by the one or more computing devices, one or more candidate routes responsive to the route request, wherein each of the one or more candidate routes comprises one or more road segments; accessing, by the one or more computing devices, a plurality of different road speed models, wherein each road speed model comprises a model of vehicle speeds on one or more of the one or more road segments during different conditions, each road speed model having been created by a road speed modeler configured to: collect observations of road speed data during a plurality of driving sessions, the road speed data including a set of sensor measurements relevant to road speed; and create the plurality of different road speed models corresponding to different conditions that impact the road speed; selecting, by the one or more computing devices, at least one of the plurality of different road speed models based at least in part on one or more conditions associated with the route request; and employing, by the one or more computing devices, the selected at least one road speed model to predict one or more travel times respectively for the one or more candidate routes.