1. A method for pre-processing roadway data, comprising: receiving, in a processor of a data processing system, at least one ordered series of roadway trajectory points, wherein for each ordered series of roadway trajectory points: the roadway trajectory points are spaced along a curve; the curve being defined in a terrestrial coordinate frame and tracking an along-roadway physical feature of a real-world roadway segment portrayed in a point cloud; each roadway trajectory point having a position in the terrestrial coordinate frame; for any arbitrary roadway trajectory point on the curve, that roadway trajectory point passes a predetermined proximity test for proximity to point cloud points discriminated as representing part of the along-roadway physical feature; each roadway trajectory point having at least one roadway attribute; in the processor of the data processing system, for each roadway trajectory point in the ordered series of roadway trajectory points: evaluating at least one predefined characteristic of the real-world roadway segment at a position on the real-world roadway segment that is spatially associated with the roadway trajectory point to generate an attribute value for each predefined characteristic; and assigning each attribute value to the corresponding roadway attribute of the roadway trajectory point; to generate an attributed roadway trajectory for use in a control system of a self-driving road vehicle.