wow, this is right up my alley! The useable runway width is measured from the (outer) edge of the painted white "edge" lines. That dimension is what is designed to be used for normal landings and take-offs. In addition, a runway is required to have safety areas, obstacle free areas/zones, runway protection zones, etc. These dimensions are determined by the size of the runway, size of the aircraft using the runway, and the approach minimums used on each runway (instrament, non-precision, or visual approach).