Postmiles are valid postmile points at 0.1 (1/10th) mile intervals, based on Office of GIS Linear Referencing System layer (TIGER linework). Exceptions that are included are the beginning and ends of routes (and other situations) that don't necessarily begin or end on the even 0.1 mile interval. This data is based upon the 31 December 2017 extraction from the Transportation System Network (TSN) database maintained by the California Department of Transportation (Caltrans) Traffic Accident Surveillance and Analysis System (TASAS) branch.