Postmile reference markers of major intersections, entrance ramps, and exit ramps for all state highways were obtained from the Caltrans Traffic Accident Surveillance and Analysis System (TASAS) and the Traffic and Vehicle Data Systems Unit (available on http://traffic-counts.
Collisions occurring on state highways have several additional fields: route number, route direction, postmile, and postmile prefix.
Events occurring in the realigned sections are likely to have the designated R or M prefix and the postmile value will differ from the original route.
Each of the potential postmile prefixes was associated with every route.
Once the separate routes have been created for each prefix version, the routes must be calibrated using the associated postmile reference markers.