 Except where prefixed with a letter , postmiles were measured on the road as it was in 1964 , based on the alignment that existed at the time , and do not necessarily reflect current mileage . R reflects a realignment in the route since then , M indicates a second realignment , L refers an overlap due to a correction or change , and T indicates postmiles classified as temporary ( for a full list of prefixes , see the list of postmile definitions ) . Segments that remain unconstructed or have been relinquished to local control may be omitted . The numbers reset at county lines ; the start and end postmiles in each county are given in the county column . 
