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See here for an explanation of the abbreviations used in the Standard column.
Disclaimer: Unfortunately, an exhaustive list of sources on the opening dates cannot be provided. These were gathered over the years from Web searches, TII reports, newspaper reports, discussion groups, and monumental stones with opening date inscriptions which are usually placed on the roadways as part of the opening ceremony. The dates can only be used as a guide and should not be seen as authoritative. Distances in square parenthesis are widenings or improvements of existing routes and so do not count as new mileage of roads.