Wikipedia
OpenLR
OpenLR is a royalty-free open standard for "procedures and formats for the encoding, transmission, and decoding of local data irrespective of the map" developed by TomTom.
The format allows locations localised on one map to be found on another map to which the data have been transferred.
OpenLR requires that the coordinates are specified in the WGS 84 format and that route links are given in metres. Also, all routes need to be assigned to a "functional road class".
The specification is licensed under a Creative Commons license.
TomTom has published a library for the format under the GPLv2.