I’m not sure if this will be of much use here, but as the current OHM copyright page on the wiki does specifically reference CC-BY-4.0 sources I thought it would be worth a mention.
The itiner-e dataset has recently been released both as an online map and as a bulk download in GeoJSON format[1]. The coverage area is the roughly the extents of the Roman Empire in 150CE. They indicate that in the future “Itiner-e will serve as a platform for an open community of editors, and the next phase of development will include a process to join this community”, so I guess it could be described as a newly formed competitor to OHM.
It is apparently derived from a number of historical sources and archaeological records and is meant to be higher resolution than previous efforts. Individual road segments are tagged with a level of certainty and sources and classified into main and secondary roads according to criteria on their documentation page.
As far as accuracy is concerned they consider their “certain” road segments to have “less than 50 m deviation in mountainous terrain, less than 200 m in the plains”.
They have a more detailed description of how the data was created here.
although the Nature “scientific data” article linked below also links to other formats for the version discussed there including SHP and GeoPackage. ↩︎