I think structurae has arrived at their  m by dividing the total length by the number of pillars    However they did not calculate the width of the pillars as a bridge span is in my understanding defined as the free space between two pillars so you are correct that they number is strange Anyway the bridges wooden arches are clearly segmental so assuming a record span is not unrealistic wwwhdgdeeurovisionenhtmlbrhtml    As for Alconetar go to Google Images You can see that the three surviving arches are segmental arches but  these look a bit like much later amendments  these segmental arches had very conventional spanrise ratio so the advantages of the seg arch bridge were not fully exploited   The arches of the Pons Fabricius look rather semicircular to me  Could you uncover your references As for Roman bridges there is the classical Colin OConnor Roman Bridges Cambridge Univ Press  ISBN