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 bridge's wooden arches are clearly segmental so assuming a record span is not unrealistic 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 span rise ratio so the advantages of the seg arch bridge were not fully exploited the arches of the pons fabricius look rather semi circular to me could you uncover your references as for roman bridges there is the classical colin o'connor roman bridges cambridge univ press isbn