Thx for the reply - I honestly am learning new stuff all the time so thought maybe there was a usecase that I didn't know about!
The 'track' highways are assumed to allow motor vehicles but don't HAVE to: it can still be a 'track' but not allow cars. Any highway classification above that (residential>unclassified>tertiary>etc) REQUIRES that motor vehicles are allowed, otherwise it is retagged as a 'pedestrian_street'.
And then service roads crash the party and everyone goes home. (i.e., I am still not 100% on correct service road usage/tagging).  *frustratedsigh*