Answer: STS

AnswerCrossword Clue
STSBourbon and Wall: Abbr.
STSMap markings: Abbr.
STSNC, ND, and NE
STSOK et al.: Abbr.
STSPeter, Paul and Mary: Abbr.
STSThe USA's 50
STSDowning and Fleet: Abbr.
STSMap abbrs.
STS16 of them start with M or N: Abbr.
STSMA and PA
STSMap lines: Abbr.
STSReligious splinter groups
STSCA and CO
STSD and C, in D.C.
STSThoroughfares: Abbr.
STSLA and MI
STSMany are named for trees: Abbr.
STSPO listings
STSParts of rtes.
STSSome block boundaries: Abbr.
STSPeter and Francis: Abbr.
STSBleecker and Beale (abbr.)
STSAve crossers
STSPeter and Paul: Abbr.
STSB'way crossers
STSBasin and B'way
STSChurch BMOC's
STSAve. crossings
STSCity rds.
STSBourbon and others (abbr.)
STSPeter and Paul (abbr.)
STSAve. crossers
STSHoly ones (abbr.)
STSAves.' crossers
STSThoroughfares (abbr.)
STSRds. or aves.
STSRds.
STSUrban rds.
STSAves.
STSCanon fodder?: abbr.