The majority of SR 100 is concurrent with other routes: US 302 from downtown Portland to northern Portland, SR 26 from northern Portland to Gray, US 202 from Gray to Augusta, US 201 from Augusta to Fairfield, SR 11 from Fairfield to Newport, and US 2 from Newport to downtown Bangor.
The majority of SR 100 overlaps with other routes: US 302 from downtown Portland to northern Portland, SR 26 from northern Portland to Gray, US 202 from Gray to Augusta, US 201 from Augusta to Fairfield, SR 11 from Fairfield to Newport, and US 2 from Newport to downtown Bangor.