- Western Terminus:
- Old Sunrise Highway and County Line Road (CR 1) at the Nassau-Suffolk county line
- Eastern Terminus:
- Great East Neck Road (CR 96) and Trolley Line Road in Babylon
- 4.42 miles
CR 12 is an east/west route that runs just south of the Babylon Branch of the Long Island Rail Road from Amityville to Babylon. Old Sunrise Highway turns into CR 12 at the Nassau-Suffolk county line. It is known as Oak Street in Amityville and Copiague, Hoffman Avenue in Lindenhurst, and Railroad Avenue in Babylon. It eventually comes to an end when it reaches Great East Neck Road (CR 96), turning into Trolley Line Road. The entire length is a two-lane undivided road with traffic lights.