Colonia is a census-designated place and unincorporated area within Woodbridge Township, in Middlesex County, New Jersey, United States. As of the United States 2000 Census, the CDP population was 17,811. In 1919, the New Jersey State Highway Commission built a new road that became part of the Lincoln Highway, an early plan to create a transcontinental highway. The stretch was constructed on the west side of the Pennsylvania Railroad from near the northeast of Dow Avenue between Colonia and Iselin to Cedar Street in Menlo Park, to avoid two railroad crossings. The old road is now Middlesex-Essex Turnpike and Thornall Street, on the east side of the tracks.