Corridors 18 and 20 combine to form the Interstate 69 Corridor from Texas to Michigan. Corridor 18 refers to the entire Interstate 69 corridor from Laredo to Port Huron, including branches to Brownsville via U.S. 77 and 281, Interstate 530 extenstion from Pine Bluff to Monticello, Interstate 94 from Chicago to Port Huron, and the Ambassador Bridge in Detroit. Corridor 20 is almost entirely absorbed by Corridor 18, as it is the Interstate 69 corridor from Laredo to Texarkana. The only part of Corridor 20 that may be separate from Corridor 18 would be in and around Texarkana, which might be overtaken by Corridor 1Interstate 49.

Website: http://www.aaroads.com/high-priority/corr18.html