What is the oldest highway in the US?

The first completed section (i.e. open to traffic) was I-70 in Kansas. From the Federal Highway Administration's website: "On Nov. 14, 1956, Gov. Fred Hall participated in a ribbon-cutting to open the newly paved road, and a sign was posted, identifying this section of I-70 as the "first project in the United States completed under the provisions of the new Federal-Aid Highway Act of 1956."