U.S. 6, from Provincetown on Cape Cod, went to Long Beach California from 1936 to 1964, and at that time was the longest highway under one designation in America

At 3,199 mi currently, as it's been modified several times. 

It was constructed in 1926

In 1964, the state of California renumbered its highways, and most of the route within California was transferred to other highways. This dropped the highway's length below that of US 20, making it the second-longest U.S. Route in the country. However, since US 20 has a discontinuity through Yellowstone National Park, US 6 remains the longest continuous U.S. Route in the country.

https://en.wikipedia.org/wiki/U.S._Route_6 

https://tywkiwdbi.blogspot.com/2024/09/a-naturalist-buys-old-farm.html