By 1945, a divided highway designated Alternate US 24 (US 24A) was opened from the state line north to Erie. After World War II, the MSHD planned to convert several highways in the state to freeways. In planning maps from 1947, the modern I-75 corridor was included in the system that later became the Interstate Highway System. It was also included in the "General Location of National System of Interstate Highways Including All Additional Routes at Urban Areas Designated in September 1955", or "Yellow Book" after the cover color, that was released in 1955 as the federal government readied plans for the freeway system.