This freeway was mostly built along the existing US 75; one of the projects in Navarro County, near Corsicana, was the first Interstate project in Texas approved under the Federal-Aid Highway Act of 1956. It was not until 1964 that I-345, extending I-45 north along the proposed Central Expressway bypass, was added as a proposed state highway. I-45 and I-345 were built and opened in the 1970s, with the final section, between Lamar Street (exit 283A) and the Central Expressway (exit 283B), opening on February 25, 1976. At the north end, before it merged into the Central Expressway (which continued to carry US 75), I-345 straddled the bridges over Bryan Street and Ross Avenue, the latter the location of the opening ceremonies in 1949. Because of their location, these two bridges were not replaced in the 1990s reconstruction of the North Central Expressway, and are the only surviving grade separations from the initial construction north from downtown. At the time the interchange with I-20 was built, the freeway that crossed I-45 was then a part of I-635; it would not be until later when, initially, I-20 was added to I-635 as a multiplex, then later still, I-635 would be truncated away from the I-45 interchange (back around to just north of what is now I-20's interchange with US 175). Reconstruction and widening to six lanes, from the Ellis-Navarro county line (between exits 243 and 244) north to State Highway 310 (exit 275), began in 1991. The last section, near the north end, was completed in 2002.