source: Danville Gazette (Danville, Indiana) - issue of Thursday, November 4, 1976 - page 1, column 1:
According to a newspaper article, on October 28, 1976, US 36 between Avon and Interstate 465 officially opened as a 4-lane highway. Ribbon-cutting ceremonies were held at the Hendricks County-Marion County line, presided over by Governor Otis Bowen. Construction had begun in 1975 to widen this stretch of road from 2 lanes to 4 lanes, with an $8.6 million contract awarded to Rieth-Riley Construction Co.
The portion of US 36 between Danville and Avon remained 2 lanes until 1985, when it was widened to 4 lanes (source: October 1985 newspaper issues).