The Bypass in Three Events, 1978–1982 — C5 Settled

the question carried since R-1, closed by decomposition

There is no single bypass year, because the bypass happened three times: on paper, in concrete, and in traffic. First, the designation: the freeway west of South Bend began in 1958 as the Bypass U.S. 20 — the St. Joseph Valley Parkway — was completed from U.S. 20 to Mayflower Road on 19 September 1963, extended past State Road 23 in late summer 1965, and reached U.S. 31 on 15 December 1967; in 1978 the BYP US 20 designation was replaced by the US 31 designation on that existing southern and western freeway. Second, the concrete: Michigan's first section, from the state line to U.S. 12, was completed and dedicated on 10 August 1979 and opened to traffic by 6 September 1979, and the freeway's own Indiana Toll Road interchange — Parkway Exit 255B, Toll Road Exit 72 — was built beginning in 1979 with the northern extension and opened on 27 October 1980. Third, the traffic: in 1982 the new U.S. 31 was completed to bypass South Bend to the west, through traffic actually left the Dixie Way corridor, and Business U.S. 31 was created on the old road the same year. The Niles bypass followed on 16 August 1987 and the Indiana portion of the parkway was completed in 1992. The register's C5 — where the record has carried "1978," "1980" and "1982" as rival answers to when U.S. 31 left this road — is therefore closed as settled by decomposition: each date is true of a different event, and no one of them is the bypass whole.

Sources: the U.S. Route 31 in Indiana route history (the parkway course 1958–1992 and the 1978 designation replacement, verbatim); Michigan Highways, U.S. 31 in Michigan (the August 10, 1979 dedication); the Indiana Toll Road exit list (Exit 72 opened October 27, 1980); Jim Grey, Down the Road (the 1982 completion and diversion).