Interstate 675 provides the primary business route into Saginaw from Interstate 75. Initially, I-675 was built as a reliever route for through traffic (between Detroit and northern Michigan) in order to avoid the existing drawbridge along I-75 at the Saginaw River. The drawbridge was eventually replaced by 1987 with the fixed high-span bridge Zilwaukee Bridge (locally referred to as the Z-Bridge).
Don Hargraves adds more information about the Zilwaukee Bridge:
The old Zilwaukee bridge was well known as a bottleneck for a few reasons. One of them was that it was a drawbridge, another was that for many years I-75 was 6-laned from Flint to just below the Zilwaukee bridge, where it shrank down to four lanes for the bridge. After that, it went back to six lanes to north of Bay City.
The reason the old Zilwaukee bridge was a drawbridge was because of a General Motors plant on the Saginaw side of the bridge that used the river for steel shipments. At the time the Zilwaukee bridge was being replaced by the present structure, General Motors stopped using the river for steel shipments.
There was a mini-disaster with one of the pylons that held the bridge up above the river during construction. The bridge is okay now, but for years after the new bridge opened you saw bumper stickers that said "I drove over the Zilwaukee Bridge AND LIVED!"
The new Zilwaukee Bridge is eight lanes wide. A bit wide, but better too wide than too narrow (as the drawbridge was).
Designated in 1969, Interstate 675 was completed by 1971.1