STILLWATER — Hundreds of onlookers gathered to celebrate the opening of the new St. Croix River bridge connecting Minnesota and Wisconsin, an enormous project that was decades in the making.

Traffic began streaming across the $646 million bridge Wednesday evening escorted by vehicles with flashing lights. Earlier Wednesday, Minnesota Gov. Mark Dayton and Wisconsin Gov. Scott Walker ceremoniously helped cut a red ribbon to officially open the four-lane bridge. Officials said the bridge is one of the largest transportation projects ever completed in Minnesota and Wisconsin.

About a mile upstream, the Minnesota Department of Transportation shut down the 86-year-old Lift Bridge to vehicle traffic. That bridge will later open to pedestrian and bike traffic.