By Tim O'Neil --- Workers opened the four lanes of their new Missouri River bridge in phases during the first week of September 1958. The bridge replaced a 1904-vintage span downriver that was overwhelmed by 16,000 vehicles daily. It opened the way for migration into St. Charles County and a clear drive across the state. At $8.5 million, the bridge was a key part of the Mark Twain Expressway, later called Interstate 70. The 24-mile, $97 million Mark Twain from downtown into St. Charles was the first spoke in the pinwheel of today’s regional system of superhighways.