West of I-70 exit 237, the name Natural Bridge Road continues westward for two sections of road: 1.4 miles (2.25 km) as the southern service road of I-70, and 2.8 miles (4.51 km) as Route B (connected by way of Lambert International Boulevard). Prior to the construction of I-70 and several phases of expansion of Lambert Airport, Route 115 continued west as far as St. Charles, which included a portion of Natural Bridge Road in Bridgeton which has been significantly impacted by recent airport expansion. Portions of the same right-of-way exist as a section of St. Charles Rock Road (which is now Route 180) west of I-270. This route was once part of Route 115.
After the Discovery Bridge on the eastern portion of Route 370 opened in 1992, the portions of Route 115 west of the airport were removed from the state highway system. In 1998, the former Route 115 bridge across the Missouri River was demolished.