Complex sentence: Much roadway was substantially rebuilt in the from the 1970s to the 1980s which included replacing the aging repurposed railroad bridges with more modern bridges; some of which are able to accommodate more than two lanes of traffic.
Split into simpler sentences: Much roadway was substantially rebuilt in the from the 1970s to the 1980s. Most of the older bridges built for railroads were replaced by more modern bridges that are able to accommodate more than two lanes of traffic.