Complex sentence: A bascule bridge was built alongside the swing bridge to increase traffic flow in 1972, and although this greatly improved traffic flow, the bridges were still inadequate for modern-day traffic requirements, leading to the parallel M5 motorway being constructed downstream.
Split into simpler sentences: A bascule bridge was built alongside the swing bridge to increase traffic flow in 1972. Although the new bridges greatly improved traffic flow, by the 1970s they were inadequate for modern-day traffic requirements, leading to the parallel M5 motorway being constructed downstream.