Complex sentence: However accidents occurred from increased traffic close to winding bends and narrow pavements; since these the central streets have been pedestrianised or partly blocked off, leading to a decline in volume of pedestrians and convenience shop closures, in favour of out-of-town supermarkets.
Split into simpler sentences: However accidents occurred from increased traffic close to winding bends and narrow pavements. Since then the central streets have been pedestrianised or partly blocked off, leading to a decline in the number of pedestrians and shop closures in favour of out-of-town supermarkets.