It doesn't though. Traffic approaching the 50mph zone at 70mph has to slowdown quickly, thereby causing more congestion behind. The congestion isn't there until the 50mph zone starts. Within the congestion, traffic is driving at between 5-20mph, thereby causing more pollution. Apart from the 50mph zones, the main other reason there is congestion on the M4 in both directions is the existence of the tunnels at Brynglas which causes queues in both directions for miles at peak times. The is more congestion on the M4 now than before 50mph zones were introduced.