I never speed in 20 zones but there are some areas where I live that have recently been dropped from 30 to 20 for, as far as i can tell, no good reason. One particular road springs to mind, it's a busy main road out of the town and gets a lot of traffic during peak hours. There's really no good reason for the limit there to be 20, and this seems to be backed up by practically nobody observing the posted limit. One day they'll find somewhere to park a van and nab quite a few people I suspect.