How 20mph speed limits affect excessive speeding.
For those who currently already ignore the 30mph limit, having a 20mph limit in place will considerably increase the ability for detection and prosecution, it also makes their offence more severe and the expected punishment to be more harsh.
… But it won’t stop people speeding.
No it won’t and neither does any speed limit stop those who flout the law from driving in the manner they do. These people need to be dealt with by the police and the courts. As a side note, 20mph limits will not lead to world peace either (but they might make a very small step in the right direction).