The 20mph campaign/s and arguments...

20mph where though? 20mph limit on a motorway isn't going to allow greater traffic throughput.
Well spotted.

The motorway is a good example, however, of how and why slower can be faster - Variable speed limits, when followed, provide for faster throughput, on the basis of minimising the concertinaing of traffic.

I would expect that 20mph zones are assessed on the basis that it would be beneficial - either in terms of traffic throughput, or safety, or even bloody mindedness.