The big argument is that motorists tend to push speed limits. 70mph on motorways tends to have a lot of drivers doing 70-80. 40mph has a lot doing 40 -50, 30mph tends to have people doing 35 to pushing 40.
Thus with a 20 mph limit, in all likelihood, you will have speeds kept to 30mph and...
I like it!
Although I tend to only go through Brighton on my bike, and cars doing 20 to 25 mph means I get a superb tow in their slipstream!
Keep up the good work people, it's doing my average speeds the world of good! :thumbsup: