I spent a lot of my time in germany. Over there many, if not most, cities have a thing called "grüne Welle". (In english= Green Wave". This is where a serial of traffic lights are connected and coordinated in such a way that once you pass through the first one you are 100% guaranteed to hit every subsequent set of lights on green on the assumption that you drive at the speed limit.
Now, In Wales, when I go to Lidl I go through about 3 sets of lights. And you can guarantee that the tw*ts who went screaming past my drive way over the speed limit as I was trying to get out are now sat in front of me at the lights. And as soon as the lights change they're off wheels spinning - to the next set of lights - where I'll join them in a couple of minutes.
Now. In Hamburg if they reduced the speed limit on that long road with 50 sets of lights forcing me to drive at 32KMPH (20MPH) then that would really slow me down because I just don't hit any traffic lights and I'd cover the whole distance at 20mph. But in Wales. Where you can guarantee I hit a LOT of lights. Then the difference between 30 and 20 is not going to add any time (at all) to my short journey to Lidl.
As I said above. My current average speed in wales, including longer faster trips, but mostly in town, is 14mph. I will eat my hat (I will go out and buy a hat first) if my average speed limit is affected, at all, by the reduction from 30 to 20.
But them I'm biased. I live on a 40mph road and it's being reduced to 30. having said that. Hardly anyone obeys the 40 so I don't see why they should obey the 30 either. Their average speed wont change either because we all spend over half of our journey time at the lights. Just the speeders spend more time waiting at the lights that those of us who drive sensibly.