Something similar happened near where I live in Morden - the limit on the A298 (a two-lane dual carriageway) was dropped from 40 to 20mph sometime within the last year. It felt very silly at the time because nobody was adhering to the new limit, minimum speed driven there was 30 and most people just carried on driving at 40. It didn't feel appropriate, but I suspect the intention was emissions rather than safety as there are a lot of houses along that road. I don't know when or why but it was then revised to 30 a month or two ago.
I don't know what the emissions difference is between 20 and 30 but it may be significant, if you lived near such a road with young children (and especially if you didn't drive) you'd probably be strongly in favour of a lower limit.