I'm kind of on the fence about this as 30K is waaaaay to easy to hit on a bike. So we'd all be speeding as well. Forcing motorists to slow down to the point they can be easily passed by cyclists.... I'm not sure how good an idea this is. Keeping in mind the average handling skill level of most cyclists, I can see more accidents as motorists (upset at slow movement), are now really competing for the same slivers of road as cyclists. Granted less peds and cyclists will be killed every year I think but I can more being injured.
It might even have a congestion effect as some motorists might not even bother with the car in the city at all because it's too damned slow, it would be cool. I hope I'm just being cynical but I'd rather see more aware drivers than slow-mo madness ones. .02$
40kph is the typical residential speed here in Toronto. 30 is mind numbingly slow in a car.