I mean, I don't think I've ever seen an ambulance stop for a stop sign. But even allowing that speed bumps will slow an ambulance down, it's not as though there would ever be 200 speed bumps delaying the ambulance by 10 minutes. If the ambulance arrives a few seconds too late, the ambulance was already several minutes too late to begin with.

I believe that ambulances and especially fire trucks have a wider wheelbase than cars and vans do, so what I've seen in a number of places is speed humps that don't cover the entire road. They're wide enough that you can't avoid them with a car, but narrow enough that an ambulance or fire truck won't notice them.

Or possibly it's for buses.

It's still easier to get through calming than it is to get through a street jammed with traffic.

source?