I have this happen to me about once a week on the school run.
I think I've had it getting confused by the bollard keep left sign on a small central island, plus when I've been past some parked cars in a lay-by, this is all in the same 30mph zone.
I too have been thinking about disabling whatever system option it is that is doing it, the only thing I'm wondering though is if it defaults to being back on every time the car is restarted...