They obviously lessen your chances of an accident - for example, most Sunday afternoons I drive around looking for a car to rear-end, I find this highly satisfactory for relieving frustrations which have built up from spending too long at home over the weekend. However, when I see one of those signs I often think twice before driving my car into the back of them, now whilst, I still proceed to try and rear-end the car, on occasion that extra few seconds does give the other car time to get out of the situation (he who hesitates is lost...), so there you go, q.e.d. the sticker reduces the chances of an accident. If you value your children, you will buy one!
(and yes, i am not particularly busy pre-bank holiday friday afternoon).