This question applies to a different product than most UX designers are used to discuss: cars.
Long story short, in my short 7 year driving experience I've be mostly driving two cars: a 2005 Peugeot 307 and an E90 2011 Series 3 BMW. For the money they cost, both cars provide excellent value although the far pricier BMW provides far more better driving experience.
When I was young, I rode a crash-without-seatbelt simulator and what I always remember since then is: wear my seatbelt while riding anything with four wheels. Thankfully, for the absent-minded, both cars I mentioned are kind enough to provide visual and audio warning when you start driving without wearing your seatbelt.
The Bavarian BMW's way of reminding you to fasten your seatbelt is a red, always-on icon on the dashboard for the visual part. The audible sound is a one-note, really "warm" and "soothing" and "calm" sound that repeats for... lets say 1 second. No variations in the volume. It just goes "bloom ... bloom" every on second or so. It goes off after some minutes. I think it's also the same sound (different pitch) used by the PDC (parking distance control) system to warn you about an obstacle on the rear while reversing. I can honestly say that sound is engineered in such a way, it almost relaxes me.
The French Peugeot's way also utilizes a visual and an audible notification. The visual sign is pretty much identical with the "Bimmer's", with the difference that: it blinks but after some minutes it goes off (it's always on in the BMW, if you don't fasten your seatbelt). But the sound... oh boy the sound! The sound is a three note, twisted and torturing sound that sounds like a mosquito on steroids that run away from hell itself. It starts low but eventually the volume becomes higher and higher until me or some of the passengers becomes so upset that threatens to damage the car if the sound doesn't stop. It goes like "too-ra-ree too-ra-ree". The sound stops after some minutes, again, if you haven't fasten your seatbelt.
Today, I've observed this pattern I've generated: When driving the BMW for distances no more than 5-10km away (no highways and high speeds) I never fasten my seatbelt despite clearly listening and distinguishing the audio warning. On the other hand, when I drive the Peugeot the moment I hear the screeching sound I almost instinctually reach for it.
So... my question is....Which notification system is considered a success in terms of UX? BMW's where the sound doesn't get on my nerves but I risk ending up dead because of an unfortunate accident (yes it can happen in a low-speed scenario too) or Peugeot's that despite the fact of the sound being unbearable to my ears and calmness, it certainly forces me to always fasten my belt, thus minimizes the possibilities of severe injury.