When my son was at the nursery I caught myself shouting a lot in the mornings because he was a dawdler and we often late late. My solution was to start going downstairs 10 minutes earlier. Once we were out of the house we were fine. Shouting at my son to be quicker was a pointless waste of my time and energy and a sure way to create bad vibes first thing in the morning. (He is still a dawdler, by the way).
Just to be clear, if I saw my toddler scooting towards a busy road, I would shout, but in panic. Even my dog understood he had to stop and pause and wait for me to catch up on one occasion he accidentally escaped from the lead. Shouting at a toddler for releasing the seatbelt is a silly waste of energy too, imo. Firstly, he can't fully understand implications. Secondly, he probably can't redo the belt himself so you still needed to stop which delayed seeing his grandparents. Thirdly, he probably had a good reason to undo the belt (in his view) -he might have simple been too hot etc.
I am not going to pretend I am a saint and I never, ever shout. However, I last lost my temper 2 years ago. I reflected on it and I apologised.
We do a lot of things in life out of habit. Once you are get into a habit of having a respectful conversation with your child, you are not going to regret it.
For example, my friend who comes from a country where smacking is an acceptable and even approved form of child discipline described a situation when a young child was repeatedly pushing an even younger child under water despite a few warnings. A dad gave this child a smack and the matter was solved.
I pointed out to my friend, that if the said dad simply told the said child to stay out of the swimming pool for the rest of the day, the child would have learnt a lesson just the same. My friend agreed.
The trouble with this approach is then the dad would have probably been forced to entertain his child more since the child would have been prevented from playing with a group of his friends in the swimming pool.
Incidentally, the rates of violence against women and children in my friends' country are much, much higher compared to the UK.
I am not saying, shouting is the same as smacking. However, both approaches come out of anger and annoyance and these are not good emotions you want to teach your child