Agree with all your replies so far, Springy, and will add 'one slow breath'. In CBT you learn to recognise that little jerk just as you're about to kick off. You can do this. If your reactions seem to be coming from nowhere - take the slow breath just in case! It will help you to notice the 'jerk' when you feel it. Then give yourself a second to evaluate your response.
I'll offer you two strategies I've learned for deciding whether to act on an impulse or feeling.
The first is from therapy: Is this feeling/impulse relevant, appropriate and helpful? Only do the action if it scores all three. It takes a bit of practice, but it's worth it imo. I do it all the time now, and am able to choose a different reaction (usually). Go me 
For example, the dawdling cyclists triggered annoyance. OK, that's relevant and appropriate but not helpful - you're unlikely to speed up the traffic, so let it go and twiddle your thumbs.
The second is from AA: Hand it over - to God, as you're a believer. It's extraordinary how much energy we waste getting emotional over things we can't control. As a matter of fact, handing it over works well for things we do control, as well - save the energy, and you're more likely to spot the solution as it appears. My advice is to practise handing it over as much as possible, while practising slow breaths :)
Ready? Off you go, then! All those slow breaths will improve your complexion, and your beatific calm will ease your stress lines. Let me know when you start seeing your results!