The real test is when you aren't thinking about unclipping and need to suddenly. Usually, that happens at a stop light and people panic as they start to tip over. One way to prepare for that sensation is to practice it by sitting on your bike in a narrow hallway so you can let yourself tip over and then try to unclip and get your foot down before fully tipping over and using the walls to catch your fall if necessary. This also happens frequently to mountain bikers who suddenly find themselves unable to pedal over a hill. Most of my bike "crashes" have been at 2 mph in this manner!