Cross bike - SPD
Mountain bike - SPD
Fixed gear - SPD
Road bike - SPD
Fat bike - SPD

I cannot think of one time during the several thousands of miles I have ridden where I crashed because I couldn't get unclipped fast enough to put a foot down. I was hit by the driver of an SUV in an alley and managed to unclip both feet before colliding with the vehicle which was my only option. From riding back country roads to riding in rush hour traffic in Chicago and everything between.... you simply figure it out. It is such a natural motion now that I don't even think about it. Having good control of your bike and a good sense of balance will often be just enough so that even if you for some reason forgot how to unclip or couldn't figure it out, you would still have enough time to get your foot unclipped and down.

So, my suggestion is to learn how to ride, how to stop or at least do a very slow roll without putting your foot down. There are several tutorial videos on YouTube on how to properly execute a track stand. Learn how to do this, and how to control your balance and your bike, and then give it a shot.