I agree that SPD is the way to go if you are using walkable MTB or touring shoes with your road bike. The factory setting on genuine SPD pedals is excessive for most riders IMHO. Use the multi-release cleats and back the retention setting way down. If you are experiencing unintentional clip-outs, increase the retention a quarter turn at a time until you can clip out easily but not accidentally. If you started with single release cleats and a factory retention setting, you are much more likely to experience failed clip-outs. The retention on my SPD pedals is set quite light and with multi-release cleats it doesn't even require a conscious effort to clip out, just step off the pedal in any direction other than straight forward or straight back. I feel a slight click as the cleat disengages but even in sudden, unplanned stops, I've never been unable to intuitively unclip. The only falls I had were before I learned about adjustable retention and multi-release cleats and even then I only had a couple of failed clip-outs.