Originally Posted by Laggard
Or you could just listen and look over your shoulder. Easy to do. I commute every day in an urban setting and can't honestly recall being suprised that there was a car behind me.
You've never been passed by a Prius? When the electric motor is engaged, they are very, very, quiet (as are many other cars with modern tires and motors).
A good mirror (e.g., the Take a Look model) provides a much greater degree of situational awareness while riding. By way of comparison, try removing the rear view mirrors in your car for a few days to see the difference in outlook.