We prefer convex mirror for observing the traffic behind us; because its field of view is much larger than the plane mirror. However, it gives erroneous idea about the speed of the vehicles behind us. But because of the first advantage we still prefer it. The view shown by a plane and a convex mirror is as shown in the figure below. It is clear from the figure that the view produced by a convex mirror is wider than that produced by a plane mirror.
