Being alert means scanning the road ahead for hazards and obstacles, looking over your left shoulder regularly so you know about overtaking cars, listening for the sound of car doors closing or engines starting, etc. An experience bicyclist uses all of these cues to better understand the road environment and to plan maneuvers in advance. If you see a hazard in the road well before you get to it, you will be able to look over your left shoulder, find a time when it is clear to move, and smoothly avoid the obstacle. Being alert is a major help in eliminating the "sudden swerve" which is the cause of so many bike-car crashes (see table). Being alert means seeing and hearing things far enough in advance that you will not be surprised. I teach kids to look for and avoid:

•  Potholes

•  Gravel, sand, broken glass

•  Sticks, piles of leaves

•  Cracks and steps, gutter/roadway seam

Many inexperienced cyclists ride far too close to the curb, even in the gutter. This means they are riding where all the debris accumulates, and they can get trapped in the gutter if there is a step up to get from the concrete gutter onto the asphalt roadway, a common sight.

I use this picture of a dumpster as an example of an unusual situation that requires extra caution. After discussing all the things that one needs to watch for (hidden parked cars, an obscured intersection, construction debris in the road ...)  one father said to me "I'll never view a dumpster the same way again".