It seems that after a car accident, someone always shouts, "they came out of nowhere!"

Sound like an exaggeration? It could actually be inattentional blindness.

Inattentional blindness happens to all of us off and on. We're constantly barraged with so much sensory information that we can't process it all. That's why we've developed attention. Attention filters out most of this sensory input, so that we can focus in on select details. Though we often think seeing is a conscious process, the truth is seeing is largely unconscious. We're inattentionally blind to the filtered out information that never reaches our consciousness.

Many factors affect inattentional blindness. One is conspicuity, or a thing's raw ability to capture attention. Your name is cognitively conspicuous. If you hear your name, your attention may be quickly diverted. Bright objects are often visually conspicuous also.

Another factor is expectation. If past experience has taught you that there's never anything in your way when you back out of your driveway, then an unexpected neon green car may slip past your attention. Then there's mental workload. You can process only so much information at a time, so if your attention is on say an upcoming deadline at work, it may overlook things like a neon green bug.