A researcher at Western Washington University says he has proof that people talking on cell phones are twice as oblivious as people who aren't.

Psychology professor Ira Hyman employed a clown riding on a unicycle to prove his point. As the clown rode through campus, just 25 percent of the people talking on cell phones saw him. More than half the people walking alone, or listening to portable music devices or walking with another person noticed the clown.

Hyman says the study also found that a person's familiarity with his or her environment does not eliminate the effects of cell phone use on navigation.

His research is being published in an upcoming issue of Applied Cognitive Psychology.