Large objects appear to move more slowly than smaller objects traveling at the same speed, an optical illustion that may be one reason 7,000 cars are hit by trains every year, a Pennsylvania State University psychologist said.

"Analysis on accidents reveals that in most cases there was clear warning of the train's approach and adequate visibility," Dr. Herschel W. Leibowitz wrote in American Scientist, "but for some unexplained reason, the driver of the vehicle chose to cross the track and was killed or seriously injured."

Because the eye judges large objects, such as trains, to be moving more slowly than they really are, drivers often believe they have more time to cross the intersection than they really have.

"In addition," Leibowitz said, "signal systems . . . are designed to anticipate the 'worst case,' so that the lights, bells and gates are activated in sufficient time to accommodate the fastest train, the slowest motorist and the worst weather."

For most drivers, that's too long a wait, and impatience leads them to try to run the signal. "The fact that in most cases we can safely ingore official warning contributed to the danger," he wrote. Shorter warning signals could lessen the problem.

About 650 people a year are killed at train crossings in the United States.