Study leader Dennis Shaffer of Ohio State University and colleagues tested more than 400 college students in three experiments. When asked to guess the length of the lines from memory, most answered 2 feet.
The real answer is 10 feet. That's the federal guideline for every street, highway, and rural road in the United States, where dashed lines separate traffic lanes or indicate where passing is allowed, Shaffer said.
The study, published in the journal Perception & Psychophysics, found even when the students were standing some distance away from actual 10-foot lines or riding by them in a car, they judged the size to be the same: 2 feet.
"We were surprised, first, that people's estimates were so far off, and second, that there was so little variability," Shaffer said in a statement.
The finding holds implications for traffic safety. Each dashed line measures 10 feet, and the empty spaces in-between measure 30 feet. So every time a car passes a new dashed line, the car has traveled 40 feet, but in this study, people consistently judged the lines and the empty spaces to be the same size, claiming that both were 2 feet, Shaffer explained.