Reported October 2006

BACKGROUND: The Virginia Department of Transportation is placing optical speed bars at regular intervals along Lee Chapel Road in Springfield, a stretch of pavement notorious for fast driving and traffic accidents. The bars are about two feet long and a foot wide and are placed at intervals that narrow from 24 feet at the start to 15 feet at the end. This creates an optical illusion -- a flipbook effect -- that tricks speeding drivers into thinking they are driving faster than are, causing them to slow down. A British study has shown that optical speed bumps reduced fatal and serious injury crashes, and the method has already been successfully tested in Texas, Kansas and Mississippi.

FLIPPING OUT: A flipbook is simply a moving picture made as a small bound book whose pages show a series of movement -- broken down into individual images. When the pages are flipped quickly, the images look as if the scene is moving. This occurs because of an effect called "persistence of vision." Images in your will stay a fraction of a second longer than what is actually shown. Our eyes and brain retain a visual impression for about 1/30th of a second. Because of persistence of vision, we don't notice that a movie screen is dark about half of the time, or a TV image is little more than one bright, fast dot sweeping across the screen. Movies show one new frame every 1/24th of a second, and each frame is shown three times during this period. The eye retains the image of each frame long enough to give us the illusion of smooth motion.

BEFORE THE SILVER SCREEN: In the 19th century, a number of optical toys were created that also gave the illusion of motion from a rapid succession of still photos. The most well-known is the zoetrope, invented in 1834 by George Horner: A zoetrope is a cylinder with vertical slits cut into the sides. On the inner surface of the cylinder were images from a motion sequence. As the cylinder spins, the user looks through the slits at the images, producing an illusion of motion much like a movie.

If you would like more information, please contact:

Ann M. Overton
Public Information
Virginia Transportation Research Council
Charlottesville, VA
(434)293-1912
ann.overton@vdot.virginia.gov

FACTOID...

From 2002 to 2004, there were 22 crashes on that stretch of Lee Chapel Road. Its average annual crash rate is 22 percent higher than the overall average rate for all of Fairfax County.

ON THE WEB...

Persistence of Vision