Highway departments across North America have been using an optical illusion to help prevent speeding and the accidents that it can cause. Optical speed bars use painted dividing bars to trick drivers into slowing down. Designers of the bars use specific distance between dividing bars to make drivers slow down. Designers slowly make the distance of stripes smaller over a specific distance. This makes drivers instinctively slow down, according to research.

Across North America, optical speed bars are used on construction zones and other areas where slow speed is vital for safety. The bars work by making drivers more conscious of their speed, even if the drivers do not glance at their speedometer. The way the brain processes the bars makes the driver think they are going faster than they actually are, which actually encourages the driver to drive slower than they intend to.

The optical speed bars work by making drivers feel that the painted yellow highway dividers are whooshing past at a faster rate. This illusion seems to work at slowing drivers down, according to research studies. Researchers have found that on highways, in rural areas, and around construction sites, optical speed bars do seem to reduce driver speed.

One study at the University of Kansas found that drivers reduced speeds by up to 29.8% when driving by optical speed bars. Researchers found that optical speed bars yield the most significant results on the morning drive 7 a.m. and 9 a.m. This may be because drivers are more tired and more receptive to the illusion at this time. Further, studies have found that drivers maintained a lower speed even after leaving optical speed bar areas.

Some municipalities are adding optical speed bars to areas where a high incident rate has occurred. Others are adding them on curved roads or high speed limit areas, where driving conditions are more dangerous. They are also very common in construction areas, where despite posted speed limits, some drivers tend to drive too fast for safety. Hopefully, these optical illusions will help prevent accidents and the personal injuries they cause. Currently, there is no research as to whether these bars prevent pedestrian accidents or bicycle accidents, but slowing down drivers is of course key to preventing most types of accidents involving vehicles.