Your brain tells you "You're going faster."
"Optical speed bars" are an experimental roadside optical illusion that tries to slow you down while you're driving.
These white bars along the side and center of the road are primarily found on roads where the traffic is traveling at highway speeds but must slow down for a curve.
The white bars are installed a certain distance apart, but when approaching an area where traffic should slow down, the bars are spaced closer together.
This gives drivers the illusion that they're going faster than their actual speed. If drivers perceive the bars traveling by at 4 seconds apart and then they see them start whizzing by at 3 seconds apart, the theory is that they'll assume their vehicle is speeding up, and they'll slow down.
Studies have shown that this low-cost speed reduction method can reduce average speeds by five miles per hour.
- Cash-strapped Oregon traffic engineers test optical illusion to slow speeders down (video), Oregon Live>>
- Low Cost Treatments for Horizontal Curve Safety, Chapter 7. Innovative and Experimental Treatments, FHWA Safety>>