The county has painted a so-called optical speed zone on a two-mile stretch of southbound Stockton Hill Road about 20 miles north of Kingman. Sets of white markings are painted perpendicular to the yellow centerline 150 feet apart for the first five intervals, reducing by 10 feet until they are 100 feet apart.
Drivers see the markings going by faster and faster, subliminally convincing them their speed has increased, causing them to either brake or take their foot off the accelerator, said Steve Latoski, the county's design engineering manager.
"The idea is to achieve speed control," Latoski said. "Speed limits don't necessarily do that. Enforcement does that but can only occur over a finite period of time. Or there can be physical controls. This is a low-cost mechanism for us just to call people's attention to their speed."
The stretch of highway chosen for the experiment is at the end of a long, rural stretch and right before it enters developed areas. Although posted at 55 mph, the averages speeds are at least 10 mph higher, Latoski said.
Since the striping was painted on July 11, average daytime speeds have decreased by 2 mph, and nighttime speeds dropped by nearly 5 mph.
"If we can demonstrate consistent, significant decrease, we will likely look at other, similar applications in the county," Latoski said.