Every fall and winter, from November to March, a thick ground fog—known as tule fog, named after the tule grass wetlands of the California Central Valley—descends on California State Route (SR) 99 in the San Joaquin Valley. It is responsible for a number of automobile pileups, including an 86-vehicle chain-reaction accident outside of Fresno in November 2007 that killed two and injured 41, as well as an 87-car pileup in 2002 that killed two people.

The dangerous situation prompted the California Department of Transportation (CALTRANS) and the California Highway Patrol (CHP) to deploy a wireless system that keeps drivers updated on upcoming speed and visibility conditions in advance, giving drivers an opportunity to slow down and adjust their driving as necessary.

“People driving on this stretch become complacent since they know the route so well, but when visibility is 20 to 50 feet, they don’t adjust their speed. It’s like running through your living room with your eyes closed,” said Jose Camarena, chief public information officer with the CHP District 6.

Armed with $12 million from CALTRANS, the CHP’s District 6 deployed the “Fog Pilot” along a 12-mile stretch of SR 99. The system integrates multiple technologies, including weather detection stations, microwave vehicle/motion detectors, visibility sensors, closed circuit television cameras and changeable message signs. All of the data and components on the system are centrally managed and controlled by ICx’s Cameleon ITS Transportation Management Software and connected wirelessly via an unlicensed WiMAX network powered by Proxim.

That means the system can detect a sudden slowdown in traffic or significantly reduced visibility, which automatically trigger different canned messages on the 40 signs positioned every quarter-mile that warn drivers to drive with caution.

“What makes this program so different than what we’ve done in the past is the fact that we are taking the human element out of it,” Camarena said. “We didn’t want to have someone having to study information and then type up messages. The key is to have pre-approved messages for each incident automatically uploaded to those signs.”

The Fog Pilot was introduced with a rather large media outreach, as well, Camarena said. When the CHP increased its presence on that stretch of highway, the local newspaper—the Fresno Bee—ran ads encouraging people to slow down, while CVS billboards sponsored eight billboards with the public-service announcement.

Camarena said the impact of the system can be measured by the fact that no fatal accidents have occurred to date this fog season. Moreover, the system is applicable after fog season ends for incidents that involve blowing dust, accidents and construction.