After a chain of crashes on a November day in 2007 killed two people and wrecked about 100 cars, the California Department of Transportation (Caltrans) set out to develop a solution. The result was a warning system called Fog Pilot, now in place on the stretch of road where that 2007 incident took place, that includes sensors and a series of electronic signs that tell drivers if traffic is slowing ahead. The components of that system, strung along Highway 99 in an area of mostly farmland, are linked via a private WiMax network.

While much of the attention on WiMax has been focused on publicly available carrier networks that use licensed frequencies, the technology can also be used for private networks and on unlicensed spectrum. The network used by Caltrans runs on the same unlicensed band used by IEEE 802.11a Wi-Fi systems and is based on fixed WiMax instead of the mobile technology that is being deployed nationally.

Between November and February each year, low-altitude areas of the Central Valley are often blanketed with a thick fog, named "tule fog" for the tule reeds that grow in the valley. At its worst, the fog reduces effective visibility to zero, meaning drivers can't see more than five feet in front of their cars. (At that point, the California Highway Patrol stops traffic and guides cars through in groups.) Tule fog typically appears in the mornings and evenings, though on about five days in a season, it stays all day, according to Jose Camarena, a Caltrans public information officer

U.S. Highway 99 is a bad place to be when it's fogged in. But it's one of the state's major north-south arteries, as well as the main route between towns and farms in the local area.

"We obviously have a contradiction, where local drivers think they can keep driving 60 to 80 miles an hour when you only have 100-feet visibility, if that," Camarena said. "Everybody always thinks, 'It's not going to happen to me.'"