WiMax Cuts Through Highway Fog

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There are deadly hazards in the fog that blankets California's rural Central Valley every winter, but a WiMax network may now help warn motorists of what looms ahead of them and prevent accidents.

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.'"

An accident between just two vehicles can snowball into a pileup when fast-moving drivers can't see that traffic has stopped ahead of them. So Caltrans set up a network of traffic and visibility sensors to detect when those conditions exist. Chameleon ITS Transportation Management Software from ICx Technologies processes the data to determine what warning signs should say. The Fog Pilot system has signs every quarter-mile that tell oncoming drivers if there is an accident, stopped traffic, or slowing ahead of them, Camarena said.

Fog Pilot has been set up in a trial deployment on a 12-mile stretch of the highway south of Fresno. For now, the sensor data comes to human operators who set up the sign messages remotely. But that takes one or two minutes, and for accident warnings, every second counts. So when the trial system is finished in a few months, it will be fully automated and able to change sign messages in 30 seconds or less, Camarena said.

Rather than lay down a wired network to link the sensors and signs, the builders of Fog Pilot chose WiMax. The Proxim Wireless Tsunami MP.11 5054 base stations and 5012 subscriber units transmit the conditions data. Fog Pilot uses a point-to-multipoint configuration for communications among devices that are close to each other and point-to-point for the quarter-mile links, said Darrell Alfaro, CEO of Moonblink Communications, the wireless contractor on the project. To send data beyond the highway network to the Internet, the system uses a handful of EV-DO (Evolution-Data Optimized) cellular radios, Alfaro said.

WiMax is a good protocol for this type of project, because it can deliver a high-bandwitch signal over distances of several miles.

Even though the sensor network uses the 5.8GHz band shared with Wi-Fi and devices such as cordless phones, interference isn't a major problem because the network is low to the ground and uses directional links, creating a narrow radio "tunnel," Alfaro said.

As configured today, the network doesn't need the bandwidth that WiMax provides, Alfaro said. But in the future, Caltrans plans to install video cameras for monitoring the situation on the highway. The WiMax network is ready to handle those data streams, he said.

Private, unlicensed networks make up only a small percentage of WiMax deployments, said IDC analyst Abner Germanow. Interference from Wi-Fi networks can be a problem. But in places where there aren't many Wi-Fi networks, such as on Highway 99, it's a realistic option, Germanow said. And for point-to-point wireless links, in general, WiMax is likely to displace proprietary systems and play a bigger role over time, he said. Standards mean many chipset and equipment vendors can make similar gear.

"The near-term reality of WiMax in point-to-point networks is that WiMax hasn't necessarily decreased the cost of that point-to-point equipment, but the longer-term promise is that as the volume of chipsets increases ... the prices on that gear will fall," Germanow said.

If the test deployment of Fog Pilot works as expected, Caltrans would like to set up sensors and signs in many other dangerous areas of the Central Valley. The system could also be used to warn drivers of other types of conditions and the accidents they may cause, Camarena said.

"The faster we can get out that information, the further we can decrease the magnitude of the accident," he said.

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