Who goes there?

California’s Central Valley—extending from Bakersfield in the south to Redding in the north—is one of the country’s largest agricultural regions. It also is a major transportation corridor, with I-5 and California Highway 99 running through the valley. CA-99 in Fresno in the project area carries more than 100,000 vehicles per day.

The region is subject to a particularly dense kind of fog, known as “Tule fog,” during the winter. In fog season, which runs roughly from Nov. 1 to March 31, Tule fog can form overnight and reduce visibility to less than an eighth of a mile, and in some cases to nearly zero. Drivers along the corridor continue to drive at unsafe speeds for the visibility, and the unsafe speed has led to large, multiple-car crashes. In November 2007, Tule fog caused a 108-car pileup. There were two deaths and nearly 40 injuries. The pileup, which included 18 big rigs, extended for nearly a mile and closed CA-99 for over 12 hours. The last vehicle collided 10 minutes after the initial crash.

In addition to the threat to life and property, these major pileups have an enormous effect on the economy of the Central Valley. In order to reduce the likelihood of future multivehicle crashes, District 6 of the California Department of Transportation (Caltrans) is implementing a pilot project to automatically detect fog and warn motorists of hazardous conditions. Construction of the Fog Detection and Warning System (FDWS) began in October 2008. Phase 1 will be completed in February 2009, and Phase 2 will be completed before the beginning of the 2009-10 fog season on Nov. 1, 2009. The project covers a 13-mile stretch of CA-99 south of Fresno.

In an effort to deploy a system before there were additional major fog-related collisions, Caltrans used emergency funds to construct and implement this system. Caltrans chose a team led by the ICx Transportation Group, a California-based systems integrator, to design and deploy the FDWS.

A fully functioning fog detection and warning system requires the ability to monitor current conditions on the roadway; a system to evaluate the data received from the roadway and determine what it means; the ability to communicate information about current conditions to transportation management staff in a timely and accurate enough manner that they can take action based on the information; and the ability to communicate current information directly to the traveling public. The Caltrans FDWS performs all of these functions.

Senses tingling

Detection and Communications

The FDWS will use sensors to detect both visibility and speed on CA-99 in the project area. To measure visibility, the team selected the PWD10 forward scatter sensor developed by Vaisala. The sensors have been installed every half-mile covering both directions of the freeway. They are installed at driver-eye level to ensure the system is reporting conditions as seen by the driver. In addition, the project team has installed SmartSensor HD radar spot-speed sensors from Wavetronix every quarter-mile through the project area. These radar units are capable of measuring traffic volume, classification, speed, lane occupancy and presence in both directions of travel.

These two sensing technologies combine to provide a more complete picture of traffic and visibility conditions than has ever been attempted on a large scale. The data from the sensors will be used to assess both visibility conditions and, equally important from the perspective of both travelers and traffic managers, speed differential at downstream locations.

Because of the relatively rural nature of the project area, dedicated wireline communications are not available. Moreover, even if they were, the cost of trenching to connect into such systems would be prohibitive. As a result, all system communications are wireless. The communications system uses Proxim wireless devices to communicate between devices in the corridor. Backhaul communication to the traffic-management center is done using Verizon Wireless EVDO modems. Also because of the scarcity of fixed infrastructure, 35% of the field equipment runs on solar power.

Data Processing

Data processing ensures that the data collected in the field is available in a useful format to travelers and traffic managers. The system was developed with two levels of data processing. Under normal system operations, all data is collected in the field and transmitted wirelessly to the Caltrans Traffic Management Center (TMC). At the TMC, it is processed using the Cameleon ITS platform developed by 360 Surveillance. If there are significant speed differentials on the freeway or if there is fog, Cameleon automatically generates messages for the data dissemination systems (discussed below). In addition, the personnel in the TMC have access to the data and can use it to ask the California Highway Patrol (CHP) to implement its traffic-slowing procedures in the fog zone.

If communication to the TMC is disrupted for any reason, the system will use the intelligence built into the field equipment to generate the appropriate messages on the changeable message signs (CMS). This ensures that the system will continue to function even should the wireless communications be disabled during periods of heavy demand.

Data Dissemination

The system uses both roadside and wide-area dissemination methods to make sure that information about current road conditions reaches as wide an audience as possible. On the roadside, CMSs are placed every half-mile on both sides of the freeway.

The signs, ADDCO Brick signs, are configurable and can be built to whatever size is most appropriate for each specific location. The CMSs have messages indicating when traffic is slower ahead and when there is fog. If an incident has occurred, the CMSs will warn drivers of slower traffic ahead in order to prevent chain-reaction collisions. It is difficult to anticipate how drivers will react to the messages. If the system works successfully, drivers will read the CMSs and slow to an appropriate speed.

Speed and visibility data from the system also will be provided to a new 5-1-1 traveler information system being developed as part of this project. The 5-1-1 system, which also will include general traffic information for the southern Central Valley, will be able to inform travelers of problems in the project area via the telephone and Internet before they reach the area, possibly before they even leave their home or office. Warnings to travelers will help reduce the danger of severe fog by minimizing the number of vehicles on the roadway.

Phasers on 2

The project team will embark on Phase 2, which should feature some or all of the following features:

• Full-matrix color message signs to provide better information;
• Full road weather information systems to monitor the full range of weather conditions, including rain, wind, humidity and temperature. The data from these sensors could potentially be used to predict fog;
• Highway advisory radio reports with alerts to travelers;
• Closed-circuit television cameras to provide more detailed information to the TMC and to the public over the Internet;
• Pulsing in-pavement lighting to be used to slow traffic down under certain conditions (such as when there is an incident ahead). The lights would not be used during low visibility until an incident has occurred for fear they would guide drivers to move at unsafe speeds;
• Thermal cameras; and
• Incident detection using advanced radar detection.

Expected outcomes

Caltrans and the rest of the project team believe that this demonstration project will both reduce the danger of fog in the project area and also provide valuable lessons for deploying fog detection systems elsewhere. Key questions that the project team expects to answer over the project period are:

• Is the detection density appropriate? Is more detection needed, or would it be possible to reduce costs by using less?
• What is the best way to communicate road conditions to travelers?
• How do drivers react to the information provided?