The Colorado Department of Transportation (CDOT) is responsible for a 9134-mile highway system throughout the state, including 3406 bridges. Each year, this system of highways handles more than 48 billion vehicle miles of travel.
CDOT maintenance forces take care of the highway system, plowing snow and repairing pavement. In fact, during the winter of 2009-2010, CDOT personnel plowed 7.2 million miles of highway. They also used more than 248,000 tons of asphalt and 178,800 gallons of liquid asphalt to repair road damage and potholes.
Information about road conditions often comes to CDOT from its own workers or the Colorado State Patrol. But for the past several years, the agency has been deploying video surveillance systems to reduce costs and provide real-time driver information on highways such as Interstate 25 in Pueblo, Colorado Springs, and metro Denver, Interstate 70 from Glenwood Canyon to metro Denver, Interstate 225 in metro Denver, and U.S. highways 6 and 85.
In 2006, CDOT wanted to extend this system and developed a plan to install a high-resolution video-based traffic surveillance and data collection system along U.S. Highway 36 between Denver and Boulder. As on other roadways, the system’s information would give CDOT monitoring personnel up-to-date information on the highway’s congestion and other conditions. It could also feed into a public Web site for consumers seeking traffic information about this busy commuter route.
Searching For The Right Signal
Having completed other projects, project manager Jill Scott of CDOT had a good idea of the equipment needed, including high-resolution pan/tilt video cameras, automatic traffic identification systems from Sirit that determine the amount of time it takes for cars to travel a certain distance, and radar equipment from Wavetronix that determines the average speed of traffic.
What was different about this project, however, was that U.S. 36 has no fiber infrastructure to carry the data and video back to CDOT’s operations center. In fact, even telephone and electric facilities are scarce along the route. This meant that the system would need some kind of wireless transport solution. CDOT worked with its primary networking equipment vendor to design a system that used 802.11-based radios in the 2.4- and 5.8-MHz frequency bands to carry traffic along the route’s 30 miles.
“We had scoped out the project based on one vendor’s claim that their radios could reach up to 22 miles,” said Jill Scott, project engineer at CDOT. “Once we discovered we would need to deploy 18 hops, we realized that it was totally impractical.” In addition, the high-resolution video-based application requires high bandwidth, and at roughly 10 to 20 Mbits/s, the Wi-Fi system couldn’t deliver sufficient capacity.
KNS Communications of Denver, a provider of microwave systems for utilities, governments, and enterprises, recommended microwave radio systems from Exalt Communications. The Exalt systems can carry up to 200 Mbits/s of Ethernet with 99.999% reliability. They also can carry traffic 30 miles or more on a straight, line-of-sight hop.
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As it happened with the curving and hilly route of U.S. 36, the project would require seven hops of Exalt gear. In the end, only five cameras were deployed. The cameras used VGA resolution (640 by 480) at 30 fps with MPEG-4 compression.
Deployment began by starting with the head end of the system near CDOT’s network operations center in Golden, Colo. Proceeding hop by hop along the route, each link was set up and tested for acceptable performance before the next link went up. The Exalt radio systems have built-in spectrum analyzers, which made it easier for technicians to characterize the wireless environment on each hop and then tune each radio for optimum performance.
The all-outdoor Exalt 5.8-GHz radio systems include built-in Ethernet hubs, which saved the cost of using a switch to connect the radar, traffic monitoring, and video equipment in many locations. In addition, the systems have Web-based remote SNMPv3 management capabilities so the whole network can be monitored and managed from a single console.
Reliable Performance From Day One
Video traffic is highly asymmetrical, with most of the bandwidth moving upstream from the cameras back to the operations center and only occasional pan/tilt/zoom commands heading downstream from the operations center to the cameras. CDOT was able to use an Exalt feature called throughput symmetry control to assign 80% of the total network bandwidth to the upstream direction.
At the operations center, employees monitor traffic on U.S. 36 using 36-in. Panasonic monitors and Camera Camelion software. Monitoring activities include collecting information on road conditions, construction zones, chain laws, and accident management (see the figure).
After extensive testing, the project went live in early 2009, and it has worked flawlessly from the first day. The results have been excellent. “The video we get from this system is very valuable,” says Scott. “You can’t tell that it’s not coming in over fiber.”
In addition to feeding the CDOT operations center, the live video also flows via custom CDOT software to a consumer Web site where users can see live streaming video of U.S. 36 and other highways in the video surveillance system to determine current traffic conditions. Commuters can click on a highway designation on the site to see live video of dozens of on-ramps, off-ramps, and interchanges throughout the state.
The system has now been operational for more than a year in all weather conditions, and the microwave backhaul system has proven its reliability and performance. At one point a new source of interference crept into one of the links, but Scott’s team was able to resolve it using the spectrum analyzer built into each Exalt radio, along with Exalt’s remote management and configuration capabilities. As for the overall project’s success, Scott has become a believer in microwave backhaul systems.
Charles Rubin is a freelance writer based in Northern California. He can be reached at [email protected].