Vehicle Detection with Wireless Sensors
Traffic detection is a fundamental component of the planning and operation of local roads and highways in California. While installing and maintaining the inductive loop detectors that are traditionally used to detect traffic can be expensive, a unique new technology called the Sensys wireless vehicle detection system (VDS) is proving to lower the lifecycle costs associated with detecting traffic. In fact, it is estimated that the wireless vehicle detection system cuts costs in half over a 15-year period.
To develop the wireless vehicle detection system, the California Department of Transportation (Caltrans) Division of Research and Innovation teamed up with the Partners for Advanced Transit and Highways (PATH) program, a research unit of the Institute of Transportation Studies at the University of California, Berkeley. Today the product is available on the market through Sensys Networks, a business founded by the researchers who developed the wireless vehicle detection system.
The Impetus for a New Technology
For nearly 50 years, the primary technology used to detect vehicles has been the inductive loop detector. Although simple, inductive loop detectors are somewhat expensive to install. They require a nearby source of electrical power, which adds to the cost of installation. They can also be expensive to maintain, as they suffer from various forms of deterioration caused by the mechanical stress of freeze/thaw cycles and vibrations, as well as oxidation. These installation and reliability issues were the driving concepts behind the development of the wireless detectors.
The Benefits of the Wireless Detectors
The California Center for Innovative Transportation (CCIT) tested and reported on the wireless vehicle detector system on behalf of Caltrans DRI in October of 2006.
The test allowed Caltrans and the CCIT team to verify the short amount of time required to install the wireless detectors and established that large benefits can be realized from reduced lane closure times. Over a 15-year period, the wireless detector system would require closure of 15 lane hours per station, as opposed to 56 lane hours with inductive loops.
Testing also determined that the wireless detectors deliver data quality similar to that of the incumbent inductive loop technology. Specific findings include:
Complete test documentation and results are available at www.calccit.org.
Overall, the initial direct cost of a Sensys wireless VDS is estimated to be approximately $9,700 including hardware, labor and installation equipment. The direct cost of an inductive loop is estimated to be nearly three times that amount, approximately $26,100. Over a 15-year lifecycle, the regular maintenance of a loop system would also be about twice that of a Sensys wireless VDS, the estimates being $9,400 and $5,000 respectively. However, due to shorter life of Sensys wireless detectors, Sensys VDS would be replaced more often. This translates into total maintenance and replacement costs of about $12,900 for a Sensys VDS and $23,500 for a loop system. In total, the Sensys VDS is estimated to cost $22,500 over a 15-year period, less than half of the $49,500 cost for an inductive loop vehicle detection system.
How the Wireless Vehicle Detection System Works
The Sensys wireless vehicle detection system includes individual wireless sensors and a local controller, which is also called an "ss point." This setup is sometimes complemented by a repeater, or several.
Figure 1 shows a typical freeway installation comprised of two vehicle detection stations. Sensors detect vehicle presence and transmit ON (vehicle detected) and OFF events (vehicle no longer detected) to the access point. The access point aggregates the data from individual sensors and can report key traffic parameters including presence, average counts, average speeds, lane occupancy, and queue length at selectable time intervals. The system also features various self-monitoring and diagnostics capabilities.
Two recent breakthrough technologies make the sensors used in the system remarkably small and energy efficient. The Sensys unit relies on a magneto resistive sensor for vehicle detection. Micro Electro Mechanical Systems (MEMS) shrink the magneto resistive sensor to a size smaller than 2-inches-tall and 4 inches in diameter. The system also uses a highly efficient low-power radio modem. The communication between the access point and the sensors uses a proprietary wireless protocol designed to minimize power consumption and extend the battery life of the sensors. The combination of these technologies brings to market a reliable, functional, flexible product that meets the data demands of traffic monitoring.
Installing the Detectors
Sensys wireless detectors are ideally placed in the middle of traffic lanes. To measure vehicle speed and length, two detectors are installed in the same lane at a typical spacing of 10 to 20 feet. The installation of each detector requires boring a circular hole approximately 4-inches in diameter and 2-1/4-inches deep. A drilling bit is available to contractors for that purpose. The hole is sealed using fast-drying epoxy. The entire operation takes around 10 minutes per detector, which is considerably less time than is required for installing inductive loops. Sensys Networks estimates the life of their wireless detectors is approximately 10 years, based on battery capacity and discharge rates.
Applications for the Wireless Detectors
There are two primary applications for the wireless VDS: traffic flow monitoring and signal control. The sensors can measure volume, speed, occupancy, presence, headway, gap, direction of travel, and vehicle length. They can support traffic monitoring stations on freeways and arterials or traffic signal control applications including stop bar and advance detection at intersections, as well as ramp management at freeway entrances.
Municipal, county, or state roadway operators could essentially use the wireless sensors wherever inductive loops are used today. The technology can be used for new vehicle detector system locations or to replace existing failed loop detector stations. The wireless nature of Sensys wireless VDS provides added flexibility in complicated configurations such as split roadways, flyovers, bridges, or when detection is required at long distances from the traffic signal controller. The relatively low cost and ease of installation also hold the potential for additional applications such as work zone management and traffic monitoring of secondary roadways.
This article is adapted from an innovation case study about Sensys Networks, Inc. prepared by the California Center for Innovative Transportation. The authors of that report are: Tia Dodson, Public Policy Analyst; David Jacobowitz, Public Policy Analyst; Virginia Lingham, Graduate Student Researcher; and J.D. Margulici, Associate Director.
Caltrans Division of Research and Innovation (DRI) www.dot.ca.gov/research
Partners for Advanced Transit and Highways (PATH) www.path.berkeley.edu
California Center for Innovative Transportation (CCIT) www.calccit.org
Sensys Networks www.sensysnetworks.com