County's traffic control enters the 21st century.
Burlington County contracted with Parsons Brinckerhoff-FG, Inc. to perform the engineering design for the project. The scope of the project included collecting traffic data, surveying each intersection, feasibility reports for the type of communication medium and type of system detector, designing the internal closed loop traffic control system, and preparing contract plans and specifications. The project required the use of three traffic analysis programs developed by the Federal Highway Administration: Transyt-7F, Passer II, and Highway Capacity. These software programs were used to establish background cycles, maximize through-band, and provide a good intersection level-of-service.
The internal closed loop traffic control system is comprised of three major components: the traffic signal controller, the on-street master, and the central computer. These components must communicate to each other to make the system operational. The communication medium is a critical link for these components. Since Burlington County did not have an established communication medium for traffic control signals, it was stipulated that a feasibility report be provided to evaluate the various types of media.
Another critical link is system detectors. System detectors provide real-time traffic volume data to the on-street master controllers. This allows the internal dosed loop traffic control system to perform as a vehicle-demand operation. A system operating as vehicle-demand allows the traffic signals to change to various background cycles and/or go to a free operation based on real-time traffic volume. Since the system detectors are critical for obtaining traffic volume data, it was stipulated that a feasibility report be provided to evaluate the various types of detectors.
These reports provided the advantages, disadvantages, reliability, functionality, and appropriateness of each type of communication medium and type of detector. Based on these reports, Burlington County selected a communication medium and a type of detector.
The communication media report evaluated the use of twisted pair copper cable, fiber-optic cable, leased lines, UHF radio band, trunked radio system, spread spectrum radio, microwave radio, satellite radio, and spread spectrum radio.
The twisted pair copper cable and fiber-optic cable require the installation of conduit between each communication node. This escalates the cost for communications quickly, especially in areas where the traffic control signals are more than 1,500 ft apart. Each cable has its own limitations. The twisted pair copper cable requires repeater stations at designated intervals and the data transmission speed is limited. The fiber-optic cable provides unlimited transmission speed, but requires repeater stations. All of these media are susceptible to being damaged by any contractor performing excavation work between two communication nodes. These media do not offer much flexibility for changes and expansion. The annualized costs are approximately $104,000 for each type.
The leased line alternative requires a third party to maintain the communication medium. Although the initial costs are relatively low, there are monthly charges to maintain the system. As the internal closed loop system increases in the number of intersections, the monthly maintenance costs will also increase. This is a huge disadvantage for a government agency that is under pressure to keep the annual operating costs at a minimum. This medium provides a lot of flexibility for changes and expansion. The annualized cost is approximately $27,300.
Radio frequencies have been used successfully for traffic control signal communication. There are, however, many disadvantages. They are as follows: FCC licensing, required clear line of sight between nodes, frequency interference problems occurring in congested areas, and the medium is difficult to expand because of additional licensing requirements. There was no annualized cost performed because this medium was deemed' inappropriate for Burlington County.
A trunked radio system was not evaluated because it provides significant delays - up to several minutes to obtain a channel.
Microwave radio provides a high capacity transmission of data at a .high rate of speed. The difficulties of microwave radio are the need for licensing, the fade-out that can be caused by atmospheric changes or long distance, and the requirement of towers to install antennae. Similar to the UHF radio, this alternative was deemed not appropriate for Burlington County's use.
Satellite radio provides the finest of communication devices; however, at an annualized cost of $760,000, it was determined to be too costly.
The spread spectrum radio is a relatively new technology in the commercial market. This type of medium does not require licensing from the FCC. The technology was first introduced by the military because it resists jamming and is difficult for an enemy to intercept. This provides a great advantage in an urban area where frequencies are likely to have interference from other radio signals or transmissions. The spread spectrum radio can bypass interference due to a method known as "frequency hopping." The radio continually transmits over a range of frequencies, normally between 902 to 928 MHZ, to provide dean data.
The power output of spread spectrum radio is limited to a maximum of one watt. This limits the range between transmissions to five miles, depending on line of sight. Since most traffic signal intersections are within this range, this does not pose a problem for communications. The most critical design parameter for installing spread spectrum radio is to ensure that there is proper line of sight between nodes. This can be easily determined by conducting a field survey.
The data transmission of spread spectrum radio is limited to 2,200 bps. This will not serve well for an expanded IVHS system; however, it does serve very well for an internal closed loop traffic control system. Since Burlington County does not intend to expand its system, this limitation is not a problem at this time. This medium provides a lot of flexibility for changes and expansion. The annualized cost for spread spectrum radio is approximately $22,700. Spread spectrum radio is currently being used in Tennessee, Georgia, and Los Angeles, as well as in New Jersey.
The recommendation from Parsons-Brinckerhoff was to use spread spectrum radio technology as the communications medium for the internal dosed loop traffic control system. Burlington County conducted several site inspections in Georgia to evaluate an existing spread spectrum radio system. The inspections showed that this medium is dependable and cost-effective. Therefore, it was chosen as the communications medium. Peek Traffic, Inc., Sarasota, Florida supplied the spread spectrum radio system.
The detector technology report evaluated the use of inductive loops, micro-loops, magnetometer, ultrasonic radar, microwave radar, active and passive infrared, and video image processing.
The inductive loop detectors are the most commonly used sensor in the U.S. and are familiar to all contractors and maintenance personnel. They are sawcut into the pavement in the area that needs the detection. Inductive loops have a wide range of operations that they can perform such as: volume, occupancy, speed, delay, etc. The main advantages are that they serve as an excellent pulse and presence detection system, they are not susceptible to failure in severe environmental conditions, and they reflect the actual time a vehicle is over the loop. The disadvantages are that they are difficult to install properly, time consuming to install, disrupt traffic during installation, difficult to maintain, and the cuts in the roadway reduce the pavement strength.
Burlington County had been using inductive loops exclusively for all of its detection requirements.
The use of micro-loops, ultrasonic radar, and active and passive infrared were quickly eliminated because they do not perform a sufficient amount of functions or provide additional features to warrant further evaluation. The video image processing equipment was deemed to be too costly as an isolated detector site.
The magnetometer is similar to the inductive loop detector since it is based on the variations in the magnetic field caused by the passage of a vehicle. The magnetometer is a probe that is installed in a drilled hole in the pavement. A lead-in cable is inserted in a sawcut back to the edge of the roadway. This device has the same advantages and disadvantages as inductive loops except that it is not a familiar type of detection to the Burlington County maintenance personnel.
Microwave radar is an over-the-road type of detector. The obvious advantage is that there is no need to perform construction in the roadway. The radar detector can be placed on the side of the road or directly in line with the lane it is detecting. The disadvantage to locating the detector over the lanes is the need for a rigid mast-arm or bridge on which to locate the detector. This increases the cost for construction and maintenance. The disadvantage of locating the detector on the side of the road is a loss of counting accuracy and an increase of false calls. Although manufacturers continue to state that a proper design and installation can overcome false detection, it is often difficult to achieve. This type of detector is considered to be less accurate than both the inductive loop and the magnetometer.
The recommendation of Parsons-Brinckerhoff was to continue the use of inductive loops. This analysis was not as obvious as the communication media analysis. The decision to use inductive loops is basically due to cost. Burlington County agreed with this recommendation. Peek Traffic, Inc. supplied the inductive loops. The installation of the computerized internal closed loop traffic control system was completed in December 1996.
Joseph M. Pavlik formerly served as Burlington County Traffic Engineer, Mount Holly, New Jersey. He is currently Senior Project Engineer with Urban Engineers, Inc., Philadelphia, Pennsylvania.
Beth Pover Angstadt is Lead Transportation Engineer with Parsons-Brinckerhoff-FG, Inc., Princeton, New Jersey.
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|Title Annotation:||Burlington County, NJ|
|Author:||Pavlik, Joseph M., Jr.; Angstadt, Beth Pover|
|Date:||Feb 1, 1997|
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