Equipment for Collecting Traffic Load Data (2004) / Chapter Skim
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From page 21... ...
3.1.1 Intrusive Technologies This section covers sensor technologies that are placed either in or on top of the pavement and, at a minimum, provide the ability to classify vehicles into passenger vehicles and trucks. Portable Operations Portable sensor technologies used for classification include • Road tubes, • Piezoelectric sensors (BL [brass linguini]
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From page 22... ...
Pressure-sensitive technologies, by themselves, have difficulty distinguishing between vehicles in the Type of Sensor Strengths Concerns Portable Vehicle Classification Sensors Road Tubes (axle-based classification) Inexpensive Very common Easy to use Inaccurate under high volumes Difficult to install on multi-lane facilities Conventional road tubes can only measure classifications in lanes next to shoulders or medians.
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From page 23... ...
Type of Sensor Strengths Concerns Permanent Vehicle Classification Sensors Intrusive Sensors (General Comments) Sensors installed in the pavement tend to be adversely impacted by poor pavement condition Poor lane discipline limits accuracy Must be reinstalled if channelization changes Snow can badly degrade lane discipline and consequently classification count accuracy Axle sensor-based systems allow use of FHWA 13-category system and similar state classification systems When traffic flow conditions are unstable, as often occurs in urban areas, simpler, more aggregated, length-based classification schemes often work more accurately than the more complex, axlebased classification systems Inductive Loop (conventional)
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From page 24... ...
Requires regular maintenance Not as accurate in multi-lane environment Little reliability data available Ultrasonic New technology - appears promising Little reliability data available Requires multiple sensor installation Accuracy deteriorates as traffic volumes increase Some environmental conditions (air turbulence) can decrease system accuracy Acoustic New technology Little reliability data available Accuracy deteriorates with increasing variability in traffic speeds
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From page 25... ...
TABLE 3.2 Sensors commonly used for WIM Type of Sensor Strengths Concerns Permanent WIM Sensors General Comments Permanent sensors are placed flush with the road surface, increasing the accuracy of the sensor outputs The accuracy of all WIM sensors decreases with decreasing pavement conditions Unstable speeds, which are common in urban areas, result in significant decreases in WIM accuracy, regardless of the technology chosen Piezoceramic Cable Easier, faster installation than most other WIM systems Generally lower cost than most other WIM systems Well supported by industry Sensitive to temperature changes Accuracy affected by structural response of roadway Susceptible to lightning Meticulous installation required Low cost and ease of installation often result in placement in slightly rutted pavements, resulting in loss of accuracy Piezopolymer Easier, faster installation than most other WIM systems Generally lower cost than most other WIM systems Well supported by industry Sensitive to temperature changes Accuracy affected by structural response of roadway Susceptible to lightning Meticulous installation required Low cost and ease of installation often result in placement in slightly rutted pavements, resulting in loss of accuracy Piezoquartz Easier, faster installation than many other WIM systems May be more cost-effective (long term) if sensors prove to be long lived Very accurate sensor Sensor is not temperature sensitive Growing support by industry More expensive than other piezo technologies Requires multiple sensors per lane Above average maintenance requirement Sensor longevity data not available Accuracy affected by structural response of roadway Bending Plate Frame separates sensor from pavement structure Entire tire fits onto sensor Moderate sensor cost Sensor is not temperature sensitive Extensive industry experience with the technology Longer installation time required than piezo systems Some systems have experienced premature failure, while others have been very long lived (continued on next page)
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From page 26... ...
Type of Sensor Strengths Concerns Permanent WIM Sensors (Continued) Load Cell Entire tire fits onto sensor Frequently considered the "most accurate" of conventional WIM technologies Some systems have demonstrated very long life spans Most expensive WIM system Requires significant construction effort to install Becomes cost effective if constructed and maintained for a long life span Fiber-Optic Promising technology Not susceptible to lightning New technology, no longevity history Not well supported yet by industry Accuracy affected by structural response of roadway Subsurface Frame Strain-Gauge System System designed to eliminate impact loads on sensor, increasing expected design life Buried design increases "time on sensor" for an axle Very new technology, currently undergoing testing in the United States No data on longevity of system, or accuracy of output using current software design Unclear if variation in structural response of pavement will affect system accuracy Expensive, long-duration installation Multiple Sensor Systems (piezo, bending plate)
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From page 27... ...
Generally needs truck isolated on bridge to weigh accurately Not actively marketed in the United States Piezo (ceramic cable, BL-polymer film) Ease of deployment Inexpensive sensor cost Susceptible to variations in temperature More accurate if used in permanent installation Capacitance Mats Ease of deployment Modest sensor cost Only measures one wheel path Creates the largest "bump" of the portable technologies
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From page 28... ...
Fiber-optic sensors are more responsive than road tubes, theoretically making them more accurate under both very slow speed conditions and very high volume conditions. The advantage of fiber-optic sensors over piezo sensors is that the former are not temperature sensitive and the sensors themselves do not conduct electricity, thus making devices using these sensors less susceptible to lightning strikes.
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From page 29... ...
Other pressure sensors generally are less expensive to purchase than piezo sensors, though installation time and effort tends to be very similar. Dual-inductance loops.
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From page 30... ...
or suffer from occlusion that occurs when a system cannot "see" one vehicle or axle because the sensor's "view" of that vehicle is blocked by an intervening vehicle. As a result of their inability to easily count axles, most non-intrusive sensors classify vehicles by overall vehicle length, similar to dual-inductance loop technology.
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From page 31... ...
Two or more consecutive zones (set at a known distance apart) can be used, just as dual-inductance loops are used, to measure vehicle speed and consequently overall vehicle length.
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From page 32... ...
Monitoring these from two consecutive sensor locations allows the computation of vehicle speed and consequently overall vehicle length. Sensor output from passive infrared appear to be unaffected by changes in weather conditions.
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From page 33... ...
The greater the amount of vertical motion exhibited by trucks, the more difficult the task for WIM systems to accurately estimate static axle loads. Thus, for all WIM technologies, a key issue for collecting accurate weight data is to select locations for data collection that minimize the dynamic motion of trucks being weighed.
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From page 34... ...
Piezoelectric Sensors (BL and Ceramic Cable) The primary alternative to capacitance mats currently used by state highway agencies for high-speed portable WIM data collection is thin-strip piezo sensors.
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From page 35... ...
However, like the capacitance mat systems, piezo cables used in portable operations suffer from significant limitations in accuracy for the following reasons: • Sensors are temperature sensitive, making it difficult to keep them in calibration when temperature changes during the day. • The sensors' narrowness allows tires being weighed to "fold" over them, meaning that at no time during the axle weighing process is an entire tire isolated on the sensor.
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From page 36... ...
Specific WIM system technologies that can be used for permanent, continuous weight data collection are 36 • Piezoceramic sensors, • Piezopolymer sensors, • Piezoquartz sensors, • Bending plates, • Hydraulic load cells, • Bridge and culvert WIM systems, • Capacitance mats, and • Other WIM technologies (fiber-optic, subsurface strain gauge, multi-sensor)
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From page 37... ...
of piezoquartz sensors, two lines plus an inductance loop, or one line of piezo sensors and two inductance loops. As with other piezo installations, each of these configurations allows for the computation of vehicle speed and, consequently, axle spacing, which in turn allows vehicle classification.
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From page 38... ...
This also negates the bridging effect from which strip sensors suffer, as well as limiting the effect that different tire pressures and tread designs have on the forces exerted on the scale platform. Tests of system performance generally indicate that bending plates are more accurate than traditional piezo cable and capacitance mat WIM systems and are roughly equivalent in accuracy to piezoquartz sensors, but are less accurate than hydraulic load cells.
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From page 39... ...
Permanently mounted capacitance mats differ from portable mats in that the former mats are placed in steel frames that are installed in the pavement surface. This allows the surface of the mats to be flush with the roadway and improves the accuracy of the system.
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From page 40... ...
40 In the case of the bending-plate system described above, the loss of one of the two right bending plates actually leaves the site as being equivalent to a conventional bending-plate WIM site in terms of sensor accuracy. Note that before such an approach is adopted, the highway agency must make sure that the vendor's data collection electronics can both handle any additional sensor inputs and correctly interpret the signals coming from sensors placed in a staggered position.
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