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Traffic Monitoring: A Guidebook

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4. Traffic Monitoring Equipment Capabilities & Limitations

Manual Recording - Tally, Mechanical, Electronic

The basic form of a manual count involves personnel recording information on a field sheet using a tally mark. It can include data related to a vehicle passing a particular point, making a particular turning movement at an intersection, the classification type of a vehicle, or the number of pedestrians utilizing a cross-walk. Tally marks should only be used for very low volume locations, but they are a relatively inexpensive form of counting.

Mechanical board counters may also be used for manual counts. This device is referred to as a multi-denominator tally and mechanically records a count from the push of a button. This device allows a person to count a higher volume location and typically can accommodate up to eight different categories. At predetermined time intervals, the cumulative information is manually recorded by personnel on a field sheet. The information can then be put into a spreadsheet for analysis purposes. Collecting more than eight categories will involve the use of more personnel to obtain the data.

Electronic board counters are similar in operation to mechanical board counters. However, the electronic board counter has internal memory and will automatically store the collected information at desired time intervals. Upon completion of a count, the data can be downloaded to a computer for analysis purposes. The electronic Board counter is supplied with a manufacturer program that allows for easy transfer and output of the field data.

Accumulative Count Recorder

Accumulative count recorders (ACRs) are automatic counters that can be set up at a location and count continuously for short periods of time (short term counts) up to several weeks. These devices utilize temporary sensors such as piezoelectric strips, tape switches, temporary induction loop detectors, or pneumatic tubes placed in the roadway. These sensors can easily be positioned for counting and then retrieved for future use at other locations. The most commonly used sensor for an ACR is the pneumatic tube or "road tube." The ACR is easy and quick to install at locations and is relatively inexpensive to operate. However, the road tube sensors will degrade over time, are not as accurate for speeds less than 10 mile per hour (mph), and are less accurate when recording on multiple lanes at one time.

Automated Traffic Recorder

Automated traffic recorders (ATRs) are automatic vehicle counters that operate for longer periods of time and are set up for more permanent function (continuous counts). These devices utilize permanent sensors, such as induction loop detectors, which are installed directly into each lane of pavement at a count location. An inductive-loop detector senses the presence of a metal object by inducing currents in the object. The ATR sensors tend not to degrade as rapidly as the ACR temporary sensors. The automated traffic recorder produces accurate data and works well where vehicle volumes are high. The data is often used to analyze travel trends at a particular location, as well as to adjust short term count data into estimates of annual, design, or average conditions (2). Power and a communication link must be available at each location to allow for the continuous operation of the ATR.

Automated Vehicle Classification Recorder

Truck and recreational vehicle traffic often generates different seasonal or day-of-week trends than do total volumes, which are likely dictated by automobile traffic trends. Automated vehicle classification (AVC) recorders can record the type of vehicle based on a defined set of length or axle parameters. These devices utilize permanent sensors (such as a set of induction loop detectors in each lane) that monitor each lane of travel at a count location. Classification is typically separated by lane, direction, and sorted into the standardized FHWA 13-category classification scheme; however, most devices allow for sorting up to 14 user-defined classifications. Classification data can be easily converted to vehicle volume counts when needed. While these devices perform best at locations with speeds around 45 mph, they do not function well for locations where traffic is moving too fast or too slow due to miscounts. Power and a communication link must be available at each location to allow for the continuous operation of the AVC recorder.

Weigh-in-Motion Recorder

Weigh-in-motion (WIM) recorders are devices that can determine the weight of a moving vehicle, in addition to recording vehicle classification and volume. These devices utilize permanent sensors (such as bending plates) or temporary sensors (such as piezoelectric strips) that monitor each lane of travel at a count location. WIM recorders are highly accurate, typically within 10 percent of gross vehicle weight (GVW). While these devices provide the most accurate travel information, they also require greater investment for equipment, installation, and site preparation, in addition to greater maintenance needs. Power and a communication link must be available at each location to allow for the continuous operation of the WIM recorder.

Other Devices

There exist other types of sensors for automated counting equipment not listed in the previous sections. These other types of sensors are generally more expensive than the ones listed above and typically require power and a communication link at each location. Video imaging and active infrared detectors utilize optical data capturing to perform vehicle classification or vehicle turning movements at intersections. Unfortunately, these devices perform badly in poor weather conditions and in high density locations due to occlusion (high profile vehicles obstructing the detector's line-of-site of other vehicles). Acoustic detectors are devices which utilize sound waves to count vehicles, but perform poorly in echo-filled environments. Remote traffic microwave sensors (RTMSs) are miniature radar devices which utilize low-power microwave signals to detect vehicle classifications. The microwave sensor is not affected by poor weather and does not have issues related to occlusion. Finally, cellular and satellite communications equipment is being utilized to provide vehicle tracking information and could be useful in remote areas, but this technology is still being developed.

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