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Non-intrusive technologies include video data collection, passive or active
infrared detectors, microwave radar detectors, ultrasonic detectors, passive
acoustic detectors, laser detectors and aerial photography.
All these technologies represent emergent fields that are expanding rapidly with
continuing advances in signal processing.
At present time such technologies are used to provide supplemental information
for selected locations or for specific applications (e.g., queue detection at
traffic signals).
Most non-intrusive systems are operationally and somewhat visually similar,
consisting of small electronics unit mounted in a weatherproof housing placed in
various locations, as shown in Fig. 1.
The first type of non-invasive detectors are roadside mast-mounted.
The detector possesses a field-of-regard covering an oblique area upstream or
downstream of the unit.
There are also multiple zones of detection defined within the overall field of
regard, or the overall zone of detection same as the field of regard, depending
on the specific detector type and technology.
Obscuration problems occur when high-sided vehicles screens lower vehicles from
the detector or the field-of-view being too large, leading to detection of
vehicles outside the desired lane.
Figure 1:
Typical non-intrusive technology configurations
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The second type of non-invasive detectors are mounted on gantries or bridge
undersides, with field of regard directly below, or at a slight oblique to the
unit.
Finally, some units, such as open-path pollutant monitors are mounted road side
at ground level, firing a beam across the road.
Such units are subject to side-by-side masking and hence most suitable for only
single lane, unidirectional flows.
- Video image detection (VID)
The traffic parameters are collected by frame-by-frame analysis of video images
captured by roadside cameras.
The following parameters are collected: Depending on the processing methodology
almost all traffic parameters are captured from video analysis.
Simple video systems often collect flow volume and occupancy.
More complex systems allow the extraction of further parameters.
Advantages
Possibility to capture all desired traffic information, including some
parameters that are not readily obtainable using other types of detectors
Possibility of a permanent visual record of the traffic flow that reviewed and
analyzed by a human operator.
Disadvantages
VID systems are susceptible to obscure issues, as with other non-intrusive
detectors.
Performance of VID systems might be degraded in bad weather or low light
conditions.
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