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Active Infrared (AIR)/Laser
Low power LED or laser diode fires a pulsed or continuous beam down to road
surface as shown in Fig. 1.
Time for reflection to return is measured.
Presence of a vehicle lowers the time of reflection.
High scanning rates provides a detailed profile for classification
determination.
Use of Doppler frequency shift from moving object allows for very accurate speed
determination.
The AIR system collected following parameters flow volume, speed,
classification, vehicle presence, traffic density.
Advantages
- Very accurate flow, speed and classifications possible.
- Laser systems work in day and night conditions.
Disadvantages
- Active near-IR sensors adversely affected by weather conditions.
- Laser systems impeded by haze or smoke.
- Some problems with tracking small vehicles reported.
- Relatively high costs compared to other units.
Precise, but limited zone of detection require additional units over other
systems.
Figure 1:
Laser radar beam geometry. (Source: FHWA vehicle detection manual)
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Microwave - Doppler and Radar
Low energy microwave radiation (2.5 to 24 GHz) is transmitted into the detection
zone.
Objects within the zone reflect a portion of the radiation back to a receiver.
Doppler units use the frequency shift of the return to calculate speed as shown
in Fig. 2.
It can’t detect the stationary objects.
The microwave system collected following parameters.
Doppler - Flow volume and speed;
Frequency-Modulated, Continuous Wave (FMCW) - Flow volume, speed and presence;
Microwave - Flow volume, speed, presence, possibly classification;
Advantages
- Very accurate. Easy to install, long ranged.
- Multiple detection zones possible.
- Day or night operation.
Figure 2:
Microwave radar operation. Source
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Disadvantages
- Possible sensitivity to spurious returns from adjacent objects
- Restrictions on use due to electromagnetic interference with other
electronics.
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