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Number of vehicles can be counted either manually or by machine depending upon
the duration of study, accuracy required, location of study area etc.
In its simplest form an observer counts the numbers of vehicles along with its
type, passing through the section for a definite time interval.
For light volumes, tally marks on a form are adequate.
Mechanical or electrical counters are used for heavy traffic.
Although it is good to take some manual observations for every counting for
checking the instruments, some other specific uses of manual counts are
following:
- Turning and through movement studies
- Classification and occupancy studies
- For analysis of crosswalks, sidewalks, street corner space and other
pedestrian facilities
These can be used to obtain vehicular counts at non-intersection points.
Total volume, directional volume or lane volumes can be obtained depending upon
the equipment available.
These are installed to obtain control counts on a continuous basis.
A detector (sensor) which responds on the passage of vehicle past a selected
point is an essential part of this type of counters.
These can be mainly grouped into contact types, pulsed types, radar types.
Among the contact type counters, pneumatic tubes are mostly used.
Air pulse actuated by vehicle wheels, pass along the tube thereby increasing
the count.
Pulsed types mainly depend upon the interruption of a beam generated from a
station located near the site, which is detected by the receiver.
In radar types, a continuous beam of energy is directed towards the vehicle.
The frequency shift of energy reflected from approaching vehicle is conceived
by sensors.
Due to tedious reduction of the voluminous amount of data obtained, use of such
counters was decreasing.
But the use of computers and data readable counters has reversed the trend.
These are used to obtain temporary or short term counts.
Generally these make use of a transducer unit actuated by energy pulses.
Each axle or vehicle passage operates a switch attached to a counter which is
usually set to register one unit for every two axles.
If significant number of multi-axle vehicles is present, an error is
introduced.
A correction factor, obtained from a sample classification count, is introduced
to reduce this error.
This can further be sub-divided into two types:
- Recording counters provides a permanent record of volumes by
printing the total volume.
These may be set for various counting intervals.
- Non-Recording Counters must be read by an observer at desired
intervals.
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