Methods of Measurement

Methods of conducting spot speed Studies are divided into two main categories: Manual and Automatic. Spot speeds may be estimated by manually measuring the time it takes a vehicle to travel between two defined points on the roadway a known distance apart (short distance), usually less than 90m. Distance between two points is generally depending upon the average speed of traffic stream. Following tables gives recommended study length (in meters) for various average stream speed ranges (in kmph)

Stream Speed	Length
below 15	30
15 -25	60
above 25	90

Following are the some methods to measure spot speed of vehicles in a traffic stream, in which first two are manual methods and other are automatic:

Pavement markings

In this method, markings of pavement are placed across the road at each end of trap. Observer start and stops the watch as vehicle passes lines. In this method, minimum two observers required to collect the data, of which one is stand at the starting point to start and stop the stop watch and other one is stand at end point to give indication to stop the watch when vehicle passes the end line. Advantages of this method are that after the initial installation no set-up time is required, markings are easily renewed, and disadvantage of this is that substantial error can be introduced, and magnitude of error may change for substitute studies and this method is only applicable for low traffic conditions.

**Figure 1:** Pavement Marking
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Enoscope or Mirror box

Enoscope consists of a simple open housing containing a mirror mounted on a tripod at the side of the road in such a way that an observer's line of sight turned through 90o. The observer stands at one end of section and on the other end enoscope is placed and measure the time taken by the vehicle to cross the section (fig 6.2). Advantages of this method are that it simple and eliminate the errors due to parallax and considerable time is required to time each vehicle, which lengthen the study period and under heavy traffic condition it may be difficult to relate ostentatious to proper vehicle are the disadvantages of enoscope method.

**Figure 2:** Enoscope Method
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Road Detector (Pressure contact strips)

Pressure contact strips, either pneumatic or electric, can be used to avoid error due to parallax and due to manually starting and stopping the chronometer or stopwatch. This is the best method over short distance it gives quite relevant data and if it is connected through graphical recorder then it gives continuous data automatically.

Doppler-Principle Meters (Radar)

This is recently developed method, it automatically records speed, employs a radar transmitter-receiver unit. The apparatus transmits high frequency electromagnetic waves in a narrow beam towards the moving vehicle, and reflected waves changed their length depending up on the vehicles speed and returned to the receiving unit, through calibration gives directly spot speed of the vehicle.

Electronic-Principle Detectors (Photography)

In this method a camera records the distance moved by a vehicle in a selected short time. In this exposure of photograph should be in a constant time interval and the distance travelled by the vehicle is measured by projecting the films during the exposure interval. The main advantage of method that, it gives a permanent record with 100% sample obtained. This method is quite expensive and generally used in developed cities. In this we can use video recorder which give more accurate result.