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R.N.G.PATEL INSTITUTE OF TECHNOLOGY

Subject : Traffic Engineering and management �Subject Code : 3160615

Topic: Methods of conducting spot speed study and presentation of spot speed study

Prepared By

Nirav P Desai

Assistant Professor

Civil Engineering Department

R.N.G Patel institute of technology Bardoli

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1. Enoscope Method

Enoscope is a mirror box supported on a tripod stand. In its simplest principle, the observer is stationed on one side of the road and starts a stop watch when a vehicle crosses that section.

An enoscope is placed at convenient distance of say 50 m in such a way that the image of the vehicle is seen by the observer when the vehicle crosses the section where the enoscope is fixed and at this instant the stop watch is stopped.

The main advantage of this method is that it is a simple and cheap equipment and at this instant the stop watch is stopped.
The main disadvantage of this method is that the progress is so slow as it is difficult to spot out typical vehicles and the number of samples observed will be less.
Spot speed by two enoscope :
In this method, the observer stand between two enoscopes.

When the image of vehicle is seen in the enoscope at A, stop watch is started and the instant, when image of vehicle is seen at B , stop watch is stopped.

2. Radar speed meters:

Radar speed meters work on principle of “Doppler effect”. According to this principle the speed of moving body is proportional to change in frequency between radio wave transmitted to moving body and radio wave received back.
The instrument directly measures speed, and an accuracy of at least ± 1.5 km.ph. is possible. The instrument is battery operated and is portable.
The speed meter is so kept that angle between direction of travel of vehicle and axis of transmission of radio wave is low as possible,say within 20°.
The instrument is set up near edges of carriage way at aheight of about 1m. The speeds of vehicles in both directions can be observed by this method.

3. Pressure contact tubes:

In this method pneumatic tubes are used as detectors to indicate time of entering and leaving the base length.
When a vehicle passes over tube laid at the first reference point, an air impulse is sent, which activates an electromagnetically controlled stop watch in hands of the observer.
When the vehicle passes over the second tube, the stop watch automatically stops. The reading is noted by observer.Alternatively, the readings can be recorded by automatic data recorders.
The main disadvantage of this method is that contact tubes are easily seen by the drivers and this may affect their behavior.

Presentation of Spot Speed Data

(1) Histogram and frequency distribution curve

(2) Cumulative frequency cure

(1) Histogram and frequency distribution curve

A frequency distribution curve, of spot speed is plotted with average values of each speed group of vehicles on X-axis and % of vehicle on Y-axis. Thus, frequency distribution curve is found by rounding off the histogram so that the area of the frequency curve is equal to the area of the histogram.
MODEL SPEED:- The speed corresponding to peak of the frequency distribution curve is called model speed.
The model speed indicate the speed at which the greatest proportion of vehicles travel.

(2) Cumulative frequency cure

The cumulative frequency curve is plotted between cumulative frequency percentage and upper limit of speed in each sped group.
It is used for determining the number of vehicles travel above or below given speed.

85^th percentile speed :
The speed corresponding to 85% of cumulative frequency in the cumulative speed distribution graph is known as 85^th percentile speed.
85^thpercentile speed is the speed at or below which 85% of the vehicles are passing point on the highway or only 15% of the vehicles exceed the speed at that spot.
85^thpercentile speed is also known as safe speed limit.
98^thpercentile speed is taken as the design speed for a highway.
15^thpercentile speed is the minimum speed.