Spot Speed Studies in Traffic Engineering: Endoscope, Measurement and Methods of Measuring Spot Speeds!
Speed Studies:
Traffic flow is influenced by the speed of the traffic on existing roads. Speed is a function of several factors such as the geometric design features, traffic conditions like the presence of other vehicles, time and place, the environment, and the driver, and the traffic system in general.
Speed studies and speed-delay studies are useful in several ways such as:
(i) Designing geometric feature of the highway
ADVERTISEMENTS:
(ii) Planning traffic regulation and control measures
(iii) Studying traffic capacity of a highway
(iv) Accident studies
(v) Cost-benefit analysis of a highway project
ADVERTISEMENTS:
(vi) Studying traffic trends for future use.
The different types of speed considered are:
1. Spot Speed:
This is the instantaneous speed of a vehicle at a specified location.
ADVERTISEMENTS:
2. Running Speed:
This is the average speed maintained by a vehicle over a given stretch of highway while the vehicle is actually in motion; the clause “while the vehicle is actually in motion” is significant because the running speed is determined by dividing the length of the stretch by the time for which the vehicle is in motion (The running time excludes that part of the journey time for which the vehicle suffers delay, for example, at controlled intersections.)
3. Operating Speed:
It is the sustained running speed at which a vehicle can travel under the existing traffic and environmental conditions.
ADVERTISEMENTS:
4. Overall Speed or Journey Speed:
This is the effective speed with which a vehicle covers a particular route between two terminals; this is, the average speed obtained by dividing the total distance covered by the total time taken including all delays, but excluding voluntary stoppages, if any.
Spot speeds are useful to study paints of congestion, for locating traffic signals, or for enforcing speed limits.
Running speeds are useful to assess the traffic capacity of highways. Journey speeds are useful to assess the adequacy of an existing road network as also the efficiency of the improvement measures and for cost-benefit studies of a project.
ADVERTISEMENTS:
For assessing the improvement measures implemented on a stretch of a highway, journey speed studies are conducted before and after the implementation. These studies are specially referred to as before and after studies.
Average Speed:
There are two different types of average speed obtained from spot speeds of vehicles- Time-mean speed and space-mean speed.
Time-mean speed is defined as the average of the speed measurements at one point in space of all the vehicles passing over a period of time.
This is given by-
Space-mean speed is defined as the average of speed measurements at a particular instant of time of all vehicles over a particular stretch of the highway.
This is given by-
Where, σ s is the standard deviation of the space-mean speed.
Methods of Measuring Spot Speeds:
The methods available for spot speed measurement may be classified as:
1. Direct observation of the time taken by a vehicle to cover a known distance.
2. Radar speedmeter method.
3. Electronic meter method
4. Photographic methods.
The direct observation method can be further classified to long-base method and short-base method.
The long-base method is sub-divided to:
(i) Direct measurement of time
(ii) Enoscope
(iii) Pressure contact tubes.
Section of the Observation Point:
The criteria for this are:
1. Appropriate location for the specific purpose of study, intersection zones
2. Minimum influence of the observers and equipment on the driver and their speeds
3. Straight and level section of the highway to minimise the effect of geometric elements and roadside developments.
For long-base methods, the Institute of Traffic Engineers (1965) has recommended certain base lengths for spot speed determination at different average speed ranges of the traffic stream.
Basic Lengths for Spot Speed Determination:
This is the simplest and most direct method for spot speed determination. Two observers are stationed at a convenient distance (50 m) such that they can see each other. The first one starts a stop-watch as a vehicle crosses the first reference point and stops it on a signal from his counterpart the moment the vehicle touches the other end of the section.
If the timing is done by a single observer, the measurement can be made from a convenient point in the section on one side of the road at a slightly higher elevation so that both the timing spots can be seen directly.
From the known distance and the measured time intervals, speeds can be calculated. A skilled observer can read a stop-watch to an accuracy of 0.2 second. If the observer can station themselves in an inconspicuous way, the speeds calculated are not influenced by the driver’s reaction to work. The disadvantages of this approach are that the readings are influenced by the observer’s reaction time and the parallax effect in observing the vehicle position.
The parallax error of the observer can be eliminated by using a simple device called ‘Enoscope’, especially in the case of a single observer. Also known as ‘Mirror box’, it is an L-shaped box, open at both ends; a mirror is set inside it at 45° to both the arms as shown in Fig. 4.8.
Light rays from the vehicle impinge on the mirror, get reflected and pass in a direction perpendicular to the incident ray. They reflected rays fall on the eye of the observer as shown. The observer can start and stop the stop-watch more accurately, with no parallax, because the line of sight is perpendicular to the direction of motion of the vehicle.
Either one enoscope can be used or two. If only one is used the device is placed directly opposite the first reference point and the observer is stationed at the other, as shown in Fig. 4.9.
The reference points are marked by fixing poles at the side of the road; a lamp may be used for night work.
As soon as the vehicle passes the first reference point or the enoscope the timer is started and it is stopped as soon as it passes the observer. If two enoscopes are used, one each is placed at either end of the reference section and the observer will be positioned midway between the two as shown in Fig. 4.10.
The observer starts the timer when a vehicle crosses the first reference point and stops it as soon as the vehicle crosses the other end of the base by using the line of sight from each of the enoscopes. This method is not suitable for heavy multilane traffic because it is difficult to associate an observed shadow with a particular vehicle. Further, it is difficult to conceal the enoscope and the observer from the driver, whose attention would be distracted, giving scope for false readings.
In this method, pneumatic tubes laid across the carriageway at the two reference act a detectors; when a vehicle passes over them, air impulses are sent to an electromagnetically controlled stop-watch in the hands of the observer, starting the time at the first reference point and stopping it at the second. The readings can be recorded by automatic date records also, reducing the manual work.
Short-Base Methods:
Here, the base length is very short, say 2 m to 3 m and electronic instruments are used along with pneumatic table detectors. Recording may be manual or automatic.
Radar Speedmeter Method:
The radar speedmeter works on the principle of Doppler Effect. Microwaves of radio frequency are transmitted to a moving vehicle. According to Doppler Effect, the speed of the vehicle is proportional to the change in the frequency of microwaves received back at the transmitter after being reflected by the vehicle.
The speedmeter is calibrated to read the speed directly in km/h, after the potential difference is amplified to enhance the accuracy of measurement to ± 3 km/h. The instrument is portable and battery-operated; it is set up near the edge of the road at about 1 m height above the road surface. This is used by the traffic police department for enforcing speed limits.
The operation zone can be around 50 m, and speeds can be measures for vehicles moving in both the directions.
This requires the use of pneumatic tube and air switch, or electric contact stripes as detectors. On actuation, the current in milliamperes is allowed to accumulate in a condenser for a period of two electric pulses from the switches on the road. Vehicle speeds can be recorded only manually but the meter is calibrated to read the speed in km/h.
Photography with time-lapse cameras has been successfully used to determine spot speeds of vehicles even in crowded zones. Photographs are taken at fixed intervals of time (say, one second per exposure) using a special camera. The passage of any vehicle can be traced with reference to time by projecting the exposed film on a screen. Video cameras also may be used for the purpose.
Presentation and Analysis of Speed Data:
Spot speed data can be analysed mathematically or graphically.
In this method, average speed or mean speed is obtained from a frequency distribution table by multiplying the number of vehicles in each speed class (30-40 km/h, 40 to 50 km/h, 50-65 km/h, and so on) by the mean speed of that class, summing up, and dividing the sum by the total number of vehicles observed.
Depending upon the instruments and techniques used for measuring spot speeds, the two kinds of average speeds, time-mean speed and space-mean speed, are obtained; these values give an idea of the traffic speeds in a certain stretch of the road to enable the traffic engineer to plan corrective measures, if necessary.
The speed test data are used to form a frequency distribution table showing details of groups covering various speed ranges and number of vehicles in each range. From this table, both the histogram and the frequency distribution curve can be generated as shown in Fig. 4.11.
For normally distributed data, the frequency curve is bell-shaped as shown. The speed corresponding to the peak is called the modal speed at which maximum number of vehicles travel.
Cumulative Spot Speeds of Vehicles:
If a graph is plotted with the average values of each speed group on the x-axis and the cumulative percentage of vehicles travelling at or below the different speeds on the y-axis, the graph appears somewhat as shown in Fig. 4.12.
This is very useful in determining the speed above or below which certain percentages of vehicles are moving.
85 Percentile Speed:
This is the speed below which 85% of the vehicles are moving at the point considered on the highway. In other words, only 15% of the vehicles exceed this speed at the point. This is considered to be the safe speed limit under the existing conditions in that zone. However, for the purpose of geometric design, 98 percentile speed is considered.
15 Percentile Speed:
This is considered to represent the minimum speed on major highways. To decrease delay and congestion and to prevent accidents, traffic which moves at a speed less than this are generally prohibited. This provides good overtaking opportunities.
Median Speed:
It is the middle or 50 percentile speed. There are as many vehicles going faster than this speed as there are ones moving slower.