5. SPOT Speed Study - Lecture notes 5 PDF

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3. SPOT SPEED STUDY 1. OBJECTIVE: To determine the instantaneous speed of the different vehicles at a given stretch.

2. EQIPMENT: Stop-watch, data sheet, measuring tape 3. BACKGROUND: 1. Design Speed: A speed determined for design and correlation of the physical features of highway that influence vehicle operation. th

2. 85 Percentile Speed: The speed below which 85% of all the vehicles travel is called 85th percentile speed and which is used for determining the speed limits for traffic regulation. 3. Modal Speed: It is the speed value occurring most frequently in a frequency distribution of speed obtained in a speed study. The modal speed would be the value with highest frequency of distribution. 4. Median Speed: It is the middle value of the set of speeds when they are arranged in an ascending order or descending order. 5. 98th Percentile Speed: It is the speed below which 98 percent of all the vehicles travel. It is used as design speed in geometric design. th

6. 15

Percentile Speed: It is the speed below which 15 percent of all the vehicles travel, is used to

determine the lower speed limit on major facilities such as Expressways. Vehicles travelling

below this speed cause interference with the traffic stream and may cause hazards.

METHODS FOR DETERMINING SPOT-SPEED The method available for measuring spot speeds can be grouped as under a) Those that require observation of the time taken by a vehicle to cover a known distance. b) Radar speedmeter which automatically records the instantaneous speed c) Photographic Method Methods under a) above can be further classified into: i) Those in which vehicles are timed over a long distance (long base) ii) Those in which vehicles are timed over a short distance (short base)

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The long-base methods commonly used are:  

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Direct Timing Procedure 

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Enoscope 

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Pressure contact tubes 

The short-base method uses vehicle detectors over a short distance 1. Direct timing procedure It involves measuring travel time over a measured distance. a) Stopwatch method using pavement marking b) Enoscope 2. Methods using pneumatic tubes a) Electrically operated meter b) Electronic meter 3. Photographic techniques a) Time-lapse photography b) Continuous photography 4. Methods based on Doppler principle a) Radar meter b) Ultrasonic meter Locations for Spot Speed Studies 

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The following locations are generally used for the different applications of spot speeds: 

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Locations that represent different traffic conditions on a highway or highways are used for basic data collection 

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Mid-blocks of urban highways and straight, level sections of rural highways are sites for speed trend analyses 

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Any location may be used for the solution of a specific traffic engineering problem. 

 

When spot speed studies are conducted, it is important that unbiased data be obtained. This requires that drivers be unaware that such a study is being conducted. Equipment used therefore, should be concealed from the driver, and observers conducting the study should be inconspicuous. Since the speeds recorded eventually will be subjected to statistical analysis, it is important that a statistically

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adequate number of vehicle speeds be recorded.

Time of Day and Duration of Spot Speed Studies The time of day for conducting a speed study depends on the purpose of study. In general, when the purpose of study is to establish posted speed limits, to observe speed trends, or to collect basic data, it is recommended that the study be conducted when traffic is free-flowing, usually during off-peak hours. However, when a speed study is conducted in response to citizen complaints, it is useful if the time period selected for the study reflects the nature of the complaints. The duration of the study should be such that the minimum number of vehicle speeds required for statistical analysis is recorded. Typically, the duration is at least 1 hour and the sample size is at least 30 vehicles. Sample Size for Spot Speed Studies The calculated mean speed is to represent the true mean value of all vehicle speeds at that location. The accuracy of this assumption depends on the number of vehicles in the sample. The larger the sample size, the greater the probability that the estimated mean is not significantly different from the true mean. It is therefore necessary to select a sample size that will give an estimated mean within acceptable error limits. For a spot speed study at a selected location, a sample size of at least 50 and preferably 100 vehicles is usually obtained (Ewing 1999) Histograms and Frequency Curves The information revealed by the frequency distribution table is best represented graphically by histograms and frequency curves. This histogram is plotted directly form the table and frequency curve is found by rounding off to histogram in such a way that the area under curve is equal to the area of the histogram. The curve is also useful in determining the pace of vehicles where the pace is the speed range, for some nominal increment of speed (usually 20kmph) which contains the most vehicles.

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CUMULATIVE FREQUENCY CURVE The cumulative frequency curve is used for determining the number of vehicles travelling above or below a given speed. Percentile speeds (i.e. that speed below which a specified percentage of vehicles are travelling) are also indicated

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4. DATA COLLECTION: Direct timing method was used to measure spot speeds.  

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Mark two reference points along the stretch based on the average speed. 

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Synchronize the stop watches. 

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The observer at beginning of the stretch waves his hand and thus gives signal to the second observer. 

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The second observer starts the stopwatch and stops it when the same vehicle crosses the observer.   Thus from the known distance and time, spot speeds are calculated.  Table 1: Recommended Base lengths for Spot speed Average speed of Traffic stream (in kmph) Base length (in m) 

Less than 40

27

40 to 65

54

Greater than 65

81

5. DATA ANALYSIS:

6. RESULTS: From graph

From Normal Distribution table

15th Percentile Speed 50th Percentile Speed 85th Percentile Speed 98th Percentile Speed

9. SPOT SPEED APPLICATIONS: 1. Geometric Design of Roads a) Design assumes uniform speeds and it is necessary to evaluate the effects of actual speed distribution on design features b) Super elevation and sight distances are directly related and vary appreciably with speed 2. Regulations and Control of Traffic Operations a) Establishing speed limits

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b) Determining the safe speed at curves and approaches to intersections c) Locating the positions of traffic signs and calculating lengths of NO PASSING ZONE. 3. Speed trends, 4. Used for analyzing the causes for accidents and identifying the relation between speed and accidents. 5. To compare diverse types of drivers and vehicles under specific conditions 6. Research studies

10. LIMITATIONS: Spot Speed is affected by environmental conditions (like weather, visibility), Enforcement, Traffic conditions, Driver, vehicle and motive of travel.

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