2.2_Traffic Studies_Speed Studies_045446.pptx

Traffic engineering project

1. Department of Civil Engineering, NIT Srinagar A presentation on Presented By: Dr. Jitendra Gurjar Assistant Professor 1

2. Speed is the ratio of the total distance (d) travelled by a vehicle to the total time (t) taken by the same vehicle. Speed, V= (d/t) m/sec. The speed survey is done to determine the speed that drivers select, unaffected by the existence of congestion. This information is used to determine general speed trends, to help determine reasonable speed limits, and assess safety. The speed of travel on the road is also used in classifying routes. The level of service based on speed is an indicator of the quality of traffic flow or mobility. 2

3. The speed of different vehicles varies with respect to time and space. To represent, these variations several types of speed can be defined. These are Spot speed Average speed Time Mean Speed (Vt) Space Mean Speed (Vs) Running speed Journey speed 3

4. Spot speed is referred to as the instantaneous speed of a vehicle at a point or cross-section. 4 Spot speed is needed to design Horizontal and vertical curve Location and size of sign Accident analysis

5. Spot speed is measured using an Pavement Marking Method Enoscope or Mirror Box Method, Radar Method Photographic Method Electronic Method 5

6. Pavement Marking Method The pavement marking method can be used to successfully complete a spot speed study using a small sample size taken over a relatively short period of time. It is a quick and inexpensive method for collecting speed data but a relatively inaccurate method. To calculate vehicle speed, we use the predetermined study length known as trap length and the elapsed time it takes for the vehicle to move through the test section in the following formula: V= (d/t) m/sec. 6

7. 7 Traffic Stream Average Speed (mile/hr) Trap length (feet) Below 25 88-100 25-40 176-200 Over 40 264-300 Pavement Marking Method The pavement marking method can be used to successfully complete a spot speed study using a small sample size taken over a relatively short period of time. Marked the recommended study lengths on the pavement based on the average speed of the traffic stream.

8. 8 Pavement Marking Method As the front of the vehicle crosses the starting section of the study length, the starting section observer sends a signal to the end section observer who starts the stopwatch. The end section observer stops the stopwatch when the vehicle reaches the end station.

9. 9 Pavement Marking Method Therefore, the time elapsed between the two sections is obtained. Repeat the procedure for recording the time of vehicles moving in the opposite direction. Calculate vehicle speeds by using the predetermined study length and the elapsed time

10. 10

11. Enscope or Mirror Box Method Enoscope, also known as the Mirror Box, is an L-shaped box, open at both ends, with a mirror set at a 45° angle to the arms of the instrument. The instrument bends the line of sight of the observer so that it is perpendicular to the path of the vehicle. The method can be used with one Enoscope or with two Enoscopes. 11

12. 12 Traffic Stream Average Speed (mile/hr) Trap length (feet) Below 25 88-100 25-40 176-200 Over 40 264-300 Enscope or Mirror Box Method Marked the recommended study lengths on the pavement based on the average speed of the traffic stream. Enoscope is placed directly opposite to the first reference point and the observer stands opposite to another reference point such that the observer and Enoscope are in the same line of sight.

13. 13 Enscope or Mirror Box Method Start the stopwatch when the image of the passing vehicle is seen through the Enoscope by the observer and stop the stopwatch as soon as it passes the observer. Therefore, the time elapsed is obtained. Repeat the procedure for recording the time of vehicles moving in the opposite direction.

14. 14 Enscope or Mirror Box Method If two Enoscopes are used then the observer stands mid-way between two reference points and notes the time taken. Calculate vehicle speeds by using the predetermined study length and the elapsed time.

15. Speed Radar Gun Method The speed gun or radar meters are used for the measurement of spot speeds much more rapidly as compared to previously discussed methods. It measures speed directly by measuring the difference in the frequency between the emitted and reflected radar wave emitted on an oncoming vehicle. 15

16. Speed Radar Gun Method The principle of the radar meter is that when it is targeted at a vehicle moving with some speed, the difference between the frequencies of the emitted and reflected beams is used to calculate the speed. 16

17. Speed Radar Gun Method This is normally referred to as the Doppler Effect which states that the difference in frequency is proportional to the speed of the oncoming vehicle. Using this principle, the instrument is programmed to calculate the speed of the vehicle. 17

18. Advantages The radar speed meter is pre-calibrated to display/record the vehicle speed in kmph. The radar speed meters are capable of measuring speeds of vehicles from about 5 kmph to over 300 kmph, as may be required for the measurement of speeds of cars during the car- race. 18

19. Advantages The present-day radar speed meters have provision for taking photographs of the vehicles along with the display of the speed of the vehicles at that instant. Simple handheld radars of small size are also available. Disadvantage One of the disadvantages of this type of equipment is that when more than one vehicle is trapped, the speed of the closest vehicle only will be recorded. 19

20. Average Speed It is the average spot speed of all vehicles passing a given point on the road. There are two types of average speeds Time-mean speed Space-mean speed 20

21. Time Mean Speed Time mean speed is the average of all vehicles passing a point over a duration of time. It is the arithmetic mean of the speed of the vehicles passing a point on a highway during an interval of time. It represents the speed distribution of vehicles at a point on the roadway. 21

22. Time Mean Speed Time mean speed Vt is given by 22 Where n= number of vehicles observed d= length of section of highway Vi =Observed instantaneous speed of ith vehicles in kmph qi is the number of vehicles in the ith speed range v i mean speed of vehicles in the ith speed range

23. Space Mean Speed The space mean speed also averages the spot speed, but spatial weightage is given instead of temporal. Space mean speed is the harmonic mean of the speed of the vehicles passing a point at the highway during an interval of time. This speed is used for flow density relationships. 23

24. Space Mean Speed Consider the unit length of a road, and let vi is the spot speed of ith vehicle. Let ti is the time the vehicle takes to complete the unit distance and is given by If there are n such vehicles, then the average travel time ts is given by, 24

25. Average Speed: Relationship between Time-mean speed and space-mean Speed where,  v t  = Time-mean speed v s  = Space-mean speed σ s  = Standard deviation in space mean speed σ t  = Standard deviation in time mean speed v i  = speed of ith vehicle q i  = frequency of ith vehicle 25

26. Average Speed The space-mean speed is slightly lower than the time-mean speed under typical speed conditions on rural highways. 26

27. Q.1 Problem - Twenty five spot speed observation were taken and were as under: 50, 40, 60, 54, 45, 31, 72, 58, 43, 52, 46, 56, 43, 65, 33, 69, 34, 51, 47, 41, 62, 43, 55, 40, Calculate : (i) Time – mean speed (ii) space-mean speed and 27

28. Q.2 Speed observations from a radar speed meter have been taken, giving the speeds of the subsidiary streams composing the flow along with the volume of traffic of each subsidiary stream. The readings are as under: Calculate (i) Time-mean speed (ii) Space-mean speed (iii) Variance and standard deviation about space-mean speed (iv) Variance and standard deviation about time-mean speed 28 Speed (km/hr) Volume of subsidiary stream (Vehicle hour) 2-5 1  6-9  4  10-13  0  14-17  7  18-21  20  22-25  44  26-29  80  30-33  82  34-37  79  38-41  49  42-45  36  46-49  26  50-53  9  54-57  10 58-61  3

29. Journey speed It is the distance divided by the total journey time. Total journey time includes all delays due to traffic. Journey speed = Distance/Total journey time 29

30. Running speed It is the ratio of the distance divided by running time. Running time is the time that the vehicle is actually in motion i.e. total journey time minus delays. Running speed = Distance/(Total journey time – delays) Both speeds are usually a result of travel time and delay study – used to evaluate road performance. 30

31. Q.3 Evaluate journey & running speeds for the following situation: Average Total travel time including delay due to traffic = 32 minutes. Average Delays at each junction: J1 = 5 min, J2 = 5 min, J3 = 3 min, J4 = 4 min, and J5 = 6 min . 31

32. Presentation of Spot Speed Data From the spot speed data of the selected samples, a frequency distribution table of spot speed data is prepared by arranging the speed groups covering desired speed ranges (such as 10 - 20 kmph, 20-30 kmph, 30-40 kmph, etc. ) and the number of vehicles in each speed range. The arithmetic mean of the measured speeds is taken as the average spot speed of all the vehicles in the stream. 32

33. Presentation of Spot Speed Data It is possible to determine either the average spot speed of any desired vehicle class (such as car, HCV, etc.) or the combined weighted average speed of all the vehicles in the traffic stream. The table gives the general information of the speeds maintained in the section and also regarding the speed distribution pattern. 33

34. Frequency Distribution Diagram A graph is plotted between the average values of each speed group of vehicles on the X-axis and the percentage of vehicles in that group on the Y-axis. This graph is called the frequency distribution diagram of spot speeds. 34

35. Frequency Distribution Diagram: Model Speed This diagram will have a definite peak value of travel speed across the section and this speed is denoted as 'modal speed'. It may be inferred that the maximum proportion of vehicles in the stream prefers to travel at about this speed on this particular location of the road. A vertical line is dropped from the peak of the curve, with the result found on the horizontal axis. 35

36. Frequency Distribution Diagram: Model Speed 36

37. Frequency Distribution Diagram: Median Speed The median speed is defined as the speed that divides the distribution into equal parts (i.e., there are as many observations of speeds higher than the median as there are lower than the median). It is a positional value and is not affected by the absolute value of extreme observations. 37

38. Frequency Distribution Diagram: Median Speed By definition, the median equally divides the distribution. Therefore, 50% of all observed speeds should be less than the median. In the cumulative frequency curve, the 50th percentile speed is the median of the speed distribution. Median Speed = v50. 38

39. Frequency Distribution Diagram: Pace The pace is a traffic engineering measure not commonly used for other statistical analyses. It is defined as the 10 km/h increment in speed in which the highest percentage of drivers is observed. It is also found graphically using the frequency distribution curve. The pace is found as follows: A 10 kmph template is scaled from the horizontal axis. 39

40. Frequency Distribution Diagram: Pace Keeping this template horizontal, place an end on the lower left side of the curve and move slowly along the curve. When the right side of the template intersects the right side of the curve, the pace has been located. This procedure identifies the 10 Km/h increments that intersect the peak of the curve; this contains the most area and, therefore, the highest percentage of vehicles. 40

41. Frequency Distribution Diagram: Pace 41

42. 'Cumulative Speed Distribution Diagram A graph is plotted with the average values of each speed group on the X-axis and the cumulative percent of vehicles at or below the different speeds on the Y-axis, as shown in the figure. This graph is called the 'cumulative speed distribution diagram’ 42

43. Frequency Distribution Diagram of Spot Speeds 43

44. Speed Dispersion The ‘15th percentile speed ’ means the speed at or below which 15 percent of the vehicles are passing the point on the highway. On some arterial roads where slow-moving vehicles are to be prohibited,15 th percentile speed may be determined from the cumulative speed distribution diagram. The 15th percentile speed is considered to represent the lower speed limit, to prohibit slow-moving' vehicles to decrease delay and congestion. 44

45. Speed Dispersion The ‘50th percentile speed ’ means the speed at or below which 50 percent of the vehicles are passing the point on the highway. 50 th percentile speed may be determined from the cumulative speed distribution diagram. The 50th percentile speed is considered to represent the mean speed. 45

46. 'Cumulative Speed Distribution Diagram of Spot Speeds The '85th percentile speed ’ means the speed at or below which 85 percent of the vehicles are passing the point on the highway. In other words, only 15 percent of the vehicles exceed this speed at that spot. The drivers exceeding 85th percentile speed are usually considered to drive faster than the safe speed under existing conditions. Hence this speed is adopted for the 'safe speed limit' at this zone. 46

47. Cumulative Speed Distribution Diagram Therefore if a 'speed limit' regulation sign is to be installed at an accident-prone location. Spot speed studies are carried out during different periods of the day, A cumulative speed distribution diagram is plotted and an “85” percentile speed is adopted as the upper-speed limit. 47

48. Cumulative Speed Distribution Diagram Speed Dispersion The ‘98th percentile speed ’ means the speed at or below which 98 percent of the vehicles are passing the point on the highway. For the purpose of checking the requirements of highway geometric design elements, the 98th percentile speed is generally taken as the 'design speed' of the existing roadway facility. 48

49. Speed Dispersion All vehicles do not travel at the same speed at a location on a road. If there is 'free flow of vehicles, the speed distribution generally follows a normal distribution curve. The amount of spread of speeds from the average speed affects both capacity and safety. 49

50. Speed Dispersion This spread in speeds is represented by 'speed dispersion'. The quality of the flow of vehicles in a traffic stream may be represented in terms, of speed dispersion. The speed dispersion may be judged by several methods such as. (a) 85th minus 15th percentile of spot speeds, (b) standard deviation of spot speeds, or (c) the coefficient of variation of spot speeds. 50

51. Q.4The table below gives the consolidated data of spot speed studies on a section of a road. Determine the most preferred speed at which the maximum proportion of vehicles. 51 Speed Range (km/hr) No. of Speed Observations 0-10 10-20 11 20-30 30 30-40 105 40-50 233 50-60 216 60-70 68 70-80 24 80-90

52. Q.5 Spot speed studies were carried out at a certain stretch of a highway with mixed traffic flow and the consolidated data collected are given below. Determine: (i) The upper and lower values or speed limits for installing speed regulation signs at this road stretch, (ii) The design speed for checking the geometric design elements of the highway, and (iii) Speed dispersion 52 Speed Range (km/hr) No. of Speed Observations 0-10 12 10-20 18 20-30 68 30-40 89 40-50 204 50-60 255 60-70 119 70-80 43 80-90 33 90-100 9

53. CA O’Flaherty, Transport Planning and Traffic Engineering, John Wiley & Sons, Inc., New York; Toronto, 2002. McShane & Roess, Traffic Engineering, Prentice‐Hall of India Private Ltd, New Delhi‐ 110001, 1990. Kadiyali & Lal, Principles and Practices of Highway Engineering, Khanna Publishers, Delhi‐ 6, 1996. Chakarborty & Das, Principles of Transportation Engineering, Prentice‐Hall of India Private Ltd, New Delhi‐110001. L. R. Kadiyali, Traffic Engineering and Transport Planning, Khanna Publishers, 2‐B, Nai Sarak, Delhi‐ 110006, 1999 . 53