Chapter 32 Traffic data coection 32.1 Overview Unike many other discipines of the engineering, the situations that are interesting to a traffic engineer cannot be reproduced in a aboratory. Even if road and vehices coud be set up in arge aboratories, it is impossibe to simuate the behavior of drivers in the aboratory. Therefore, traffic stream characteristics need to be coected ony from the fied. There are severa methods of data coection depending on the need of the study and some important ones are described in this chapter. 32.2 Data reuirements The most important traffic characteristics to be coected from the fied incudes sped, trave time, fow and density. Some cases, spacing and headway are directy measured. In addition, the occupancy, ie percentage of time a point on the road is occupied by vehices is aso of interest. The measurement procedures can be cassified based on the geographica extent of the survey into five categories: (a) measurement at point on the road, (b) measurement over a short section of the road (ess than 500 metres) (c) measurement over a ength of the road (more than about 500 metres) (d) wide area sampes obtained from number of ocations, and (e) the use of an observer moving in the traffic stream. In each category, numerous data coection are there. However, important and basic methods wi be discussed. 32.2.1 Measurements at a point The most important point measurement is the vehice voume count. Data can be coected manuay or automaticay. In manua method, the observer wi stand at the point of interest and count the vehices with the hep of hand taies. Normay, data wi be coected for short interva of 5 minutes or 15 minutes etc. and for each types of vehices ike cars, two wheeers, three wheeers, LCV, HCV, muti axe trucks, nonmotorised traffic ike buock cart, hand cart etc. From the fow data, fow and headway can be derived. Modern methods incude the use of inductive oop detector, video camera, and many other technoogies. These methods heps to coect accurate information for ong duration. In video cameras, data is coected from the fied and is then anayzed in the ab for obtaining resuts. Radars and microwave detectors are used to obtain the speed of a vehice at a point. Since no ength is invoved, density cannot be obtained by measuring at a point. Introduction to Transportation Engineering 32.1 Tom V. Mathew and K V Krishna Rao
observer x enoscope Base ength Figure 32:1: Iustration of measurement over short section using enoscope 32.2.2 Measurements over short section The main objective of this study is to find the spot speed of vehices. Manua methods incude the use of enoscope. In this method a base ength of about 30-90 metres is marked on the road. Enoscope is paced at one end and observer wi stand at the other end. He coud see the vehice passing the farther end through enoscope and starts the stop watch. Then he stops the stop watch when the vehice passes in front of him. The working of the enoscope is shown in figure 32:1. An aternative method is to use pressure contact tube which gives a pressure signa when vehice moves at either end. Another mosidey used method is inductive oop detector which works on the principe of magnetic inductance. Road wi be cut and a sma magnetic oop is paced. When the metaic content in the vehice passes over it, a signa wi be generated and the count of the vehice can be found automaticay. The advantage of this detector is that the counts can be obtained throughout the ife time of the road. However, chances of errors are possibe because noise signas may be generated due to heavy vehice passing adjacent anes. When dua oops are used and if the spacing between them is known then speed aso can be cacuated in addition to the vehice cost. 32.2.3 Measurements over ong section This is normay used to obtain variations in speed over a stretch of road. Usuay the stretch wi be having a ength more than 500 metres. We can aso get density. Most traditiona method uses aeria photography. From a singe frame, density can be measured, but not speed or voumes. In time apse photography, severa frames are avaiabe. If severa frames are obtained over short time intervas, speeds can be measured from the distance covered between the two frames and time interva between them. 32.2.4 Moving observer method for stream measurement Determination of any of the two parameters of the traffic fow wi provide the third one by the euation = u.k. Moving observer method is the most commony used method to get the reationship between the fundamenta stream characteristics. In this method, the observer moves in the traffic stream unike a other previous methods. Consider a stream of vehices moving in the north bound direction. Two different cases of motion can be considered. The first case considers the traffic stream to be moving and the observer to be stationary. If n o is Introduction to Transportation Engineering 32.2 Tom V. Mathew and K V Krishna Rao
Figure 32:2: Iustration of moving observer method the number of vehices overtaking the observer during a period, t, then fow is n0 t, or n 0 = t (32.1) The second case assumes that the stream is stationary and the observer moves with speed v o. If n p is the number of vehices overtaken by observer over a ength, then by definition, density k is np, or or n p = k (32.2) n p = k.v o.t (32.3) where v 0 is the speed of the observer and t is the time taken for the observer to cover the road stretch. Now consider the case when the observer is moving within the stream. In that case m o vehices wi overtake the observer and m p vehices wi be overtaken by the observer in the test vehice. Let the difference m is given by m 0 - m p, then from euation 32.5 and euation 32.7, m =.t k.v o.t (32.4) This euation is the basic euation of moving observer method, which reates, k to the counts m, t and v o that can be obtained from the test. However, we have two unknowns, and k, but ony one euation. For generating another euation, the test vehice is run twice once with the traffic stream and another one against traffic stream, i.e. m w =. + k.v w Introduction to Transportation Engineering 32.3 Tom V. Mathew and K V Krishna Rao
=. + k. m a = t a k.v a.t a =.t a k. where, a, w denotes against and with traffic fow. It may be noted that the sign of euation 32.5 is negative, because test vehice moving in the opposite direction can be considered as a case when the test vehice is moving in the stream with negative veocity. Further, in this case, a the vehices wi be overtaking, since it is moving with negative speed. In other words, when the test vehice moves in the opposite direction, the observer simpy counts the number of vehices in the opposite direction. Adding euation 32.5 and 32.5, we wi get the first parameter of the stream, namey the fow() as: Now cacuating space mean speed from euation 32.5, = m w + m a + t a (32.5) m w = k.v w.t = v v w [ ] = v = (1 v 1 ) = (1 t avg ) If v s is the mean stream speed, then average trave time is given by t avg = v s. Therefore, m w = (1 t avg ) = t avg t avg = m w = v, Rewriting the above euation, we get the second parameter of the traffic fow, namey the mean speed v s and can be written as, v s = (32.6) mw Thus two parameters of the stream can be determined. Knowing the two parameters the third parameter of traffic fow density (k) can be found out as k = (32.7) v s For increase accuracy and reiabiity, the test is performed a number of times and the average resuts are to be taken. Exampe 1 The ength of a road stretch used for conducting the moving observer test is 0.5 km and the speed with which the test vehice moved is 20 km/hr. Given that the number of vehices encountered in the stream whie the test Introduction to Transportation Engineering 32.4 Tom V. Mathew and K V Krishna Rao
Sampe no. m a m o m p m(m o m p ) t a = ma+mw t a+ u = k = ma v 1 107 10 74-64 0.025 0.025 860 5.03 171 2 113 25 41-16 0.025 0.025 1940 15.04 129 3 30 15 5 10 0.025 0.025 800 40 20 4 79 18 9 9 0.025 0.025 1760 25.14 70 vehice was moving against the traffic stream is 107, number of vehices that had overtaken the test vehice is 10, and the number of vehices overtaken by the test vehice is 74, find the fow, density and average speed of the stream. Soution Time taken by the test vehice to reach the other end of the stream whie it is moving aong with the traffic is = 0.5 20 = 0.025 hr Time taken by the observer to reach the other end of the stream whie it is moving against the traffic is t a = = 0.025 hr Fow is given by euation, = 107+(10 74) 0.025+0.025 = 860 veh/hr Stream 0.5 speed v s can be found out from euationv s = = 5 km/hr Density can be found out from euation as 0.025 10.74 860 k = 860 5 = 172veh/km Exampe 2 The data from four moving observer test methods are shown in the tabe. Coumn 1 gives the sampe number, coumn 2 gives the number of vehices moving against the stream, coumn 3 gives the number of vehices that had overtaken the test vehice, and ast coumn gives the number of vehices overtaken by the test vehice. Find the three fundamenta stream parameters for each set of data. Aso pot the fundamenta diagrams of traffic fow. Sampe no. 1 2 3 1 107 10 74 2 113 25 41 3 30 15 5 4 79 18 9 Soution From the cacuated vaues of fow, density and speed, the three fundamenta diagrams can be potted as shown in figure 32:3. 32.3 Summary Traffic engineering studies differ from other studies in the fact that they reuire extensive data from the fied which cannot be exacty created in any aboratory. Speed data are coected from measurements at a point or over a short section or over an area. Traffic fow data are coected at a point. Moving observer method is one in which both speed and traffic fow data are obtained by a singe experiment. Introduction to Transportation Engineering 32.5 Tom V. Mathew and K V Krishna Rao
speed u 40 25.14 15.04 5.03 density k speed u 800 1760 1940 860 fow fow 20 70 129171 density k Figure 32:3: Fundamenta diagrams of traffic fow 32.4 Probems 1. In the moving observer experiment, if the density is k, speed of the observer is v o, ength of the test stretch is, t is the time taken by the observer to cover the road stretch, the number of vehices overtaken by the observer n p is given by, (a) n p = k.t (b) n p = k. (c) n p = k v o.t (d) n p = k.v o.t 2. If the ength of the road stretch taken for conducting moving observer experiment is 0.4 km, time taken by the observer to move with the traffic is 5 seconds, number of vehices moving with the test vehice in the same direction is 10, fow is 10 veh/sec, find the mean speed. (a) 50 m/s (b) 100 m/s (c) 150 m/s (d) 200 m/s 32.5 Soutions 1. In the moving observer experiment, if the density is k, speed of the observer is v o, ength of the test stretch is, t is the time taken by the observer to cover the road stretch, the number of vehices overtaken by the observer n p is given by, (a) n p = k.t Introduction to Transportation Engineering 32.6 Tom V. Mathew and K V Krishna Rao
(b) n p = k. k v o.t (c) n p = (d) n p = k.v o.t 2. If the ength of the road stretch taken for conducting moving observer experiment is 0.4 km, time taken by the observer to move with the traffic is 5 seconds, number of vehices moving with the test vehice in the same direction is 10, fow is 10 veh/sec, find the mean speed. (a) 50 m/s (b) 100 m/s (c) 150 m/s (d) 200 m/s Soution: Given that =0.4 km, =5seconds, m w =10,=10 veh/sec, substituting in euation,v s = =100m/s. mw Introduction to Transportation Engineering 32.7 Tom V. Mathew and K V Krishna Rao