Applications of Global Positioning System in Traffic Studies. Yi Jiang 1

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1 Applications of Global Positioning System in Traffic Stuies Yi Jiang 1 Introuction A Global Positioning System (GPS) evice was use in this stuy to measure traffic characteristics at highway intersections an freeway work zones in Iniana, USA. GPS technologies have provie traffic engineers a new tool for obtaining accurate measurements relate to vehicle motions. The GPS evice use on a test vehicle is capable of recoring the test vehicle s positions (latituinal an longituinal coorinates) an spees at specifie time intervals. When the test vehicle travels at the normal spees of the majority of the vehicles in a traffic flow (perceive by the test vehicle river), the exact trajectory of the test vehicle is ientifie through satellite signals an recore by the GPS evice. Each ata point recore by the GPS evice inclues the vehicle position, spee, time, an the istance between the current an the last time points. Because the test vehicle travels at the representative spees of the traffic flow, the GPS ata epicts a spee profile of the traffic flow as well as the test vehicle on the traverse highway section. In aition, when GPS ata is use at a work zone, the ata can also provie values of traffic elay, vehicle queue, average travel spee, an vehicle acceleration an eceleration before an after the work zone. Data Collection With GPS A section of SR-26 (State Roa 26) in Lafayette, Iniana, USA, was selecte for ata collection with GPS to stuy traffic elay at intersections. The length of this roaway section is about three kilometers (from US-52 to I-65) an there are a series of ten intersections on the section. To examine the traffic patterns at the Iniana freeway work zones, seven work zones (three crossover work zones an four partial closure work zones) on Iniana s interstate highways were selecte for traffic ata collection. A GPS evice, connecte to a laptop computer, was place on a test vehicle (a normal size passenger car) to perform the ata collection. Two persons, a river an a GPS recorer, were neee to perform the ata collection. The river rove the test vehicle at his perceive spee of the traffic flow to obtain a spee profile pertinent to the traffic flow. The GPS recorer s task was to control the GPS program an to recor each lanmark point by pressing a specifie button on the computer keyboar. Figures 1 an 2 present some examples of such spee profiles. Figure 1 is a spee profile along the roaway section on SR-26 an Figure 2 is a spee profile at a work zone on I-74 (Interstate 74) between the Iniana/Illinois State Line an the Wabash River. As can be seen, the vehicle spee profiles capture the whole moving process of the traffic flow going through the roaway section with ten intersections as well as along the work zone on I-74. Figure 1 epicts the vehicle spee changes when 1 Ph.D., PE, Assistant Professor, Department of Builing Construction Management, Purue University, West Lafayette, Iniana 47907, USA. 1

2 passing the intersections. Figure 2 clearly shows that the traffic flow ecelerate when approaching the work zone, an travele at lower spees within the work zone, an accelerate after exiting the work zone. In aition to the spee profile, the roaway geometric layout is also reaily available from the GPS ata. Figure 3 epicts a plan view of the test vehicle s course of travelling through the I-74 work zone. The recore test vehicle s travelling route is nothing but the geometric layout of the work zone an the ajacent roaway. It shows that the test vehicle starte at the on-ramp to merge into the roaway, passe through the work zone, an then exite the roaway at the off-ramp. With this layout, the traffic engineers can easily locate the prominent features along the roaway, such as curves, ramps, an work zone limits. The locations of these features are often useful or even essential for analyzing traffic characteristics. Traffic Conitions at Intersections 1. Total Delay Cause by Intersections: The spee limit on this roa was 64 kilometers per hour (40 miles per hour). The GPS collecte ata inicates that the average spee was lower than the spee limit on most of the sections with a few exceptions (three out of nine sections) where it was higher than the spee limit. The values of average spees can be use to estimate traffic elays if the travel spee when vehicles are unaffecte by intersections are known. The unaffecte travel spee can be measure where the istance between two intersections are relatively far away. The total elay can be calculate using the following equation: D total S = T (1) V u where T = the GPS recore travel time to traverse the roaway section with intersections. S = the istance of the roaway section. V u = the unaffecte travel spee. With the GPS ata an observe unaffecte travel spee, the total elays of the whole roaway section of any section between two intersections can be obtaine through Equation 1. Table 1 presents the elay values on the SR-26 section, incluing elays on iniviual sections between any two consecutive intersections as well as on the whole section with 10 intersections. 2. Stoppe Delay at Intersections: In aition to total elays, stoppe elays are also important for measuring the efficiency of traffic control facilities. The uration of each vehicle stop is nothing but a stoppe elay. Therefore, the values of the stoppe elays can be rea irectly from the GPS recore ata. In this case, the vehicle stoppe in front of the sixth an the seventh intersections. The values of the stoppe elays are 41 secons for the sixth 2

3 intersection an 15 secons for the seventh intersection. These are the recore time urations corresponing to the spee values of zero. Traffic Conitions at Freeway Work Zones 1. Vehicle Spee at Work Zones an on Freeway Sections: As iscusse above an shown in Figure 2, at each secon in time there is a GPS recore value of vehicle spee an a value of istance. The average spee at a work zone can be obtaine by calculating arithmetic mean of all the spee values within the work zone. That is, Vi V = (2) N where, V = average spee at work zone V i = the ith spee value within work zone N = total number of spee values or total number of ata points within work zone On the other han, the average travel spee at a work zone can also be calculate with the values of istance an time, which is the value of work zone length ivie by travel time. Travel time is efine (McShane an Roess 1990) as the total time to traverse a given highway segment, incluing the time uring which the vehicle is stoppe an the time uring which the vehicle is in motion. With the GPS ata, the average travel spee at a work zone is compute with the following equation. U D 1 0 = = (3) T t1 t0 where, U = average travel spee at work zone D = istance traverse T = travel time 1 = istance at the en of work zone 0 = istance at the beginning of work zone t 1 = time at the en of work zone t 0 = time at the beginning of work zone Similarly, Equations 2 an 3 can also be use to compute the average spee (the arithmetic mean) an the average travel spee on the freeway sections without work zones. In general, Equations 2 an 3 are not equivalent. However, it can be shown that, with the GPS ata obtaine in this stuy, the two equations are equal because the GPS ata was recore at equal time intervals (one-secon intervals). 3

4 Although both Equation 2 an Equation 3 can be use to obtain the average travel spee, if a etaile spee stuy is neee, it is avantageous to use Equation 2 with the iniviual spee ata to calculate travel spee an other relate statistical values. This is because it can provie more information on spee characteristics, such as spee consistence or fluctuation in terms of stanar eviation, istribution, maximum an minimum values. 2. Vehicle Deceleration an Acceleration: In an earlier stuy (Jiang 1999), it was foun that traffic elays at a work zone inclue elays cause by eceleration of vehicles while approaching the work zone, reuce vehicle spee through the work zone, time neee for vehicles to resume freeway spee after exiting the work zone, an vehicle queues forme at the work zone. Vehicle eceleration an acceleration rates are neee to estimate the elays before the vehicle enters the work zone an after the vehicle exits the work zone. However, at the time of that stuy, the values of eceleration an acceleration coul not be precisely measure an etermine an the elays were calculate with assume eceleration an acceleration rates. Now, with the GPS vehicle position an spee ata, the vehicle eceleration an acceleration rates can be reaily an accurately calculate. If a vehicle starte to ecelerate at time t with a spee v an it arrive at the beginning of the work zone at time t b with a spee v b, then the eceleration rate of the vehicle was v v b = (4) tb t r Similarly, if a vehicle arrive at the en of the work zone at time t e with a spee v e an it accelerate to its freeway spee v a at time t a, then the acceleration rate was v v a e a = (5) ta te r Using Equations 4 an 5, the mean values of the eceleration an acceleration rates for ifferent work zone types are calculate an presente in Table Traffic Delay at Work Zones: The GPS ata contains the information of the istance between any two points on a roaway section an the actual time taken for the test vehicle to traverse the istance. In aition, as previously shown, the average travel spee on freeway sections can be calculate with the GPS ata using Equation 2 or Equation 3. To calculate the elay cause by a work zone, two points shoul be chosen along the roaway, one before the vehicle starte eceleration when approaching the work zone an the other after the vehicle resume freeway spee after exiting the work zone. That is, the two points shoul be chosen on the freeway sections (one before an one after the work zone) where the vehicle travels at the freeway spee. Because the work zone is within the roaway section between the two points, the elay cause by the work zone is the ifference between the actual travel time an the estimate travel time for 4

5 the vehicle to pass the two points at the freeway spee. If the average travel spee on a freeway sections is U fw, the GPS istance an time values for the two points are an t, an fw2 an t fw2, respectively, then the equation for the work zone elay is T elay fw2 = ( t fw2 t ) (6) U fw In this equation, ( t fw2 t ) is the actual travel time, ( fw2 ) is the istance 2 between the two points, an is the time neee to traverse the same fw U fw roaway section if there is no work zone. This elay value from this equation is the elay experience by one vehicle at the work zone. Such a value shoul be multiplie by the corresponing hourly traffic flow rate to obtain the total vehicle elay per hour at the work zone. 4. Vehicle Queue Length uner Traffic Congestion: Traffic congestion occurs when the traffic flow rate excees the work zone capacity. Once the flow rate of arrival vehicles exceee the work zone capacity, the number of vehicles arriving woul be larger than the number of eparting at the work zone. Then a vehicle queue woul form upstream of the work zone. The length of a vehicle queue can be estimate through the values of traffic flow rates an work zone capacity. Because of the complexity an ranomness of traffic conitions, the estimation of vehicle queue length is usually not as accurate as esire. Furthermore, the vehicle queue length can not be measure with the conventional traffic counters. However, with the GPS recore ata, a vehicle queue can be ientifie on the spee profile curve. Traffic congestion occurre at the work zone on I-65 at SR-25 uring the ata collection. The recore GPS ata inicate that the test vehicle stoppe (with a spee value of 0) at the istance point of meters. At that moment, the test vehicle was the latest vehicle joining the vehicle queue an therefore the location was the en of the vehicle queue in front of the work zone. From the marke points in the GPS ata, it can also fin that the beginning of the work zone was at the istance point of meters. Thus, the vehicle queue length is the istance between the en of the vehicle queue an the beginning of the work zone. That is, the vehicle queue length was = meters at the moment when the test vehicle joine the queue. If the time when the vehicle joine the queue is of interest, it can also be foun from the GPS recore ata. In this example, the test vehicle joine the queue at 10:03:57 a.m., June 28, Through this example, it is clearly shown that the GPS position an spee ata can provie very precise an etaile information on vehicle queues at work zones. Conclusions The ynamic feature of the GPS collecte ata enables the traffic engineers to obtain the precise profile of vehicle spee along roaway sections with intersections an 5

6 work zones. The geometric layout can also be rawn using the GPS positioning ata. With the GPS ata, several essential traffic characteristics of intersections can be obtaine, such as the average travel spee, total travel elays an stoppe elays cause by intersections. The average travel spee on a given roaway section with work zones can be calculate through Equation 2 or Equation 3. The two equations woul yiel the same results because the GPS position an spee values are recore at equal time intervals. It is also presente that the vehicle eceleration an acceleration rates, which coul only be roughly estimate in an earlier stuy, can be reaily calculate with high accuracy using the GPS ata. Moreover, traffic elay at a work zone can be obtaine by comparing the GPS recore actual travel time through a roaway section with the work zone an the travel time neee to traverse the same section at the freeway spee. In aition, uner traffic congestion conition at a work zone, the GPS ata provies the precise values of time, position an spee of the test vehicle s movement to ientify the exact queue length in front of the work zone. References Feeral Highway Aministration (FHWA) (1989). Construction costs an safety impacts of work zone traffic control strategies, volume II: informational guie. Report FHWA-RD FHWA, U.S. Department of Transportation. Jiang, Y. (1999). Traffic characteristics an estimation of traffic elays an user costs at Iniana freeway work zones. Final Report FHWA/INDOT/SPR Iniana Department of Transportation. McShane, W. R., an Roess, R. P. (1990). Traffic engineering. Prentice Hall, Englewoo Cliffs, New Jersey. 6

7 Section Table 1. Travel Delays at Intersections Delay between Intersections (secons) Average Delay (secons/km) From intersection 1 to From intersection 2 to From intersection 3 to From intersection 4 to From intersection 5 to From intersection 6 to From intersection 7 to From intersection 8 to From intersection 9 to Total Delay = secons Average Total Delay = secons per kilometer Table 2. Mean Values of Vehicle Deceleration an Acceleration Rate Work Zone Type Merging Type Mean Deceleration Rate (km/h/s) Mean Acceleration Rate (km/h/s) Crossover Merging (Meian Crossover Direction) Non-Merging Crossover Merging (Opposite Direction) Non-Merging Merging Partial Closure Non-Merging

8 80 70 Travel Spee (Km/hr) In ters ectio n Travel Spee Distance (m) Figure 1. Vehicle spee profile along roaway with intersection 120 W ork Zone 100 En of W ork Zone Beginning of Taper En of Taper 80 Spee (km/h) Distance (m eters) Figure 2. Vehicle spee profile at a work zone on I-74 Diverging of Off-ramp En of Taper M erging of O n-ram p En of W ork Zone Beginning of Taper I-74 E ast B oun N Figure 3. The roaway layout create with GPS ata 8