EVALUATING THE SAFETY OF PLATOONED HEAVY VEHICLES: A CASE STUDY

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1 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. EVALUATING THE SAFETY OF PLATOONED HEAVY VEHICLES: A CASE STUDY Marco Guerrieri 1, and Raffaele Mauro 1 1 Department of Ciil, Enironmental and Mechanical Engineering, Uniersity of Trento, Italy Euro-Mediterranean Institute of Science and Technology (IEMEST), Palermo, Italy marco.guerrieri@tin.it ABSTRACT In the right lane of motorways with trucks oertaking prohibition is ery common the formation of platoons of heay ehicles. Although this traffic controls strategies can increase the capacity of passing lane, in the right lane may occur increases of rear-end collisions. The purpose of this research was to ealuate the safety of the platooned heay ehicles by means a closed-form stream model. The case study of the Italian motorway A was examined. The sampling of platoons was performed in four obseration sections.in the research hae been inestigated platoons with 0 heay ehicles for each platoon. Many traffic parameters hae been ealuated: frequencies of the number of platoons, minimum mean and maximum headways and speeds between heay ehicles of each platoon, etc. We hae found that the percentages of platoons whose ehicles trael with an aerage headway of less than 3 seconds are in the range 37.1% 66.9%, depending on the motorway section. In addition, it was performed a comparatie analysis between the minimum safety spaces s and the mean intra-ehicular distances s a of platooned heay ehicles. The results show that the percentages of platoons whose ehicles trael, on aerage, in safety conditions ( s = s-s a >0) are in the range 3% 43%, depending on the motorway section. Keywords: motorway, platoons, heay ehicles, safety. INTRODUCTION The traffic processes in which heay-duty ehicles trael with small headways gie rise to the reduction of fuel consumption through the decreasing of aerodynamic drag (except for the leader ehicles). Bonnet and Fritz [1] hae shown that at a spacing of 10 m and at a elocity of 80 km/h, the fuel consumption reduction is about 1% of the fuel consumption of the trail truck in isolation. This reduction, determines meaningful enironmental and economic adantage. Furthermore, under specific traffic condition, traffic processes with closely-spaced ehicles can gie benefits in terms of increase of road capacity. This occurs, for example, in the Automated Highway Systems (AHS), in which, instrumented ehicles with ITS technologies flow out in platoons on pre-selected lanes of highways. The utilization of this kind of ehicles proides quicker and more accurate responses than human driers. In addition, driers capability to identify ariations in ehicle gaps, trajectories, accelerations, etc. limits speed and precision of reaction. The lane s capacity of a traditional highway with human driers generally is below.00 per hour []. Instead, the lane s capacity of the AHS is higher, as inferable by following equation [3]: N C 3600 (1) [(L N) i (N 1) I)] where: C is the lane capacity [eh/h], is the speed [m/s], N is the number of ehicles in one platoon, L is the ehicles length [m], i is the spacing between ehicles in a platoon (intra-platoon spacing, generally 1 m) [m], I is the spacing between platoons (inter-platoon spacing, generally m) [m]. The alues calculated with eq. (1) are in accordance to the capacity-speed cures obtained by Shladoer [4] in which the maximum capacity for a lane it is reached in the case of 0-ehicle platoons and speed around 30 m/s [1]: C = eh/h. Platoons analyses turned out to be important in the study of car accidents and road safety [5]. The formation of platoons of heay ehicles is ery common in the right lane of motorways with truck lane changing restrictions or trucks oertaking prohibition. This traffic control strategies can increase the capacity of passing lanes but, at the same time, can increase the collisions in the right lanes. The paper addresses the safety analysis of the platooned heay ehicles using a closed-form stream model, based on empirical data of the Italian motorwaya. Many traffic sureys were done with the aim to estimate frequency of platoons, number of heay ehicles for each platoons, minimum, mean, maximum alues of intra-ehicular headways, speeds, theoretical safety spacing between ehicles, operating spacing. The percentages of platoons that trael in safety conditions hae been calculated by means the comparison of the aforementioned spacing. THE A MOTORWAY The A motorway, Autostrada del Brennero, linking the Po Valley and the A1 Freeway with Austria and Germany. The A is one of the principal axes of the Italian highway network and belongs to the TEN-T Network (Trans-European Road Network, corridor Helsinki La Valletta) [6]. 1561

2 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. The A is a typical diided four-lane motorway, with two hard shoulder; its oerall length is about 313 km. Figure-1shows the oertaking prohibitions along the A. Figure-1. A motorway layoutand oertaking prohibitions. The Annual aerage daily traffic (AADT) is between and ehicles per day, as shown in Table-1. N. Initial (Km) Table-1. AADTs alues. Final (Km) Name Brennero - Vipiteno Vipiteno - Bressanone Bressanone- Bressanone Bressanone - Chiusa Chiusa Bolzano nord Bolz. Nord- Bolz. sud Bolzano sud - Egna Ora Egna S.Michele S. Michele - Trento nord Trento nord - Trento centro Trento centro - Trento sud Trento sud - Roereto nord Ro.to nord - Ro.to sud Ro.to sud Ala Aio Ala Aio Affi Affi Verona nord Verona nord - int. A4 int. A4 - Nogarole Nogarole - Mantoa nord Mant. nord Mant. sud Mant. sud - Pegognaga Pegognaga - Reggiolo Rolo Reggiolo Rolo - Carpi Carpi - Campogalliano Campogallianoint. A1 AADT (eh/day)

3 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. CAPACITY ESTIMATION AND DISTRIBUTIONS OF HEAVY VEHICLES BETWEEN LANES The capacity analysis was done for two obseration periods: 5 11 May 014 and 8 14 December 014. The obseration sections are gien in Table-. The macroscopic flow parameters (i.e. flow q, speed and density k ) hae been deducted for interals of 5 minutes and 15 minutes. Traffic flows were estimated in terms of passenger car unit (pcu). In all, has been carried out N5 = 4.19 couples (; k), (q; k), (; q). Obseration section Table-. Obseration sections. Location Horizontal alignment Kofler Tangent S. Michele Tangent Portale Affi Cure: R = 1000 m Mantoa Sud Tangent Vertical alignment; slople (%) Cure: R = m; slope =0,41 % Tangent, slope =0,03 % Cure: R = m; slope = 0,3 % Cure: R = m; slope = 0,00 % For each section, the flow diagrams are obtained by means of May model [7, 8, 9, 10] in which the relationship between speed () and density (k) is gien by the following equation: 1 k ( ) kjam f e () A typical fundamental diagram for the right lanesis gien in Figure- [11]. Table-3. Traffic flow parameters (section San Michele). Lanecarriageway f San Michele (km ) - northbound roadway kjam (pcu/lane/km) C (pcu/h) jam right lane passing lane carriageway Table-4. Traffic flow parameters (section San Michele). Lane - carriageway f San Michele (km ) - southbound roadway kjam (pcu/lane/km) C (pcu/h) jam right lane passing lane carriageway Table-5. Traffic flow parameters (section Adige). Figure-. Speed - flow scatter plot for the right lane, section San Michele, northbound roadway. The Tables 3-4proide the traffic flow parameters (free-flow speed f, capacity C, jam density k jam ) obtained for the section San Michele. For comparison, Tables 5, 6 show the same types of information for the section Adige, but deducted for traffic of the year 003. Lane - carriageway f Adige (km ) - northbound roadway kjam (pcu/lane/km) C (pcu/h) jam right lane passing lane carriageway

4 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. Table-6. Traffic flow parameters (Section Adige). Lane - carriageway f Adige (km ) - southbound roadway kjam (pcu/lane/km) C (pcu/h) jam right lane passing lane carriageway The Figures 3-6 show the frequency distribution histograms of the percentage of heay ehicles in the traffic stream on the right lane and on the oerpassing lane, respectiely for weekdays and weekends (section San Michele). It is immediate to erify that the oertaking prohibitions are generally respected by heay ehicles oer 7,5 t, thoughthis leads to the formation of many platoons. Figure-5. Frequency distribution of the percentage of heay ehicles. Section San Michele, northbound roadway, right lane, weekends. Figure-3. Frequency distribution of the percentage of heay ehicles. Section San Michele, northbound roadway, right lane, weekdays. Figure-6. Frequency distribution of the percentage of heay ehicles. Section San Michele, northbound roadway, passing lane, weekends. ANALYSIS OF PLATOONS A heay-ehicles platoon is a group of heay ehicles that trael in close proximity to one another (cfr. Figure-7) [1, 13]. The leader ehicle is followed by a number of other ehicles that closely match their speed. Figure-7. Example of two-ehicle platoon where 1 is the lead ehicle and is the follower ehicle [13]. Figure-4. Frequency distribution of the percentage of heay ehicles. Section San Michele, northbound roadway, passing lane, weekdays. The main adantages of heay ehicles platoons are: potential increases of the lane capacity; reduction of the wind resistance (except for the leader ehicle) and fuel saings; lower freight costs; less seerity of accidents. The most important disadantages are the following: 1564

5 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. different types of heay ehicles will hae differing braking and acceleration characteristics, whit increase of the related rear-end collisions risk; unobstructed iew may be limited by heay ehicles platoons; some driers may attempt to drie in a platoon without the right skills; while the accidents seerity might decrease, the accidents rate might increase. Figure-9. The size of platoons rarely exceeds ten ehicles (i >10). In all, the number of annual rear-end collisions between platooned heay ehicles on the A motorway was of 38 in the years 014 and 015 (cfr. Figure-8). Figure-9. Frequency of platoons as function of the number of ehicles for each platoon. Section San Michele, northbound roadway. Figure-8. Rear-end collisions between platooned heay ehicles. For the A the sampling of platoons has been carried out at obseration sections shown in Table- for the day 7 May 014 (Wednesday). The flow parameters of platoons, with 0 heay ehicles for each platoon, hae been inestigated by collecting data regarding speeds, headways and mass of the ehicles. Among others, hae been calculated the following parameters: frequencies of the number of platoons size of i heay ehicles (i = 1,, 0); minimum headway ( min), mean headway ( mean), and maximum headway ( max) between ehicles of each platoon; minimum speed (V min), mean speed (V mean), and maximum speed (V max) of ehicles of each platoon; the number of platoons whose ehicles trael with headways comprised within predetermined alues (from 0 < 1 seconds, to 1< 0 seconds). Figure-10 shows the trends of minimum headway ( min) and mean headway ( mean), whilst the Figure-11 shows the trends of maximum headway ( max). The mean headway is characterized by low fluctuations; instead, the minimum headway increases monotonously as function of the platoons size. Figure-1 shows the speed range of ehicles as function of the platoons size. The amplitude of the range of speeds decreases systematically and significantly with the increases of the number of ehicles within platoons. In Table-8 are gien the number of platoons whose ehicles trael with headways comprised within predetermined classes ( I j). We hae found that a great part of the ehicles trael with ery low headways and, for this reason, in potential unsafe conditions. In fact, the percentages p of platoons whose ehicles trael with an aerage headway of less than 3 seconds are: p = 37.1%, section Kofler, northbound roadway; p = 66.9%, section Kofler, southbound roadway; p = 40.9%, section San Michele, northbound roadway; p = 39.4%, section San Michele, southbound roadway; p = 36.6%, section Portale Affi, northbound roadway; p = 44.0%, section Portale Affi, southbound roadway; p = 39.1%, section Mantoa Sud, northbound roadway; p = 43.8%, section Mantoa Sud, southbound roadway. Table-7 shows the results for the section San Michele north bound roadway. The relatie frequencies of the number of platoons with i ehicles are deliered in 1565

6 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. Number of platoons Table-7. Platooned heay ehicles flow parameters -Section San Michele, northbound roadway. Vehicles for each platoon min max mean Vmin Vmax Vmean 4 0,50 33,30 4,71 73,50 11,50 89, ,35,65 4,88 79,67 111,67 88, ,87 1,03 3,94 69,5 103,00 85, ,13 11,40 3,49 77,40 106,00 85, ,58 1,80 4,5 71,83 101,33 85, ,75 9,7 3,9 74,9 98,9 84, ,7 7,94 3,8 77,38 91,88 83, ,65 13,66 3,75 76,67 90,89 83, ,53 7,38 3,74 81,00 9,10 84, ,4 5,67 3,46 80,7 89,7 84, ,19 4,38 3,3 77,17 85,58 8, ,18 5,6 3,36 76,08 87,54 83, ,75 4,05,84 81,00 84,64 8, ,4 5,34 3,70 78,87 87,87 8, ,08 5,08 3,98 71,31 88,75 78, ,50 5,69 3,73 75,88 87,88 8, ,0 3,0 3,0 83,44 83,44 83, ,97 3,04 3,01 7,89 83,47 76,74 0,90 3,1 3,05 74,70 83,5 78,98 Figure-10. Relationship headway platoons size (red line: min; blue line: mean).section San Michele, northbound roadway. Figure-11. Relationship maximum headway platoons size. Section San Michele, northbound roadway. 1566

7 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. Figure-1. Mean speed of ehicles in each platoon (each dot represents a platoon). Section San Michele, northbound roadway. Table-8. Number of platoons whose ehicles trael with headways comprised within classes of headway. Section San Michele, northbound roadway. i j *=( i + j )/ N V m 0 1 0, ,8 1 1, ,07 3,5 5 84, , , , , , , , , ,5 7 9, , , , , ,5 6 90, ,5 6 87, ,5 4 94, ,5 5 89, ,5 1 81, ,5 3 88, ,5 90, , , ,5 0 - Figure-13. Mean speed of ehicles trael with headway falling within the class with mean alue *.Section San Michele, northbound roadway. Figure-14. Relatie cumulatie frequency of platoons whose ehicles trael with headway falling within the class with mean alue *. Section San Michele, northbound roadway. EVALUATION OF MINIMUM SAFETY SPACE The minimum safety spaces s between platooned heay ehicles can be calculated as follows. Figure-15 shows the locations of the leading and following ehicles, traelling in platoons, at moment in which the leading ehicle begins to decelerate; at the end of the stopping maneuer of the following ehicle is required a safety margin (s 0) [14]. Using the following notations can be found the relationship between speed, deceleration and spacing s [4]: initial speed of the two ehicles; d 1 deceleration rate of the leading ehicle; d deceleration rate of the following ehicle; perception-reaction time of the following ehicle; s 0safety margin at the end of deceleration phase (s 0 = 0,5 m) ; s (t) is the space of the following ehicle during the perception and reaction time s (t) ); s 1(t) is the distance coered during the deceleration of the leading ehicle; 1567

8 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. s (t) is the distance coered during the deceleration of the following ehicle; L is the length of ehicles (L = 5,5 m). s 1 0 s (t) s (t) s (t) s L (3) In which: s (t) (4) 1 d 1 s (t) (5) d Introducing equation (4) and equation (5) in the equation (3), we obtain: () d d s 0 1 s L (6) Regime Table-9.Safety regimes definitions. Deceleration of leading ehicle Deceleration of following ehicle a d n b d e d n c d e d d l=d f e (no braking) With the alues of Table-9, and taking into account the Italian model for ealuation the perception-reaction time of the following ehicle (,8 0,036 ) [15], hae been obtained the relationships a) s()= (,8 0,036 ) + + s 0 + L (7) d n b) s()= (,8 0,036 ) s 0 + L (8) d d n e c) s()= (,8 0,036 ) + + s 0 + L (9) d e d) s()= (,8 0,036 ) + s 0 + L (10) s()= s = s 0 + L (11) Figure-15. Safety spacing between platooned ehicles. The following alues of deceleration are of interest for the operation s safety leels: d n = normal or comfortable deceleration; d e = emergency deceleration; = instantaneous deceleration. The first one (normal) is related to passenger comfort. The instantaneous deceleration is a theoretical alue that occurs when an accident or a stalled ehicle comes within the perception filed of the subject ehicle. Can be considered the safety regimes shown in Table-9. In the light of the aboe considerations, denoting with s a the mean intra-ehicular distances (s a = 3,6 Vm/ *) and with s the safety distances obtained using the eq. (9), the safety conditions of platoons are ensure if s > s a (or s = s-s a>0). Therefore hae been ealuated number and percentage of platoons whose heay ehicles trael in safety conditions. As example, in Table-10 are gien the results for the section San Michele. Figure-16. Safety space ersus speed (eqs. (7) (11), d n = m/s ; d e = 7,3 m/s ; s 0 =0,9 m, L = 5,5 m). As summarized in Table-10, up to headway (on aerage) * = 3,5 seconds, (classes min = 3 sec., max = 4 sec.) heay ehicles trael in potentially unsafe conditions. With the aid of the cumulatie frequency distributions (shown in Table-10),it deries that the percentage of platoons whose ehicles trael, on aerage, in safety conditions ( s = s-s a>0) is equal to 37% of the total for the section San Michele, northbound roadway. Similarly, the following alues hae been obtained: 1568

9 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. = 43%, section Kofler, northbound roadway; = 19%, section Kofler, southbound roadway; = 35%, section San Michele, southbound roadway; = 43%, section Portale Affi, northbound roadway; = 36%, section Portale Affi, southbound roadway; = 4%, section Mantoa Sud, northbound roadway; = 3%, section Mantoa Sud, southbound roadway. The maximum operating conditions, relatiely to the smallest alue in the aerage headway for which all ehicles trael safely ( *=4,5 sec.), are the same for all the sections analyzed. The alue of the flow associated withmaximum operating conditions is Q = 3600/ *=800 eh/h. min max Table-10. Safety ealuation of platooned heay ehicles -Section San Michele, northbound roadway. * Number of platoons Cumulatie frequency Relatie Cumulatie frequency (%) Vm sa (m) s (m) s = sa-s (m) Safety ensured 0 1 0, ,86 87,8 1,1 93,91-81,79 NO 1 1, ,01 85,07 35,45 91,4-55,80 NO 3, ,89 84,60 58,75 90,68-31,93 NO 3 4 3, ,07 85,51 83,14 91,78-8,64 NO 4 5 4, ,58 86,5 107,81 9,66 15,15 YES 5 6 5, ,38 88,06 134,54 94,86 39,68 YES 6 7 6, ,47 90,45 163,3 97,79 65,53 YES 7 8 7, ,81 9,45 19,61 100,6 9,36 YES 8 9 8, ,04 89,86 1,18 97,06 115,1 YES , ,65 94,04 48,17 10,3 145,94 YES , ,40 90,31 63,39 97,60 165,79 YES , ,14 87,36 79,06 94,01 185,06 YES , ,64 94,5 37,6 10,49 4,77 YES , ,6 89,47 335,50 96,58 38,9 YES , ,38 81,50 38,6 86,98 41,8 YES , ,75 88, 379,85 95,06 84,79 YES , ,00 90,9 416,70 98,36 318,34 YES , , , , , , CONCLUSIONS The traffic flow processes and the ehicles distribution ( free-moing or platooned ehicles) on highway and motorway hae always had fundamental importance in highway engineering, with especially reference to topics like traffic operations, car accidents, road safety and air pollution emissions. Heay ehicles platoons formation in a traffic stream is ery common in the right lane of motorways and highways with truck lane changing restrictions or trucks oertaking prohibition. This traffic control strategies can increase the capacity of the passing lane but, at the same time, can increase the collisions in the right lane. In light of this, the study suggests a method for ealuating the safety of platooned heay ehicles using a closed-form stream model. The case study of the Italian motorway A, with an oertaking prohibitionsfor heay ehicles,was examined. To this aim, first were determined the macroscopic flow parameters (free-flow speed f, capacity C, jam density k jam) in four different road sections. The oertaking prohibitions are generally respected by heay ehicles oer 7,5 t; this leads to the formation of many platoons. In the research hae been inestigatedonly platoons with 0 heay ehicles for each platoon. We obtained the alues of many parameters, as thefrequencies of the number of platoons, the minimum mean and maximum headways and speeds between heay ehicles of each platoon, etc. 1569

10 VOL. 11, NO. 1, NOVEMBER 016 ISSN Asian Research Publishing Network (ARPN). All rights resered. The results of the analysis show that a great part of ehicles trael with ery low headways. The percentages of platoons whose ehicles trael with an aerage headway of less than 3 seconds are in the range 37.1% 66.9%, depending on the motorway section. The comparatie analysis between the minimum safety spaces s and the mean intra-ehicular distances s a allowed to determine the percentages of platoons whose ehicles trael, on aerage, in safety conditions ( s=s-s a >0). These percentages are between 3% and 43%depending on the motorway section. In conclusion, despite the brake reaction time of professional driers may be better than the general driing population, considering the mean speeds, generally the intra-ehicular distances within platoons are too small to aoid collisions. ACKNOWLEDGEMENTS The Authors wish to thanks Dr. Eng. Walter Pardatscher, CEO of the Autostrada del Brennero SpA, for the constant willingness during the deelopment of this research. REFERENCES [1] Bonnet, C., Fritz,H. Fuel consumption reduction in a platoon: Experimental results withtwo electronically coupled trucks at close spacing. In Proceedings of the Future Transportation Technology Conference, Costa Mesa, USA, 010, SAE Technical Paper [] Ioannou P. A., et al Automated Highway Systems. Springer Science+Business media, LLC. [3] Carbaugh J., Godbole D. N., Sengupta R Safety and capacity analysis of automated and manual highway systems. Transportation Research Part C: Emerging Technologies. 6C (1-): [4] Shladoer S. E Potential Freeway Capacity Effects of Automatic Vehicle Control Systems, Applications of Adanced Technologies in Transportation Engineering, ASCE Conference, Minneapolis, MN, [8] Mauro, R. Traffic analysis, deelopment of models and systems for estimating reliability on the A. Technical report 003, Part I. Autostrada del Brennero, Trento, Italy (in Italian). [9] Mauro, R. Traffic analysis, deelopment of models and systems for estimating reliability on the A Freeway, Italy. Technical report 005, Part II, Autostrada del Brennero, Trento, Italy (in Italian). [10] Mauro R Traffic analysis, deelopment of models and systems for estimating reliability on the A Freeway, Italy. Technical report 007, Part III, Autostrada del Brennero, Trento, Italy (in Italian). [11] Guerrieri M., Mauro R Capacity and safety analysis of hard-shoulder running (HSR). A motorway case study. Transportation Research Part A: Policy and Practice, 9, [1] Mauro R., Branco F., Guerrieri M Contribution to the platoon distribution analysis in steady-state traffic conditions. Periodica Polytechnica: Ciil Engineering. 58(3): [13] Alam A., Gattami A., Johansson K.H., Tomlin C.J Guaranteeing safety for heay duty ehicle platooning: Safe set computations and experimental ealuations. Control Engineering Practice. 4(1): [14] Papacostas C. S., Preedouros P. D. 009.Transportation Engineering and Planning (3 rd Edition), HPI Learning. [15] Italian Ministry of Infrastructures and Transports Guidelines for the design of road infrastructures: D.M. n. 679, 5/11/001 - Rome, Italy. [5] Appert C., Santen L Accidents in Platoons of Vehicles, in A. Schadschneider, T. Poschel, R. Kuhne, M. Schreckenberg and D.E. Wolf, (Eds.), Traffic and Granular Flow '05, [6] Regulation (EU) No 1316/013 of the European Parliament and of the Council of 11 December 013. [7] May A.D Traffic flow fundamentals. Pearson Education (US). 1570