A Two-Point Vehicle Classification System

178 TRANSPORTATION RESEARCH RECORD 1215 A Two-Point Vehicle Classification System BERNARD C. McCULLOUGH, JR., SrAMAK A. ARDEKANI, AND LI-REN HUANG The counting and classification of vehicles is an important part hours required, however, the cost of such a count was often of transportation engineering. In the past 20 years many auto high. To offset such costs, many techniques for the automatic mated systems have been developed to accomplish that labor counting, length determination, and classification of vehicles intensive task. Unfortunately, most of those systems are char have been developed within the past decade. One popular acterized by inaccurate detection systems and/or classification method, especially in Europe, is the Automatic Length Indi methods that result in many classification errors, thus limiting cation and Classification Equipment method, known as the accuracy of the system. This report describes the devel "ALICE," which was introduced by D. D. Nash in 1976 (1). opment of a new vehicle classification database and computer This report covers a simpler, more accurate system of vehicle program, originally designed for use in the Two-Point-Time counting and classification and details the development of the Ratio method of vehicle classification, which greatly improves classification software that will enable it to surpass previous the accuracy of automated classification systems. The program utilizes information provided by either vehicle detection sen systems in accuracy. sors or the program user to determine the velocity, number Although many articles on vehicle classification methods of axles, and axle spacings of a passing vehicle. It then matches have appeared in transportation journals within the past dec the axle numbers and spacings with one of thirty-one possible ade, most have dealt solely with new types, or applications, vehicle classifications and prints the vehicle class, speed, and of vehicle detection systems. For the actual classification of wheelbase lengths. It also tabulates and prints totals and aver the vehicles detected, most have depended on the classifi age speeds for each vehicle type. This paper describes the cation method developed by D. D. Nash (1) for his Automatic database built and utilized, as well as a roadside experiment Length Indication and Classification Equipment system. conducted to test the accuracy of the database and the clas Therefore this paper begins with an overview of that system, sification program, showing the classifications to be highly which is referred to as the ALICE method. accurate. In 1912, the U.S. government realized that a vast network of THE ALICE METHOD public roads had to be built and maintained for automobile travel. Today, that network consists of a staggering 3.8 million In 1974, during a project known as the West London Area miles of highways. Unfortunately, that growth has not been Traffic Control Experiment in England, planners encountered able to keep up with the steadily rising number of automobiles a major problem. Owing to the traffic composition changes in the United States. Thus many urban areas (and even some that occurred at various times, it was determined that the rural areas) are plagued with congestion problems. vehicular counts to be utilized in the experiment would have To prevent such problems from occurring, and to alleviate to be weighted by vehicle type. An investigation of the traffic those that do occur, careful planning and monitoring are needed. instrumentation equipment available at the time revealed that This is often accomplished through the use of traffic surveil nothing met the requirements, and thus the planners set out lance and control systems or traffic flow data collection sys to create such a system. What they created was termed the tems. Those systems are also used in pavement management Automatic Length Indication and Classification Equipment, and maintenance to estimate pavement loads due to various or ALICE, method. It was introduced in an article by D.D. types of trucks, depending on the axle configuration. Such Nash (1). systems are used in a similar way in bridge maintenance and ALICE utilizes two loop detectors and an axle detector, management. A common feature of such systems is a method which are placed in sequence in a short (about 5.5-m) segment of counting and classifying the number of vehicles using the of highway (Figure 1). facility. For many years this was commonly done by roadside As a vehicle enters the segment, it is detected by the first observers who spent long hours sitting in a vehicle along the loop detector and then by the second loop detector after a facility, manually recording and totaling the number and type short time interval, called tl. The axles of the vehicle then of vehicles going by. The job was tedious, and the pay was trip the axle detector as they pass; and the time interval between usually minimum wage; because of the manpower and man- axles, called t2 , is recorded. The time intervals are measured using a rather complex circuit consisting of a crystal oscillator, B. C. McCullough, Jr. , F. R. Harris, Inc. S. A. Ardekani, Virginia a comparator, and several counters. The distance between Polytechnic Institute and State University, Blacksburg 24061. Current affiliation: University of Texas at Arlington, Arlington 76019. the leading edges of the two loop detectors, called dl, is L.-R. Huang, University of Texas at Austin, Austin 78712. known and is related to tl and the velocity of the vehicle. McCullough et al. 179 r-d1--I ./ Axle detector DIRECTION OF TRAVEL ILOOP A I ILOOPB I - [DJ ~d2~ OUTPUT _J DETECTOR A OUTPUT DETECTOR-----' B i- t2 -j O~~r~T ___________.n fl_ DETECTOR - TIME FIGURE 1 Principle of the ALICE method. Likewise, t2 is related to the velocity of the vehicle and the If the axle data of the vehicle fit within the spacing ranges distance between the two axles, termed d2. Since this pro of one of those groupings (Figure 2), then the vehicle is classified duces two equations with two unknowns, d2 can be deter as a member of that group and recorded. mined. This calculation is performed within the circuit, which results in the recording of the length, axle spacings, speeds, and gap distance for each vehicle that passes through the THE TWO-POINT-TIME-RATIO METHOD segment. The ALICE system then compares the axle numbers and The Two-Point-Time-Ratio, hereafter referred to as TPTR, spacing data of the vehicle with a database, consisting of the method of vehicle classification is similar to the ALICE method, axle number and spacing ranges of fourteen vehicle groups, except that only two vehicle detectors are needed to determine using a logic program. The database includes the following the same data (except for vehicle length, which is not essential classification groups as termed by ALICE (and as commonly for classification purposes); thus, it is more efficient in its use referred to): of hardware. The detector sensors are placed in a short section of highway, similar to the ALICE sensors; but more flexibility 1. Motorcycles, is offered because in TPTR, unlike ALICE, any combination 2. Private cars and light vans, of passage loop detectors, presence loop detectors, or axle 3. Two-axle heavy goods vehicles (two-axle single-unit detectors may be utilized. For simplicity the first detector in trucks or vans), the sequence is referred to as sensor A and the second detec 4. Three-axle rigid goods vehicles (three-axle single-unit tor, as sensor B, as shown in Figure 3. trucks), As a vehicle enters the segment, its front axle is initially 5. Four-axle rigid goods vehicles (four-axle single-unit detected by sensor A and then by sensor B after a short time trucks), interval, which is referred to as 1U (and is equivalent to the 6. Three-axle articulated lorries (three-axle tractor variable tl in the ALICE method). The time interval(s) between trailers), axle detections at sensor A are also determined, with n 7. Four-axle articulated lorries (two-axle tractors with two representing the interval between the first and second axles, axle trailers), 12 representing the interval between the second and third 8. Five-axle articulated lorries (two-axle tractors with three axles, and so on. axle trailers), As the work progressed on the circuitry necessary to inter 9. Four-axle articulated lorries (three-axle tractors with face the TPTR sensors with a microcomputer, the focus shifted one-axle trailers), to possible classification programs. The first such program 10. Five-axle articulated lorries (three-axle tractors with that was considered was one that would utilize the method two-axle trailers), ology of the ALICE system; it was determined, however, that 11. Six-axle articulated lorries (three-axle tractors with three- such a program would not be well suited for use with the axle trailers), TPTR system for several reasons. One of the predominant 12. Road trains (single-unit trucks with trailers), reasons was the limited number of vehicle classes covered by 13. Cars with caravan/trailer, and the ALICE database. Since the database resulted from obser 14. 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