TRANSPORTATION RESEARCH RECORD 1225 Headway and Speed Data Acquisition Using Video M. A. P. TayroR, W. YouNc, eNp R. G. THonlpsoN Accurate knowledge of vehicle speeds headways and on trallÌc ment (such as a freeway) before this study, so there was an networks is a fundamental part of transport systems modelling. excellent opportunity to evaluate the system and suggest mod- Video and recently developed automatic data-extraction tecñ- ifications to it. This equipment also made niques have the potential to provide a cheap, quick, easy, and it feasible to inves- accurate method of investigating traflic systems. This paper pre- tigate the relationship between vehicle speeds and location in sents two studies that use video-based equipment to investigate the car parks. character of vehicle speeds and headways. Investigation oÌ head- rvays on freeway traffic allows the potential of this technology in a high-speed environment to be determined. Its application to the THE VIDEO SYSTEM study ofspeeds in parking lots enabled its usefulneis in low-speed environments to be studied. The data obtained from the video was Using film equipment compared to traditional methods of collecting headway and speed to obtain a permanent record of vehicle data. movements is not a new concept. However, considerable recent developments have occurred in collecting data using video. Digital image-processing applications offer the potential to In particular, ARRB has developed a trailer-mounted video automate a large number of traffic surveys. It is, therefore, recording system (3). This relatively new equipment has until not surprising that considerable interest has been directed at recently experienced only a limited range of applications. It developing procedures for vehicle classification and number consists of a gas-operated extendible mast (with video camera plate recognition (1 , 2). However, there appear to be several attached), a power supply, and other video accessories con- obstacles preventing these systems from being used regularly cealed in an inconspicuous, covered trailer. A range of lenses in traffic surveys. Some of these difficulties are the relatively is available that enable it to view various aspects and areas. high cost of the systems, problems of accuracy, quantity of The extraction of the data from the video could be carried data collected, and the time required to process the raw data. out by a human observer, but this is time consuming. A pref- Fortunately, systems are being developed that can be used erable method is to extract the data automatically. A pro- for specialized applications. This paper presents one such sys- cedure for carrying out this task has been developed by Trout- tem. It is a video-based system that allows vehicle speed and beck and Dods (3). The position of the vehicle is recorded as headway data to be extracted. it passes through a particular point on the screen. These points, Video-based techniques offer the means to overcome many termed detection points, are identified in the data collection of the difficulties of collecting information on speeds and stage. The movement of a vehicle through these points is headways. The basic idea is to create a video recording of determined by a change in the intensity of the image and is traffic, then extract data from it. The video approach has a recorded on computer tape for further analysis. The basics of number of advantages including being unobtrusive and requir- VADAS are as follows. The image-processing system relies ing a small labor component. Its major advantage is the pro- on the observation that most vehicles have a video illuminance duction of a permanent, complete record of the traffic scene. level much greater than that of the road pavement on which This may be reanalyzed at any stage, and provides an account they are traveling. The general luminance of the scene is of each traffic event observed. monitored by locating a reference point just off the carriage- The oft-quoted disadvantage of the video technique is the way. Thus, changes in ambient light levels due to cloud cover large amount of time and effort needed for data extraction. The and the angle of inclination of the sun can be accommodated. use of an automatic data-extraction (image processing) system Besides the reference point, VADAS allows the analyst to can overcome this problem. The video vehicle detection system define up to L6 detection points within the video frame. The (VADAS) (3) developed by the Australian Road Research illuminance level at each point is then monitored by VADAS, Board (ARRB) offers a sophisticated, high-technology system and a change in luminance above a set threshold is taken to useful in data extraction. VADAS was developed for use in mean the passage of a vehicle over that point. The basic steps studies of the performance of unsignalized intersections, involved in data extraction using VADAS are the positioning particularly roundabouts where complex traffic maneuvers of detection points on the screen and the adjustment of the take place. It had not been applied to a high-speed environ- sensitivity threshold. Once the data have been logged, they can be edited. The output from this process is a computer file of times and detector numbers that can be used in subsequent Department of .Civil-Engineering, Monash University, Clayton, data analysis. The speeds and headways can be calculated Victoria, Australia 3168. from data on the times when vehicles were detected. 131 Taylor et al. HEADWAY STUDY Such correlations should be expected on arterial roads where there are signalized intersections. There is no clear evidence A convenient way to describe the inherent variability within one way or the other for freeway traffic flows. a traffic stream is to consider it as a stochastic process and Another phenomenon of interest is the distribution of traffic examine the headways between vehicles in a traffic stream. between lanes on a multilane road. The conceptual model is Headways are defined as the time separation between the that traffic uses (indeed, under some highway codes, should same point on two successive vehicles in a traffic stream. This use) the outside lanes of a carriageway unless overtaking, definition holds for both single-lane and multilane traffic when vehicles will use the inside (median) lane(s). Under light streams. Statistical distributions can be fitted to represent traffic volumes, this model works satisfactorily. But, as traffic headways. The reciprocal of mean headway is equal to the volume increases, this flow regime breaks down. Vehicles in mean flow rate. the outside lane that overhaul a slower vehicle in that lane Headways are perhaps the most important of all traffic and wish to overtake find increasing difficulties in changing variables since a Iarge amount of traffic research and simu- Ianes. For instance, Mahalel and Hakkert (8) examined traffic lation exercises make use of headway models in the analysis flows on a twolane carriageway in Israel and found that the of traffic problems. Buckley (4) stated that "headways are median lane carried more than 50 percent of the traffic flow the building blocks with which the concepts of volume and for flow rates above 1,150 vph. Chen (9) found that the median capacity, and indeed the entire traffic stream are con- lane attracted the largest volume of traffic on a. threelane structed." The ability of traffic models to replicate traffic carriageway in Ohio, for total carriageway flows exceeding systems realistically will be enhanced through more efficient 4,500 vph. This tendency for increased proportions of total and accurate techniques of collecting headway data. flow to use the faster lanes as flow builds up leads to a greater A number of theoretical models of headways exist. Schuhl tendency for bunching in that lane, and perhaps more insta- (5) and Buckley (4) provided some of the earlier research in bility in the traffic flow. this area. The most comprehensive theoretical study of head- Although the negative exponential distribution has not proven way distributions was provided by Cowan (ó), who derived a to be a good descriptor of single-lane headways, there is a family of four headway models. The first three members of strong possibility that it could represent multilane traffic flows this family are the negative exponential distribution, the shifted since headways close to zero are feasible. Buckley (4) and exponential distribution, and the mixed exponential distri- Gerlough and Barnes (10) both suggested that the negative bution. These three models show increasing levels of sophis- exponential model could be used for multilane unidirectional tication from the simple random traffic model (the negative traffic flow. exponential distribution) through the shifted exponential dis- Given these observations, the data collection scheme was tribution (which allows for the finite length of vehicles trav- designed to include the defined flow regime; traffic volumes eling in a single lane by imposing a minimum headway) to in the identified flow regime were spread out over different M3, which represents traffic as a mixture of free and following times of day and different days to account for the known vehicles. Cowan fitted the models to data collected in a single variations in traffic flow levels over hours of the day and days lane on an arterial road. He concluded that the M3 model of the week. Spreading the data-collection periods would pro- (and the fourth member of the headway model family, M4) vide the means to avoid misinterpretations based on fluctua- could provide a reasonable description of traffic flow. tions or deviations from the normal traffic pattern. A number of problems emerge in the application of statis- Theories of traffic flow and models of traffic system per- tical distributions to traffic headways. There are two theo- formance require comprehensive, accurate, and unbiased data retical issues of importance.