This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 1 The AD4CHE Dataset and its Application in Typical Congestion Scenarios of Traffic Jam Pilot Systems Yuxin Zhang, Cheng Wang, Ruilin Yu, Luyao Wang, Wei Quan, Yang Gao, Pengfei Li Abstract— Autonomous driving has attracted considerable attention from research and industry communities. Although prototypes of automated vehicles (AVs) are developed, remaining safety issues and functional insufficiencies hinder their market introduction. To obtain reasonably foreseeable scenarios and study human driving policies, many naturalistic driving datasets are proposed. However, no open-source dataset filled with congestion scenarios is publicly available. The paper presents the Aerial Dataset for China’s Congested Highways & Expressways (AD4CHE). It contains 5.12 hours of aerial survey data from four different cities in China, with a total driving distance of 6540.7 km. Moreover, overlap and non-overlap cut-in scenarios are distinguished to better describe driver behavior in congestion scenarios. Both types of cut-in scenarios are extracted and parameterized. The Kernel Density Estimator (KDE) is utilized to generate parameter distributions for the scenario-based testing method. Furthermore, the driving behavior in overlap cut-in scenarios is intensively analyzed. The results reveal that the drivers have an evasive maneuver during overlap cut-in of challenging vehicles, and the preferred following distance varies with the relative longitudinal velocity. Both scenario parameterization and driving behavior analysis can contribute to developing and verifying Traffic Jam Pilot (TJP) systems deployed in Chinese traffic situations. The dataset is available at https://auto.dji.com/cn. Index Terms— Aerial dataset, autonomous driving, Chinese highways and expressways, congestion scenarios, driver behavior, scenario-based testing. I. INTRODUCTION S AFETY assurance of autonomous vehicles (AVs) is currently a challenge. Although it is nowadays not a rare case to see AVs being tested on public roads, it is considered inefficient due to rare occurring critical situations. To accelerate the testing, various simulation tools [1] and environmental sensor models [2] have received much attention. Due to the model validity in simulations, simulation-based testing is only partially functional. Consequently, the scenario-based approach This research was supported by JLU-DJI Collaborate Research Project (HQ-RD-201117-02), National Natural Science Foundation of China (No. 52075213), and Industrial Technology Basic Public Service Platform Project 2020 from Ministry of Industry and Information Technology of China (No. 2020-0100-2-1). (Corresponding author: Cheng Wang). Y.X. Zhang, R.L. Yu and L.Y Wang are with the State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun, 130025 China (e-mail: yuxinzhang@jlu.edu.cn,yurl21@mails.jlu.edu.cn). C. Wang is with the Autonomous Agents Research Group, University of Edinburgh, Edinburgh, EH8 9AB United Kingdom (e-mail: cheng.wang@ed.ac.uk) W. Quan, Y. Gao and P.F. Li are with the DJI Automotive, Shenzhen, 518063 China (e-mail: moritz.quan@dji.com, tena.gao@dji.com, xiaofei.li@dji.com) is motivated and studied in German Pegasus [3] and Japanese SAKURA [4] projects. The scenario-based approach aims at abandoning irrelevant scenarios to reduce the test scenario space resulting from the open world. A similar concept known as scenario engineering [5] [6] is used to achieve trustworthy AI by keeping system parameters at reasonable levels. Obviously, data sources are essential for scenario-based testing. One way to obtain data for the method comes from naturalistic driving. To determine the contribution of naturalistic driving data, the following provides a detailed comparison of traffic accident data, field operation data and data from expert knowledge. Using traffic accident data to study traffic behavior has been applied for decades. With the advent of AVs, testing AVs in those accident scenarios has broadened the application scope of traffic accident data, as it is valuable to determine whether an AV could prevent a collision in scenarios that results from human drivers. For instance, criticality phenomena [7] associated with increased criticality are extracted from the GIDAS dataset to identify which factors in scenarios influence traffic accidents. Despite the numerous applications for traffic accident data, the amount of data available is limited. Additionally, some essential attributes, such as lighting conditions to describe a scenario, are missing because the data was not originally specialized for AVs. Currently, a popular way to collect testing data for AVs is field operation data. In this method, an AV operates in the real world with a safety driver onboard. The data, such as perception data, is either continuously recorded or manually triggered to save by a safety driver, or automatically saved by a technique like “silent testing” [8]. The data collected is critical in improving the performance of AVs. However, the behavior of surrounding traffic participants may be affected by an AV in mixed traffic because of unusual equipment installed on the AV. Moreover, inefficiency and high costs limit its widespread application because of the safety drivers. In contrast, data from expert knowledge [9] can be generated in a less time-consuming manner. A developer with expertise in some domains of AVs is likely to be aware of the limitations. This valuable experience can be used to create corresponding scenarios. Nevertheless, the validity of the generated data is a concern. In particular, unknown scenarios can never be covered since experience is posterior. Unlike traffic accident data, much naturalistic driving data is available and simple to obtain. By recording the driving data © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 2 during a daily drive, naturalistic driving data is then generated. The China-FOT dataset [10] belongs exactly to this category. They recruited volunteers to drive their vehicles for one and a half years. The collected data was then utilized to investigate drivers’ behavior. Additionally, such data can be collected by mounting a sensor, such as a camera, in a fixed position. For instance, a camera mounted on a building records vehicle trajectory data [11]. A relatively new method to generate naturalistic driving data is using a drone. Such a concept is successfully applied in the PEGASUS project, in which the traffic data on a section of a highway is recorded by a drone. The HighD dataset [12] is then born. Nevertheless, there are few congestion scenarios in the HighD dataset, and the average speed is 100.67 km/h. As a result, it is inapplicable when the focus is to study driver behavior in traffic jam scenarios in order to develop traffic jam pilot (TJP) systems. A TJP as a level 3 [13] automated driving system, which can be activated in a traffic jam or slow-moving highway traffic up to 60 km/h. When the traffic jam pilot is activated, the system will take over the control, and drivers no longer need to continuously monitor the vehicle. They only have to remain alert and be ready for taking over the driving task again when the system prompts them at any time. Several original equipment manufacturers (OEMs) have introduced their TJP systems into the market. However, there is currently no TJP available in the Chinese automotive market. On the one hand, corresponding regulations in China are not yet in place. On the other hand, the traffic in China is more complicated than in Europa. Considerable effort must be expended to adapt the TJP to Chinese traffic situations. To support the adaption and investigate how Chinese drivers behave in traffic jams, this paper proposes a dataset filled with congestion scenarios on Chinese highways and expressways. The naturalistic driving data is recorded by a drone. After data processing, we provide detailed information for each recorded vehicle and road information, such as lane markings. Moreover, we distinguish between overlap and non-overlap cut-in scenarios and discovered that overlap cut-in scenarios are relatively common in Chinese traffic jam scenarios. To compare the two types of cut-in scenarios, scenarios are extracted and parametrized. Using the kernel density estimator (KDE) [14], the foreseeable parameter distributions are obtained, which facilitates the scenario-based testing method for a TJP system. Meanwhile, the Chinese driver behavior has been identified and can be used to validate and improve driver performance models such as the fuzzy safety model (FSM) [15]. These driver performance models are regarded as a reference for AVs because an AV is expected to outperform a careful and competitive driver [16]. The contributions to the paper are: an open-source dataset focusing on Chinese highway and expressway congestion scenarios is proposed. Unlike other existing naturalistic driving datasets, ours is the first open-source dataset that focuses specifically on congestion scenarios. As a result, it can aid in the development of TJP systems; Because of congestion, particular cut-in maneuvers emerge. We define overlap cut-in to distinguish it from non-overlap cut-in scenarios (common cut-in) and extract both types of scenarios in order to determine reasonably foreseeable scenario parameter distribution in congestion scenarios; the driving behavior of Chinese drivers in overlap cut-in scenarios is intensively analyzed, and new findings are discovered by answering three research questions in order to contribute to the development of driver performance models in congestion scenarios. The structure of this paper is as follows: Section II introduces the related datasets and works; Section III focuses on the data collection process in four Chinese cities and associated postprocessing. Section IV presents the results of extracted scenarios and the driving behavior analysis in congestion scenarios; finally, the discussion is carried out in section V and conclusions are given in section VI. II. RELATED WORK This section first investigates currently available naturalistic driving datasets and compares them to our dataset. Following that, works on scenario parametrization are introduced. Finally, the role of driver models in AVs is presented. A. Related datasets Various types of datasets [17] are published with the emergency of AVs. Traffic accident datasets were originally collected to analyze traffic safety issues aiming to derive measures or regulations for vehicles, infrastructures, and medical aspects. This type of data has recently been used to construct critical scenarios for AVs [18] [19]. Field operational datasets, on the other hand, are generated by AVs. Sensor data such as images and point clouds are recorded in this type of dataset. As a result, they are rather suitable for offline testing perception algorithms. The KITTI dataset [20], the Apollo dataset [21] and the Waymo open dataset [22] are examples of this type of dataset. Vehicles equipped with sensors are driven on roads in these datasets, and the data is known as field operation data. To limit the scope of the literature review, we concentrate on naturalistic driving (ND) datasets. A popular dataset recorded by a drone is the HighD dataset [12], which records the trajectories of approximately 110,000 vehicles on a 420-meter-long highway. Since then, a series of similar datasets, such as the inD [23], roundD [24] and exitD [25] datasets, have been published. The inD records the vehicle trajectories in an intersection, whereas a roundabout is a location in the roundD dataset. The exitD dataset is motivated for including merging scenarios. One common aspect of these datasets is that only the German traffic is recorded. For ND datasets recorded in America, the NGSIM dataset [11] is the most popular one to record trajectory data using a fixed camera. Besides, the INTERACTION dataset [26] includes trajectory data for intersections, roundabouts and merging scenarios. The Stanford drone [27] and Interstate-24 MOTION [28] datasets use a drone to generate trajectory data. The former is primarily concerned with campus scenarios, whereas the latter is captured on highways at high speeds. The China-FOT dataset [10] recorded 32 drivers’ daily driving in Shanghai, China. Vehicle CAN data, pedal data and © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 3 video images were recorded. Since the cameras were installed inside the vehicles, a minor modification is required. Since then, the SIND dataset [29], which was recorded at a signaled intersection, has emerged. The last dataset is ours, which is known as the aerial dataset for China’s highways and expressways (AD4CHE). Compared to other existing naturalistic driving datasets, as shown in Table I, our dataset has the following three special properties: Open source: both the dataset and the code for this paper are open-source; Congestion: the dataset is filled with congestion scenarios by recording during rush hours. Congestion stimulates overlap cut-in, This uncommon type of cut-in is valuable for developing TJP systems; Chinese highways and expressways: the traffic data focuses on traffic situations on Chinese highways and expressways, whereas few existing ND datasets take this into account. TJP and automated valet parking (AVP) systems are considered the next generation of automated driving functions for intelligent vehicles. Studying how Chinese drivers behave in congestion scenarios to support TJP systems’ design motivates us to generate the valuable AD4CHE dataset. To the best of our knowledge, this is the first aerial dataset with these three special properties. TABLE I COMPARISON OF DIFFERENT NATURALISTIC DRIVING DATASETS Dataset COUNTR Y RELEASED Y Y Y 2018 2019 2020 Y 2021 Partial Y 2021 Y 2020 trajectory data, digital maps Y 2019 TYPICAL SCENARIOS DATA CONTENT trajectory data trajectory data trajectory data trajectory data, digital maps trajectory data, digital maps trajectory data HighD [12] InD [23] RoundD [24] Germany Germany Germany drone drone drone fast straight driving intersections roundabouts ExitD [25] Germany drone entries and exits Automatum [30] Germany drone pNEUMA [31] drone NGSIM [11] Greece Germany , China, USA USA static camera Stanford drone [27] USA drone INTERACTION [26] OPENSOURCE RECORDING TYPE drone straight driving, entries and congestion urban congestion intersections, roundabouts and merging entries and exits campus and straight driving YEAR trajectory data Y 2006 Trajectory data Y 2016 Interstate-24 MOTION [28] China-FOT [10] USA drone highway fast driving Trajectory data N 2020 China vehicle sensors urban driving N 2014 SIND [29] China drone intersections Y 2022 drone highway and expressway congestion vehicle CAN data trajectory data, digital maps trajectory data, digital maps Y 2022 AD4CHE (Ours) China B. Scenario generation As an aerial dataset, it can be applied for different purposes. Its contributions to AV verification and validation are particularly noteworthy. One great benefit of aerial datasets is their application in scenario-based testing for AVs. Because of the scenarios’ validity, extracting scenarios from those datasets and using them to verify AVs is credible. In general, there are three steps to generate concrete scenarios [32] from a dataset. They are scenario definition [33] [34], scenario parameterization [35] [36] and parameter space estimation [37] [38], respectively. Ontology-based approaches are commonly used for scenario definition. Due to the necessity of prior knowledge in ontology, searching methods such as stress testing [39] [40] attempt to discover critical scenarios directly. In scenario parameterization, common trajectory parameters such as velocity and position are intuitive but may be inappropriate for characterizing scenarios; for example, it is unknown if the ego’s initial velocity or maximum velocity is better to describe its behavior in a cut-in scenario. According to E.d. Gelder et al. [35], defining too few parameters leads to oversimplification, while defining too many parameters has a dimensional problem. Therefore, they used the singular value decomposition (SVD) to choose the optimal parameters. Nevertheless, the method is only demonstrated by two simple scenarios, and assumptions are made to prove its effectiveness. In contrast, Karunakaran et al. [36] derived the parameters to describe a cut-in scenario by comparing simulated trajectories govern by control points to real-world trajectories. Aside from scenario parameterization, the next step in generating logical scenarios [32] is parameter space estimation. Many works [41] [42] have been done using publicly available datasets. For example, Zlocki et al. [38] analyzed the parameter distributions of five vehicle kinematic variables in the HighD and the SAKURA datasets to determine their correlations. To generate final concrete scenarios, sampling techniques like important sampling [43] [44] and risk-index based sampling [45] are applied to reduce the parameter space. Although the generated logical scenarios provide valuable guidance for the scenario-based testing method, their application for safety verification and validation of AVs is limited due to the dataset diversity requirement. To complement this, the parameter analysis in the AD4CHE dataset is meaningful, particularly for testing TJP systems. © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 4 C. Driver performance models In addition to scenario generation, analyzing driver behavior in congestion scenarios is also crucial. On the one hand, it can guide the parameter determination and optimization for AV decision-making [46] [47]. For instance, driving data is used in [48] to optimize vehicle motion state parameters for a generated trajectory. On the other hand, driver behavior can be used to calibrate safety metrics such as responsible sensitivity safety (RSS) [49] to balance safety and aggression. These safety metrics are commonly used as a safety checker [50] to ensure the safety of a planning module. However, safety metrics that do not use human driver behavior as a calibration reference typically result in conservative decisions that have lower acceptance and are intolerable in congestion scenarios due to frequent cut-in. Consequently, we are motivated to perform the driver behavior analysis in congestion scenarios. Additionally, driver performance models are inevitable for simulation-based testing [51], as they are required for valid simulation results. Because real-world testing brings huge test effort, verifying and validating AVs in simulations is a viable alternative. Lastly, driver performance models can be used as a guide when introducing AVs to the public. Since the UNECE Regulation No.157 [52] introduced a careful and competent driver to assess the safety of a level 3 automated lane keeping system (ALKS), various driver models, including the fuzzy safety model (FSM) [15] and the stochastic cognitive model (SCM) [53], have been studied to provide a more realistic reference for releasing AVs. However, the driving behavior in China differs significantly from that of European countries due to driving culture, infrastructure, etc. The differences are thoroughly discussed in [54]. They concluded that adaption is necessary when applying automated driving systems in Chinese traffic. As a result, the driver behavior analysis in the AD4CHE dataset could help with this adaptation. Fig. 1. The road network of the recorded data. The number in each lane represents the lane number assigned to each vehicle. Vehicles on exit or entrance ramps have a lane number greater than 100. The blue lines are road markings. The positions of these lane marking points are available. The coordinate system is located in the upper left corner, with the x-axis to the right and the y-axis to the bottom. III. AERIAL DATA PROCESSING In this section, we introduce the basic steps to process the recorded data. First, a brief introduction to the recorded sites is given. The techniques used to handle the data are described. Lastly, we present the available information in the dataset. A. Recording sites Because driving behavior is influenced by a variety of factors, including road structures, lane marking quality and driving culture, etc., it is necessary to go across several different areas in China to increase the dataset coverage. As a result, we chose two cities in the north of china and two in the south of China due to their different geographies and climate conditions. The two northern cities are Xi’an and Changchun, whereas the two southern cities are Hefei and Shenzhen. Because we aim to support the development and verification of TJP systems, whose use cases are highway and expressway scenarios with traffic flow speed from 0 to 80 km/h, four highway and expressway sections in those four cities are selected. To capture congestion, we wait for the appropriate time, such as rush hour, to fly the drone. Flight approvals are obtained prior to data collection to avoid no-fly zones and data security risks. B. Recording and pre-processing According to the design requirements of an L3 TJP system, the data must be accurate enough to reasonably model the driving behavior of traffic participants in traffic jams. Unlike the HighD dataset’s settings, which have a hover height of over 300 m, we define the hover height as 100 m and focus on collecting congested road traffic during peak commuting hours. To improve the accuracy and consistency across cities, we developed a unified aerial data collection standard to guide the data collection process, which includes a pre-flight check of the drone status, battery and camera parameter settings, etc. Even though the drone has an excellent anti-shake platform, there are still minor jitters that cause road sections to be inconsistent between images. To address this issue, we perform video alignment pre-processing on the captured video data. By matching the same road features, such as lane markings in the images of different frames, images in the entire video are aligned to the beginning frame. Consequently, the road position will not change while playing a video, and all recorded vehicles have a unified reference. In addition, the pixel size error analysis is performed by dimensional calibration in a controlled © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 5 environment with ideal ground conditions. The overall position accuracy is about 5 cm, and the maximum error is below 10 cm. Fig. 1 illustrates an example of lane segmentation and the lane number assigned to each lane. Since enter and exit ramps are not the focus, we assign them a large number to distinguish them from the main roads. In particular, as the blue lines show, we also provide the detected lane markings, which can be used for lane-changing analysis and lane width determination. The coordinate system is in the image’s upper left corner. Fig. 2. One example to illustrate our detection results. . Object ID and velocity are visualized above each tracked object. Cars are visualized by a red bounding box, while non-cars are visualized by a green bounding box. Because of the accurate yaw angle, objects on the on/off ramps are correctly captured. Moreover, severe congestion scenarios are observed, distinguishing our dataset from other existing naturalistic driving datasets. C. Objects detection and tracking After data recording and pre-processing, object detection and tracking is the remaining task. Due to the superior performance of the convolutional neural network (CNN) in object detection [55] [56], a CNN model is utilized to detect objects and segment lanes. The training data consists of 10k images for lane markings and 50k images for vehicles, which are obtained through semi-automatic annotation. Since different object classes are distinguished during annotation, our model provides the object class together with the object state. To validate the model’s performance, test data that differs from the training data is determined, and two metrics, precision and recall, are chosen to evaluate the model. By computing the intersection over union (IoU) [57] for each labeled object and detected object in the test data, the precision and recall of the detection results are 0.97 and 0.93. The high values of these two metrics indicate the superior performance of our model. To increase the accuracy of the results further, a forwardbackward extended Kalman filter [58] is applied because no time and computation limit exists offline compared to online tracking. In addition, for the size of a vehicle that should remain constant during the movements of a vehicle, the detected size when the vehicle is strictly under the drone is chosen to avoid image projection errors. The position error in the X and Y directions is within 7 cm for object detection at a length of 5 m. Fig. 2 shows the detection results. Each vehicle is represented by a bounding box with its ID and speed above it. D. Data format Based on the processed results, the state (position and velocity, etc.) and vehicle class at each frame are provided. Besides, essential information such as time-to- collision (TTC) and time-headway (THW) to describe the traffic flow is also given. Further, we give not only the information that the HighD dataset provides, but also four additional parameters. They are the lane angle, vehicle orientation, yaw rate and ego offset. The ego offset represents the deviation between a vehicle’s and lane’s center. This information can, for example, be used to analyze the offset distribution when a vehicle keeps in a lane. Such distribution plays a vital role in determining when a risk perception point begins, as described in UNECE Regulation No.157 [52], which is the first L3 AV regulation in the world. Besides the ego offset, the orientation could be helpful in analyzing the subtle driving behavior of human drivers. A lane segmentation image is also provided along with the data, which benefits the extraction of lane markings. Generally, the provided information is sufficient to accurately extract various functional scenarios and convert them to popular simulation data formats like OpenSCENARIO [59]. IV. DATASET ANALYSIS In this section, we first give some statistical information about the dataset. Then, the overlap cut-in scenario is defined and extracted. Based on the extracted scenarios, scenario parameterization and parameter distribution are presented. Lastly, the driver behavior in those overlap cut-in scenarios is analyzed to provide guidance in designing human-like TJP systems. © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 6 A. AD4CHE at a Glance To present a general overview of our dataset and highlight its differences to the HighD dataset, some crucial statistical information is given below in Table II. Due to a longer recording duration in the HighD dataset, the total number of vehicles is higher. However, when we observe the average speed attribute, the value in the AD4CHE dataset is 29.25 km/h, which is much lower than that in the HighD dataset. It implies that our dataset contains a large number of congestion scenarios on the recorded highways and expressways. This conclusion can also be drawn by looking at the “vehicle number per kilometer” attribute. It indicates that the vehicle density in our dataset is nearly four times that of the HighD dataset. TABLE II STATISTICAL INFORMATION OF THE AD4CHE DATASET AND ITS DIFFERENCES TO THE HIGHD DATASET Attributes HighD AD4CHE Recording Duration [hours] Lanes (per direction) Road Length [m] Number of Vehicles Number of Cars 16.5 2-3 400-420 110 000 90 000 5.12 5-6 ≈130 53761 42516 Number of Trucks 20 000 10306 Number of Buses 0 939 Driven distance [km] 45 000 6540.7 Vehicle Number per km 2.44 8.22 Average Speed [km/h] 100.67 29.25 Driven time [h] 447 223.65 overlap cut-in. As summarized in [60], lane-changing is divided into four phases: keeping, changing, arrival and adjustment. Cut-in, as opposed to lane-changing, emphasizes the interaction of two vehicles. Fig. 4 illustrates a cut-in maneuver. When driving in a lane, a vehicle usually has a wandering zone. The wandering zone has been statistically studied by experts in Japan and is defined as 0.375 m [52]. When the challenging vehicle leaves this zone, the timestamp is considered the beginning of a cut-in maneuver and is described as 𝑇1 . When the challenging vehicle is within the wandering zone again in an adjacent lane, we use 𝑇5 to represent this timestamp. During this period, the moment of crossing the lane marking is defined as 𝑇3 . Based on these time variables, as depicted in Fig. 4, we can distinguish overlap cut-in and non-overlap cut-in. Fig. 4. The illustration of a cut-in maneuver. 𝑇1 is the timestamp when a cut-in begins. 𝑇3 is the timestamp when a lane marking crossing is happening. 𝑇5 is the end timestamp of a cut-in maneuver. We use the variables to define overlap cut-in scenarios, which are critical in congestion due to limited free space. TABLE III PARAMETERS THAT ARE USED TO DESCRIBE CUT-IN SCENARIOS Parameters Variables Units Initial ego velocity Initial challenging vehicle velocity Initial relative longitudinal distance Initial relative lateral distance Lateral challenging vehicle velocity 𝑣ego,0 𝑣cha,0 𝑑rel,𝑥0 𝑑rel,𝑦0 𝑣cha,𝑦 m/s m/s m m m/s Start time of cut-in 𝑇1 𝑇3 𝑇5 s Time to cross a lane marking End time of cut-in Fig. 3. The speed distributions in the AD4CHE and the HighD datasets. The speed in the HighD dataset is generally higher than that in the AD4CHE dataset. It indicates that the AD4CHE dataset is more suitable to analyze congestions. For a TJP system, our dataset provides the data required to analyze driver behavior in congestion scenarios, whereas the role of the highD dataset is limited. In addition, the AD4CHE dataset is recorded on highways and expressways within the operational design domain (ODD) [13] of a TJP system. Meanwhile, rich interactive driving behavior is included due to the congestion, which is valuable for TJP development. Fig. 3 compares the speed distributions of the two datasets. B. Overlap cut-in Compared to a car-following scenario, a cut-in scenario is more challenging for a TJP system. In particular, the cut-in maneuvers in congestion scenarios are quite different due to less available space. To clearly express an overlap cut-in scenario, we distinguish lane-changing, non-overlap cut-in and s s The non-overlap cut-in maneuver occurs frequently on highways where a challenging vehicle cuts in with its rear ahead of the ego’s front. However, this type of cut-in occurs rarely in congestion scenarios where a driver is prone to cutting in even when falling behind. This type of cut-in is more critical in congestion scenarios due to the severely limited free space and is more challenging compared to the non-overlap cut-in. As a result, we concentrate on this type of cut-in scenario in our scenario analysis and define it as: Overlap cut-in: when a cut-in maneuver begins, the rear of the challenging vehicle is still behind the ego’s front. This can be mathematically expressed by 𝑙ego + 𝑙cha (1) |𝑑ego,𝑥 − 𝑑cha,𝑥 | ≤ 2 where 𝑑ego,𝑥 and 𝑑cha,𝑥 are the longitudinal position of the ego and the challenging vehicle, respectively. 𝑙ego and 𝑙cha represent their length. The overlap cut-in scenarios are those that satisfy equation (1), while the rest are non-overlap cut-in scenarios. One of our aims is to compare the parameter distributions of these two types of cut-in scenarios in order to provide design guidance for © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 7 TJP systems. To this end, the scenario parameters defined in Table III are used. Since the variable 𝑣cha,𝑦 changes continuously during a cut-in process and its maximum value represents how urgent a lateral movement is, 𝑣cha,𝑦,max is used to perform the parameter analysis. C. Scenario extraction Fig. 5 illustrates the scenario extraction process. First, we check if a vehicle has 𝑇3 to find lane-changing maneuvers. Then, we use the wandering zone to determine the start timestamp of a likely cut-in maneuver. Similarly, the end timestamp of a potential cut-in maneuver is defined if the challenging vehicle enters the wandering zone again in an adjacent lane. Then, we observe which vehicle is behind the challenging vehicle after cut-in and has no lane-changing during the cut-in process to identify the ego vehicle. 90 overlap and 286 non-overlap cut-in scenarios are found. TABLE IV PARAMETERS OF THE RSS MODEL Parameters Variables Units Reaction time 𝜌 Maximum acceleration 𝑎max,accel Minimum deceleration 𝑎min,brake Maximum deceleration 𝑎max,brake 0.75 3 6 m/s2 m/s2 6 m/s2 s According to UNECE Regulation No.157 [52], one of the critical scenarios that should be used to test TJP systems before approval is the cut-in scenario. When compared to the paper's defined overlap cut-in scenario, the overlap cut-in scenario is more difficult because lateral perception, rather than longitudinal perception, is required to accurately track and predict the behavior of the cut-in vehicle, and lateral perception is typically built with fewer and less powerful sensors. To emphasize this challenge, we utilize one exemplary overlap cutin scenario to illustrate the utility of this type of scenario in potentially improving existing TJP systems. As illustrated in Fig. 6(b), when the challenging vehicle starts a cut-in maneuver (as 𝑑rel,𝑦 decreases), it is still partially behind the ego vehicle (as 𝑑rel,𝑥 is negative). Fig. 6(a) shows the actual cut-in process. The challenge of detecting adjacent rear vehicles and identifying their intentions shall be overcome when deploying TJP systems in the regions where drivers tend to cut in even when they fall behind. Fig. 5. The overlap and non-overlap cut-in scenario extraction process. To determine if a lane-changing scenario is a cut-in scenario, the time-to-collision (TTC) metric is usually used to filter out the scenarios where the challenging vehicle is so far ahead that the ego vehicle is unaffected. A TTC threshold of 5 s [15] is used to find hazardous cut-in scenarios in the highD dataset. However, due to the limited longitudinal space in congestion, the ego vehicle reacts to a challenging vehicle despite large TTC or even negative TTC values. Consequently, the TTC metric is not appropriate in our case. Instead, we use a distancebased metric. A distance of 75 m is utilized in [61] based on 200 observations to delete the scenarios where a challenging vehicle is irrelevant to the ego vehicle. To avoid assigning such a deterministic value and thereby losing generality, we use the RSS model to calculate the minimum safe distance at which a reaction of the ego vehicle is required. With a set of conservative parameters suggested by [15] as described in Table IV, we will not miss scenarios requiring ego reaction. If a cut-in scenario is valid, we then investigate if non-cars, such as trucks or buses, are involved in the cut-in. This paper only considers car-involved cut-in scenarios; non-car-involved cut-in scenarios are ignored. Based on the definition of overlap cut-in, we further divide the cut-in scenarios into overlap and non-overlap cut-in scenarios. Following the steps outlined above, we finally obtained 376 cut-in scenarios. Specifically, Fig. 6. One example of an overlap cut-in scenario to demonstrate the dataset values to TJP system development. D. Parameter distribution The parameters listed in Table III are extracted in both overlap and non-overlap scenarios. Because the distributions of these parameters are unknown in advance, it is difficult to predefine the function form of the signal and then fit parameters to this function form. Instead, we employ kernel density estimation (KDE) to calculate each parameter’s probability density function (pdf). Due to the non-parametric characteristic of KDE, the shape of the pdf is automatically adapted under the given data, and the KDE is highly flexible in terms of the actual shape of a pdf. In our case, the Gaussian kernel function is applied to obtain the pdf of four parameters, as illustrated in Fig. 7. For the relative longitudinal distance 𝑑rel,𝑥0 at timestamp 𝑇1 , most challenging vehicles perform cut-in maneuvers with a small 𝑑rel,𝑥0 , which is consistent with the phenomenon in congestions due to small longitudinal space available. In overlap cut-in scenarios, the value of 𝑑rel,𝑥0 is less than zero. Apparent differences between overlap and non-overlap scenarios can be © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 8 found in the relative longitudinal velocity 𝑣rel,𝑥0 at timestamp 𝑇1 . The 𝑣rel,𝑥0 are mostly greater than zero in overlap cut-in scenarios, whereas the 𝑣rel,𝑥0 in non-overlap cut-in scenarios are normally distributed with a mean value of about -0.5 m/s. This indicates that the challenging vehicle is faster than the ego vehicle during the overlap cut-in maneuvers. In contrast, no preferred 𝑣rel,𝑥0 is identified during non-overlap cut-in maneuvers because the challenging vehicle is entirely ahead of the ego. Regarding the ego velocity 𝑣ego,0 at timestamp 𝑇1 , most cutin scenarios occur when the ego velocity is very low, with only a few cases occurring when 𝑣ego,0 is greater than 15 m/s. This is because the traffic in several lanes is less congested. In addition, the velocities of most ego vehicles in overlap cut-in scenarios are lower than in non-overlap cut-in scenarios. This is most likely one of the motivations for the challenging vehicle to perform an overlap cut-in. The maximum lateral velocity of the challenging vehicle during a cut-in maneuver 𝑣cha,𝑦,max also shows some distinctions between these two types of cut-in. The 𝑣cha,𝑦,max in most overlap cases is around 0.3 m/s, which is roughly half of the majority of 𝑣cha,𝑦,max in non-overlap cutin scenarios. Using the pdf generated by KDE, we can define logical scenarios for the scenario-based testing method aimed at testing a TJP system. Because no collision occurs in these reasonably foreseeable parameter distributions in the dataset, a TJP shall also have no crash in the generated scenarios derived from the pdf to achieve a comparable level of safety as human drivers. To effectively assess a TJP, the important sampling [43] [44] technique can be applied to emphasize the scenarios with higher criticality based on our findings. Fig. 7. The parameter distributions extracted from overlap and non-overlap cut-in scenarios. First row: the relative longitudinal distance is negative, and the challenging vehicle is faster than the ego vehicle in overlap cut-in scenarios. Second row: overlap cut-in occurs when the ego velocity is relatively low; the lateral velocity of the challenging vehicle is small during the overlap cut-in. E. Driver behavior analysis It is essential to understand how the driver behaves when confronted with an overlap cut-in. Although low longitudinal velocity is unlikely to cause fatal accidents, as observed in our dataset, it does cause issuance issues when a TJP cannot deal with this type of cut-in. Therefore, we analyze the following three research questions: RQ1: Does the ego driver evade laterally when confronted with an overlap cut-in? RQ2: Does the ego driver brake despite positive relative longitudinal velocity in the event of an overlap cut-in? RQ3: What is the ego driver’s preferred distance when a challenging vehicle enters the ego’s lane in overlap cut-in scenarios? To answer these three research questions and facilitate the © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 9 design of a human-like TJP, we analyze further the 90 overlap cut-in scenarios extracted from the dataset. Regarding RQ1, we analyze the lateral ego offset during the period [ 𝑇1 , 𝑇3 ] to identify if the ego will evade laterally to allow the challenging vehicle to cut in. We use linear regression to fit the time sequence lateral offsets, and the slope is utilized to determine if the lateral offsets shift or remain relatively constant. Finally, 64 ego vehicles show an increasing or decreasing lateral offset depending on the cut-in direction among 90 overlap cut-in scenarios, whereas 19 vehicles have a constant lateral offset. Because 𝑇3 equals 𝑇1 in 7 cases, no general trend can be drawn where the lateral offsets of the challenging vehicles already reach the position of 𝑇3 when entering the detection zone of a drone. To determine if the lateral inverse TTC (reciprocal of TTC) and the longitudinal ego velocity influence the lateral offset, we illustrate their relationships at timestamp 𝑇1 , because the challenging vehicle is adjacent to the ego at this timestamp, i.e., they are so close to each other in the longitudinal direction that the ego would evade laterally if the challenging vehicle cuts in. The results are shown in Fig. 8. The figure shows that shifting offset occurs more frequently when the ego velocity is within [5, 20] m/s. 20 m/s is the upper limit of velocity in the discovered overlap cut-in scenarios due to the congestion. In the low-speed area, no strong correlation is found when the lateral inverse TTC is also very low. Due to the low ego velocity in uncritical overlap cut-in scenarios, some drivers tend to leave space when a challenging vehicle cuts in, while others attempt to stop the cut-in of the challenging vehicle. Therefore, drivers tend to evade laterally to an overlap cut-in vehicle, especially when the ego velocity is slightly higher. distance headway (DHW) when reaching the maximum decelerations. The results are shown in the lower subplot in Fig. 9. Even though the deceleration values are negligible due to small velocities, the braking maneuver is preferred to be applied under small DHW values. Fig. 9. The upper subplot shows the average acceleration distribution when the challenging vehicle enters the ego’s lane. Braking maneuvers are identified despite positive relative longitudinal velocity. The lower subplot illustrates the DHW values when reaching the maximum deceleration. Braking is more frequent when the corresponding DHW is small. Fig. 10. The relationship between relative longitudinal velocity and the DHW. A linear relation is identified by analyzing the data points representing the timestamp at which the challenging vehicle completes its cut-in. Fig. 8. The influence of lateral inverse TTC and ego velocity on the lateral ego offset. The ego has a lateral offset when its velocity is larger than 5 m/s. No strong correlation between lateral inverse TTC and lateral ego offset is found. For RQ2, due to the small relative longitudinal distance, deceleration is applied in some cases, despite the challenging vehicle being mostly faster than the ego, as illustrated in the upper subplot in Fig. 9. During the period [𝑇3 , 𝑇5 ], the ego driver shall brake when necessary because the changeling vehicle enters the ego’s lane. It turns out that about half of the cases studied exhibit a deceleration process. To determine under what conditions braking is applied, we analyze the To facilitate the calibration of a suitable following distance for a TJP to avoid frequently cutting in by other challenging vehicles, we study the following distance when the challenging vehicle finishes cutting in, i.e., 𝑇5 timestamp is reached, to answer the third question RQ3. By drawing the corresponding relative longitudinal velocity along with the DHW, as shown in Fig. 10, we found that DHW tends to be small if the relative longitudinal velocity is small. Conversely, large DHW values are shown with large relative longitudinal velocities. As a result, the following distance shows a linear relationship with the relative longitudinal velocity at the timestamp of 𝑇5 with a correlation coefficient of 0.72. Many works [62] [63] have pointed out that safety metrics must be calibrated to achieve a tradeoff between safety and efficiency; our findings provide the © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 10 opportunity to calibrate those metrics to make motion planning more human-like. safety of AVs by building an automatic closed loop “recordprocess-testing”. V. DISCUSSION VII. REFERENCES We generated the first open-source dataset for supporting the design and verification of TJP systems. Unlike the HighD dataset, we recorded the trajectory data in four Chinese cities during peak hours. Most of the relative longitudinal distances are smaller than 10 m in congestion scenarios when a cut-in maneuver begins, which contrasts sharply with the results obtained at high driving speeds [64]. By distinguishing overlap from non-overlap cut-in scenarios, we demonstrated their parameter differences. In overlap cu-in scenarios, the challenging vehicle drives faster than the ego to successfully cut in. An overlap cut-in maneuver usually occurs when the ego is very slow. This is probably because the challenging driver feels safe cutting in under this condition. Meanwhile, the challenging vehicles also show smaller lateral velocities in overlap cut-in scenarios. Due to less available longitudinal space, they gamble with the ego drivers to understand if the ego driver would allow them to cut in. To interpret how the ego driver behaves in those overlap cutin scenarios, we analyzed the driver behavior by answering three questions. We discovered that most drivers evade laterally when the challenging vehicles cut in. It means that most drivers are willing to give way. Moreover, despite negative TTC in overlap cut-in scenarios, most drivers apply braking when the DHW is too small, which matches well with the margin distance described in [65] to maintain safety after braking. This following distance also depends on the relative longitudinal velocity. Their incremental relationship reflects the driving policy of drivers. The code for the analysis is available on GitHub (https://github.com/ADSafetyJointLab/AD4CHE). Although the dataset provides rich information about driving behavior in congestion scenarios, few critical scenarios are found. A future research direction could be to superimpose the driving environment or behavior in traffic accidents on the overlap cut-in scenarios to generate critical overlap cut-in scenarios. A driver model can also be studied to simulate human driver behavior in congestion scenarios. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] VI. CONCLUSION This paper proposed the first open-source Chinese largescale naturalistic traffic flow dataset focusing on congestion scenarios. During the processing and analysis of the dataset, we have the following findings: 1) the dataset has very high accuracy, and the data information such as yaw and lane marking positions, which are usually not provided in other open source datasets, are also available in our dataset; 2) drivers behave differently in overlap and non-overlap scenarios, and a gaming process exists in overlap cut-in scenarios; 3) ego drivers brake when the relative space is small, and they prefer to keep a large following distance when the relative velocity is high. 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Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply. This article has been accepted for publication in IEEE Transactions on Intelligent Vehicles. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/TIV.2023.3260902 12 [62] [63] [64] [65] N. Nadimi, H. Behbahani, and H. Shahbazi, “Calibration and validation of a new time-based surrogate safety measure using fuzzy inference system,” Journal of Traffic and Transportation Engineering (English Edition), vol. 3, no. 1, pp. 51–58, 2016. A. Rodionova, I. Alvarez, M. S. Elli, F. Oboril, J. Quast, and R. Mangharam, “How safe is safe enough? Automatic Safety Constraints Boundary Estimation for Decision-Making in Automated Vehicles,” in 2020 IEEE Intelligent Vehicles Symposium (IV), 2020, pp. 1457–1464. H. Nakamura et al., “Defining reasonably foreseeable parameter ranges using real-world traffic data for scenario-based safety assessment of automated vehicles,” IEEE Access, vol. 10, pp. 37743–37760, 2022. C. Wang, C. Popp, and H. Winner, “Acceleration-Based Collision Criticality Metric for Holistic Online Safety Assessment in Automated Driving,” IEEE Access, vol. 10, pp. 70662–70674, 2022. Yuxin Zhang (Member IEEE) received the Joint Ph.D. degree in Vehicle Engineering from Jilin University, China and UC Berkeley, USA, in 2016. From 2016 to 2019, he worked as a postdoc researcher in Systems Engineering in Jilin University, China, and also a Safety Researcher in UISEE, Beijing, China. Since 2019, he worked as an Associate Professor at State Key Laboratory of Automotive Simulation and Control, Jilin University, China, and also as a Safety Researcher in DJI Automotive. His main research interests include automated driving systems safety engineering, functional safety, and safety of the intended functionality. Prof. Zhang serves as Chair of SAE International Automated Driving FuSa and SOTIF Seminar from 2021, Expert Member of SAE International Automated Driving Safety Technical Committee from 2020, Technical Representative of Scenariobased Automated Driving Safety Standard ISO 34502, Functional Safety and SOTIF Standard ISO 26262 and ISO 21448, Safety for the Evaluation of Autonomous Products Standard UL 4600, and STPA Recommended Practices Standard SAE J3187 from 2019. He is also a Functional Safety Professional certified by TÜV SÜD, Germany from 2018. Cheng Wang received the B.Sc. degree from the school of automotive engineering at Wuhan University of Technology, Wuhan, China, in 2014 and the M.Sc. degree from the school of automotive studies at Tongji University, Shanghai, China, in 2017 and the Ph.D. degree from the institute of automotive engineering at the Technical University of Darmstadt, Darmstadt, Germany, in 2021. He currently works as a research associate at the University of Edinburgh. His research interest includes explainable AI and safety verification and validation of autonomous vehicles. Ruilin Yu received B.S. degree in vehicle engineering from Hefei University of Technology, Hefei, China, in 2021, and is currently pursuing a master's degree in Jilin University. His research interests include vehicle trajectory data analysis, SOTIF and perception system of automated vehicles. Luyao Wang received B.S. degree in vehicle engineering from Xi'an University of Technology, Xi’an, China, in 2021. Currently, he is a graduate student majoring in vehicle engineering in State Key Laboratory of Automotive Simulation and Control, Jilin University. His research interests include automated driving safety and planning. Wei Quan received the B.Sc in automobile engineering from Tongji University and the University of Applied Sciences for Engineering and Economics Berlin, German with double degree. After that, he received M.Sc in Automotive and Engine Engineering from the University of Stuttgart, Stuttgart, German. During his studies, he worked on the project of detection and tracking of ARS441 (Advanced Radar Sensor) in Continental AG, the research project of machine learning optimization processes in the University of Stuttgart and the project of ultrasonic sensor modeling in Robert Bosch GmbH. After his studies, he joined DJI Automobile and took part in the project of AD4CHE (Aerial Dataset for Congested Highway & Expressway). Yang Gao received the M.Sc. degree from Huazhong University of Science and Technology (HUST), Wuhan, China, in 2019. He is now a member of DJI Automotive. His research interests include deep learning and monocular SLAM. Xiaofei Li received the M.Sc. degree in automotive engineering from Chang’an University, Xi’an, China, in 2019. In the same year, he joined DJI Automotive and engaged in Safety of the Intended Functionality (SOTIF) related work. His mainly research interests include application of SOTIF methodology based on V model in the development process of Autonomous Driving Systems (ADS), safety analysis of planning and control algorithms and improvement of algorithm performance, and the scenario-based testing methodology used to complete the verification and validation of ADS. © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information. Authorized licensed use limited to: JILIN UNIVERSITY. Downloaded on March 27,2023 at 08:25:58 UTC from IEEE Xplore. Restrictions apply.