Analyzing and Modelling Drivers’ Deceleration Behaviour from Normal Driving

Most research in vehicle automation has mainly focused on the safety aspect with only limited studies on occupants’ discomfort. In order to facilitate their rapid uptake and penetration, autonomous vehicles (AVs) should ensure that occupants are both safe and comfortable. Recent research however revealed that people felt uncomfortable when AVs braked. This may be due to their robot-like braking performance. Existing studies on drivers’ braking behaviour investigated data either from controlled experiments or driving simulators. There is a dearth of research on braking behaviour in normal driving. The objective of this paper is therefore to examine drivers’ braking behaviours by exploiting naturalistic driving data from the Pan-European TeleFOT (Field Operational Tests of Aftermarket and Nomadic Devices in Vehicles) project. On a fixed route of 16.5km long, 16 drivers were asked to drive an instrumented vehicle. A total of about eleven million observations were analysed to identify the profile, value and duration of deceleration events. Since deceleration events are nested within trips and trips within drivers, multilevel mixed-effects linear models were employed to develop relationships between deceleration value and duration and the factors influencing them. The results indicate that the most used profile of the deceleration behaviour follows a hard braking at the beginning when detecting a danger and then becomes smoother. Furthermore, they suggest that the speed, the reason for braking and the deceleration profile mostly affect the deceleration events. Findings from this study should be considered in examining the braking behaviour of AVs. Deligianni, Quddus, Morris, Anvuur, Reed 3 INTRODUCTION Research on autonomous vehicles (AVs) has attracted a significant interest from the research community worldwide in recent years. Fundamentally, vehicle automation aims to eliminate or decrease human involvements from the routine tasks of driving (1). Ensuring human-like driving performance is however a basic condition for the wide acceptance of (semi)autonomous vehicles (2). Despite the remarkable research and development progress in the area of (semi)autonomous vehicles over the last decade, there is a concern that occupants may not feel comfortable due to the unnatural driving performance of the current technology which sometimes differs from the average human driving (2). Driver comfort is understood as a state which is achieved by the removal or absence of uneasiness and distress. Even though many factors may affect driver’s comfort (such as lateral and longitudinal acceleration, time headway etc.), one of the most important is the braking. In other words, a (semi)autonomous vehicle should decelerate in the style which could avoid mental discomfort to the people both inside and outside of the vehicle. Regarding the braking performance, stress and nervousness are apparent if the timing at which the vehicle automatically brakes differs from the driver’s own judgment or whether the level of deceleration is greater than the driver’s expectation or the deceleration profile does not follow the one that the driver is used to (3). It is therefore imperative to fully understand drivers’ braking behaviour with respect to the level of braking, the duration of braking and the deceleration profile in the context of normal driving. It should however be noted that hard deceleration is sometimes necessary in the case of emergency situations in order to avoid a collision. As a result, passenger tolerance to longitudinal deceleration would influence the design of the vehicle’s braking system (4). An efficient approach in designing (semi)autonomous vehicle would be to monitor and identify how human drivers perform the driving tasks and then analyse and characterise such behaviours with the aim of developing various thresholds so as to implement them into the system (5). Vehicle automation with respect to braking is then possible to be designed in such a way that it emulates human behaviour. This paper therefore focuses on the analysis of the deceleration events observed within normal driving with the aim of identifying the braking behaviours under different driving and operational conditions. The definition of normal driving is ‘subjective’ and there is no generic one in the literature. Moreover, perceptions of normal driving differ from country to country. In this paper, normal driving means that the drivers execute the driving tasks under ‘normal’ driving conditions i.e. the absence of any safety critical events such as ‘near misses’ or ‘crashes’. In particular, the paper aims to develop a statistical relationship between braking patterns, human factors, traffic and road network conditions. The purpose of this analysis is to inform vehicle manufacturers about the deceleration behaviour observed during normal driving so as to ensure comfortable and safe braking operations. The rest of the paper is organised as follows: firstly, the existing literature and its main findings are reviewed. This is followed by a presentation of the data used in the analysis. After that, the estimation of the deceleration profile and the statistical analysis are presented and the results are discussed. Finally, the last section summarises the main conclusions of the study. Deligianni, Quddus, Morris, Anvuur, Reed 4 LITERATURE REVIEW The analysis of the braking behaviour has gained attention in the literature over the past decades. A braking event is normally described using the deceleration value, the speed at the beginning and at the end of the event, the perception/reaction time and the duration (6). The aim of this review was to synthesise existing studies on drivers’ braking behaviour and identify thresholds related to comfortable braking performance. A gap in knowledge was also identified. Braking behaviour was studied during the design and implementation of Adaptive Cruise Control (ACC). It is perhaps the most studied feature of advanced vehicle systems (1; 5; 7). Goodrich et al. (5; 7) tried to identify the braking behaviour and apply the results to the design of ACC. In both studies a driving simulator and a controlled test track were used. The braking behaviour was characterised by the perceptual trajectory using time-to-collision (TTC) versus time headway. In their first study Goodrich et al. (5) conducted a series of experiments from which they concluded that in order to produce a comfortable performance, ACC designers need to develop controllers that emulate a trajectory, which does not violate the smooth counter–clockwise characteristics of the human–generated one. In the second study it was further postulated by Goodrich et al. (7) that in order to achieve a human interaction with the ACC system which results in safe and comfortable vehicle dynamics, the automated braking behaviour should match that of a skilled human operator. Consequently, the design and programming of the advanced vehicle system can benefit from a careful analysis of the human actions and the interaction between human and automation. Some studies have dealt with the factors which affect the braking behaviour (3; 8-10). For instance, Rakha et al. (8) conducted a study to analyse field data and to characterise driver deceleration rates at the onset of a yellow-phase transition on high-speed signalised intersection approaches. The study concluded that deceleration rates are sensitive to the roadway grade, the age, and the gender of the driver. In contrast to Rakha et al. (8) who examined braking behaviour during normal driving, Loeb et al.’s (9) study was conducted in a simulator and analysed the differences in emergency braking performance between novice teen drivers and experienced adult drivers. Their results showed significant differences both in performance and quality of braking between novice teens and experienced adults, with novice teens decelerating on average 50% less than experienced adults on the same scenarios, indicating a poor response. Haas et al. (10) evaluated driver deceleration and acceleration behavior at stop sign–controlled intersections on rural highways in southern Michigan. Their results seemed to indicate that drivers showed wide variability in rates of acceleration and deceleration and that the initial speed had a strong and statistically significant dependence on the deceleration rate while the other examined factors (e.g. driver demographics and time-ofday) had not. Finally, Kazumoto et al. (3) conducted a discriminant analysis on factors which influence the braking behaviour of drivers. The factors were the speed of the following vehicle, the distance between the two examining vehicles, the relative velocity, the TTC, the rate of change of visual angle. They found that the rate of change of visual angle, which is the inverse of TTC, is the most closely related factor to a driver's judgment about when to apply the brakes. On a more theoretical base, a categorisation of the factors that influence braking behaviour includes driver factors such as awareness, expected/unexpected need of action and experience, vehicle factors, and situational factors such as external environment (i.e. other road users, weather and traffic conditions) (11). Another classification relates to initiating and Deligianni, Quddus, Morris, Anvuur, Reed 5 mediating factors (12). Initiating factors have immediate effect on the driver’s comfort and are based on the driver’s direct interaction with the system, the environmental cues and the actual risk. On the other hand, mediating factors are more subjective but they may have a greater influence on how the driver feels when the brake is applied. These factors emerge from exposure to a system in conjunction with perceived risks, motivational factors (e.g. willingness to use automation), attitudes/biases (e.g. driving styles, trust in automation, and overall system use) and experiences. There are some studies focussing on m