3. Risk-based analysis
3.1. Concepts and Terminology
3. Risk-based analysis
two or more road users, in which the action of one user causes the other user to make an evasive manoeuvre to avoid a collision‖ (30).
With such definition, it is possible to distinguish with the terminology of ―hazard‖.Obviously, the terminology of harzard cover cases of not only vehicle-vehicle but vehicle-obstacles also.
In this research, conflicts are defined as those situations where the vehicle (the group of vehicles) are forced to change its speed (accelerate/decelerate) and/or its trajectory (swere).
Many researches (e.g. Bald 1991, Bald et al. 2008) define safety as the absence of possible damage, or implies freedom from danger. The ultimate level of safety desired by human beings is to be in a situation without any risk of personal accident, injury or material damage. In reality, this is impossible because a widespread set of dangers cannot be avoided completely. So safety generally refers to the level of danger that is socially acceptable in a real-life situation. The safety performance of a technical system (transportation system) is the measurable consequence of the extent to which it behaves as expected, with and without the interaction of human beings. The objective is to come as close as possible and reasonable to the ideal safety performance.
Traffic safety can be described best by risk value. In Bald |1991|, risk is explained as the product of the probability and the extend of a possible damage.
Risk can be described as the combination of this product for all possible extend values of damage.
The important characteristics of the risk term are:
- It is a probability statement about the future.
- The risk is higher, the more often the damage occurs.
- The risk is higher, the bigger the damage will be.
Bilal M. Ayyub (2003) and Mohammad Modarres (2006) also stated that risk is ameasure of the potential loss occuring due to natural or human activities. Risk assocuated with an event or a scenario of events, therefore, has two primary attributes of interset: risks are the occurrence likelihood and occurence consequence of an event. Such definitions are also relevant with the above definition of risk.
External risks and internal risks. The research aims at breaking new ground in road traffic micro-simulation for safety assessment, particularly applying in MD traffic flow. The new method is supposed to be constructed with the co-operation of micro and macro simulation methods. That means, in a MD traffic flow, each group of motorcycles (classifying into groups based on specific driver behaviours) will be treated as an independent object.
External risks are defined as risk level of a motorcycle group in the context of potential conflicts/damages with other vehicle groups. Internal risks, in turn, are determined as levels (and its probability) of potential conflicts/damages among vehicles in the same group.
In the 1980s Mahalel [1983] proposed to analyse the whole system ―road traffic‖ to describe the correlation between reference values and accidents. Bald et al. (2008) defined: Risk analysis in general is a systematic, model orientated and modular approach to analyse safety issues and impacts on roads.
Risk analysis methods define safety as a state of very low probability to suffer damage (mathematically risk is a number considering the probability and the extent of (negative) consequences). Historical data which describe the past may give hints to find values for these probabilities. The distribution probabilities are assumed to describe the future. Most of risk-based methodologies aim at modelling the cause-and-effect chain. The most accepted and most generally formulated among them is the Failure Mode Effect Analysis (FMEA).
Human factors
The ITE Traffic Engineering Handbook (Pline, 1999) cites a definition of ―traffic engineering‖ as ―that branch of engineering which applies technology, science, and human factors to the planning, design, operation and management of roads, streets, bikeways, highways, their networks, terminals, and abutting lands.‖ Thus the discipline of human factors is recognized as an integral contribution to traffic engineering practice.
However, many highway designers and traffic engineers do not have a clear understanding of what human factors is and how its principles are relevant to their work.
Human factors is the scientific discipline that studies how people interact with devices, products, and systems. It is an applied field where behavioural science, engineering, and other disciplines come together to develop the principles that help assure that devices and systems are usable by the people who are meant to use them. The field approaches design with the ―user‖ as its focal point. Human factors practitioners bring expert knowledge concerning the characteristics of human beings that are important for the design of devices and systems of many kinds. The discipline contributes to endeavours as complex as space exploration and to products as simple as a toothbrush. In the field of transportation engineering, there have been numerous important contributions from human factors, but these are not always self-evident. Sight distance requirements, work zone layouts, sign placement and spacing criteria, dimensions for road markings, colour specifications, sign letter fonts and icons, signal timing – these and many more standards and practices have been shaped by human factors evaluation.
Driver behaviour aims at describing and analysing driver behaviour in situ (as safe or unsafe, legal or deviant,…), to identify the internal factors (relating to the driver himself, such as his experience) and the external factors (the technical and social environment of driving) that account for this behaviour, and to reveal the psychological processes (perceptual, cognitive, motivational,…) that govern drivers´activity.
In the research, there raise the requirement of distiguishing between two different types of chain:
chain of actions based on time order, and chain of driver behaviour.
Chain of actions can be defined as a chain of activities that a normal driver usually conducts in the proceed of traffic participation. For exmaple, when conducting left-turning maneuvre at intersections, the driver has to approach the intersection, signalize his desire to turn, wait (if needed), enter the intersection area, move inside the intersection area and leave the intersection. In other words, ―chain of actions‖ describe ―which activities do drivers conducts‖
Bald (1991) describes chain of driver behaviour as follows: The driver obtains information by processing optical pictures which he receives within his “driving space”. Driver behaviour therefore depends a lot on his driving space (including infrastructure, traffic flow, traffic control and bound
conditions). In some specific cases, driver behaviour depends much more on traffic control and operation.
Driver behaviour is obviously the results from consideration to balance between acceptable risks (probability of accidents/conflicts) and urgent need to reach destination. Acceptable risk is higher, faster goes the driver. Otherwise, the driver will ride the vehicle slower but safer.
Briefly speaking, chain of driver behaviour describes ―which type of reaction does a specific driver conduct in order to respond to a specific traffic situation‖.