Road Safety Manual
A manual for practitioners and decision makers
on implementing safe system infrastructure!

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8.1 Introduction

The Challenge

Roads are provided to cater for the movement of people and resources between destinations, i.e. to provide for mobility and access. Particularly in LMICs, roadside trading and social interaction continues to be an essential third function for parts of the road network. In these countries, the benefits of setting aside areas of public space, where sociability rather than mobility is the priority, are being increasingly recognised. Mobility, accessibility, and commercial/social interaction are therefore the three key human uses that roads have to be designed and managed to serve.

Earlier chapters introduced the concepts of the Safe System, and of safe travel as a product, which requires certain actions to produce it. Safe products must match the needs, capacities and expectations of their human users and roads are no exception. This chapter outlines how to create a road system that takes account of human characteristics and meets Safe System requirements.

Human factors are a well-established scientific endeavour that has influenced developments in many areas of technology. Its application to road safety issues in a formal sense goes back to at least the 1930s (e.g. Forbes, 1939). Contemporary understanding of issues, such as the time it takes to perceive and detect any critical location that challenges the driver to adapt his driving programme and to make new decision, the desired luminance, size and contrast between objects and the background needed to resolve detail, and the rate that information is absorbed, should underpin key standards in road design. Other important demands for road design arise from the holistic perception of the road user within the road environment. From there essential design principles of "Gestalt" have to be included in the technical design considerations. This understanding of the laws of human perception and activity regulation that includes the decision-making capacity of the road user allows for the development of design and operational specifications for the road system. This includes elements such as:

  • such as sight distance requirements
  • lighting criteria
  • design and dimensions of road signs, and
  • spacing between successive decision points

By effectively do so, the road users can navigate the road system safely and comfortably. Since knowledge in Human Factors continues to evolve, many of its findings remain to be absorbed in technical standards and guidelines. This chapter seeks to introduce the concept of Human Factors, relate it to Safe System principles, and explain how Human Factors can be applied to create a safer road system. Human Factors is the generic term for those psychological and physiological threshold limit values which are verified as contributing to operational mistakes in machine and vehicle handling. It deals with general and stable subconscious reactions of common road users and excludes temporary individual reactions and conditions. From there can be derived essential conclusions for basic design principles that are until yet not well established in current national design guidelines (PIARC Report 2012R36EN). Figure 8.1 Human Factors in the system of road safety

In road safety, human factors is concerned with the interaction of human road users with the roadway system elements, including vehicles. The distinction is often made between unintended errors and disobedience of road rules. Unintended errors tend to occur when road users misperceive some aspect of the road system, they do not have enough time to react to changing situations, or they are confronted with unexpected situations. These issues and the means of remedying are discussed in Designing Infrastructure to Encourage Safe Behaviour.

Disobedience of the road rules often occurs when the road system does not adequately meet road users’ needs, e.g. when there are long waiting times to cross at signalised pedestrian crossings. However, disobedience of the road rules may also occur when users do not understand what they are supposed to do or understand the benefits of compliance. This is particularly the case in LMICs as they rapidly motorise and upgrade their road networks. Disobedience may also occur because some road users believe they can gain a benefit (such as a faster journey or a more convenient parking or unloading spot) without incurring any penalty. These issues and some of the implications they have for infrastructure provision are discussed in Other Means of Encouraging Road Users to Behave According to the Rules

Human Factors is not the same as we commonly understand human behavior or human performance to be. So, the questions of personality traits like aggression, the will to violate traffic rules consciously or mistakes because of medication or age has to be regarded separately. From there can be derived essential conclusions for basic principles of driver's education, campaigns for influencing driving behavior and enforcement.

  • Design measures to create a self-explaining and failure-forgiving road according to Human Factors needs of road users, are described in Chapter 8.2
  • Measure of communication, education and enforcement including special warning signs and campaigns are described in Chapter 8.3

Human Factors and the Safe System

Human Factors have a key role to play in achieving Safe System requirements.

Safe System principles require that no road users are killed or seriously injured. In an ideal system, collisions would not occur because the road is designed according to the needs of perception, cognitive processing and motor response for all the users. This is unlikely to be achieved as long as humans directly control vehicles and many roads are not designed consistent with the road users needs. Even with the advent of automonous and connect transportation human control is still likely for some time into the future.

Efforts should therefore be made to help road users perceive the road correctly and to make decisions about driving, riding or walking that are safe for themselves and other road users. Applying the Human Factors principles described in the remainder of this section should go some way to achieving a collision-free road network, but it must be recognised that improving guidance will not always succeed in preventing collisions. That being the case, space to correct mistakes should be provided where possible, e.g. by having lane widths that allow some manoeuvre space, providing sealed shoulders, or by having stop lines some metres in advance of the walkway on pedestrian crossings. Adequate recovery space will reduce the number and severity of impacts; however, it will not always succeed in preventing impacts. Therefore, the Safe System requires forgiving infrastructure and forgiving vehicles so that when collisions do occur, they will not result in fatalities or non-recoverable injuries.

Key Sources on Human Factors and Road Design

The NCHRP report, Human Factors Guidelines for Road Systems (HFGRS) (Campbell et al., 2012) is a comprehensive source on Human Factors and road system design and management concerning the reaction time needs of road users. It is intended to supplement the primary design references and standards, so that designers who do not have an extensive Human Factors background will be better able to take account of road user reaction time capabilities and limitations in the application of these standards.

The World Road Association (PIARC) has published a Human Factors documents – Human Factors Principles of Spatial Perception for Safer Road Infrastructure (HFPSP) (PIARC, 2019). It is the most comprehensive approach to illustrate in a practical way the needs of road users for proper reaction time, for reliable guidance and stabilization of user’s field of view and to pre-program user’s expectations so that mistakes can be avoided by design. It is based on the state of the art of human sciences, especially on the Gestalt principles of the Gestalt psychology.  Gestalt is the impression of content of perception that is clearly distinguishable from its background of scenery and the details of which are so integrated as to constitute a functional unit with properties not derived from the summation of its parts. It aligns well with Safe System principles and advocates a proactive approach to safety management, with the aim of designing roads so that crashes are unlikely. The HFPSP guide provides a powerful and convenient method for applying Human Factors principles to a wide range of situations that are likely to be encountered by drivers. It does not explicitly consider pedestrians or other vulnerable road users as active participants in the traffic system. However, the essential principles are also applicable to these road users, and the reader is encouraged to do so in cases where it is appropriate in their own practice. The Human factors guidelines for a safer man-road interface (PIARC, 2016). Human Factors concept presented in the guidelines highlights how road characteristics that influence a driver's right or wrong driving actions. The guideline explains the relationship between several road characteristics that trigger wrong perception and therefore also wrong driving reactions, most of which happen subconsciously. The guideline containes detailed examples and sketches allow those using the guideline to gain an understanding between misleading and irritating road character-istics and operational mistakes. Better understandig the human factors relationship to road safety allows for an "on-the-spot" investigation of black spots or single vehicle accidents or in road safety inspections (RSI). This information is also very valuable in the planning and design processes in road safety audits (RSA). 

These documents provide powerful and convenient methods for applying human factors principles to a wide range of situations that are likely to be encountered by drivers. The following illustration outlines the damage and prevention oriented accident approaches (Birth, Sieber and Staddt, 2004). 

Figure 8.2  Damage and Prevention Oriented Accident Approaches

Reference sources

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