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 and critical location that challenges the driver to adapt his driving programme and to make new decision, as well as the desired luminance, size and contrast between objects and the background needed to resolve detail, and the rate that informatioin 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 cpacity 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 comfrotably.

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. The US National Cooperative Highway Research Program (NCHRP) report, Human Factors Guidelines for Road Systems (Campbell et al., 2012), provides a definition:

Human factors is an applied, scientific discipline that tries to enhance the relationship between devices and systems, and the people who are meant to use them. In road safety, human factors is concerned with the interaction of human road users with the roadway system elements, including vehicles. PIARC's 2012 Human Factors in Road Design the definition is Human Factors is the generic term for those psychological and physiological threshold limit value which are verified as contributing to operational mistakes in machine and vehicle handling. It deals with the 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.  Since the focus of this Manual is on the application of Safe System principles, the first definition shown above was used as it emphasises the role of human factors in connecting system elements.

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 quesitons of personality traits like aggression, the will to violate traffic rules consciously or mistakes because of medication or age has to be regarded separately

As highlighted in Scope of the Road Safety System pedestrians, cyclists and motorcyclists make up more than half of the world’s road fatalities; and make up considerably more than half in some regions of the world. At best, these vulnerable road users can expect limited protection from impact forces in the event of a collision; e.g. from helmets, some protection from the engine casing and front screen of a motor scooter, or ‘forgiving’ front end designs of cars and vans to mitigate impacts with pedestrians. Reducing death and serious injury and providing safe travel conditions for this group of road users is one of the major challenges facing the roads sector.

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. The closer we can get to collision-free roads, the safer the system will be. Even with the advent of automonous and connect transportation human controll 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. 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 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, 2015) and the Human factors guidelines for a safer man-road interface (PIARC, 2016). The 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). 

Damage and Prevention Oriented Accident Approaches

The HFPSP guide considers three key requirements, presented below.

Requirement No 1: the driver must have sufficient time to react

It takes about six seconds for a driver to adapt to a changed situation on the road and to re programme their driving actions. Figure 8.1 shows the stages and the approximate time required for each action; note that there may be some overlap between these times. First, the driver or rider has to notice that something requires attention and to identify the situation; second, they need to decide on and plan the driving action required and to mentally test that it is appropriate; third, time is required for the corrective action itself and for the vehicle’s systems to take effect. In the first two stages, it is assumed that drivers will continue to travel at their original speed, but the time and distance required to stop in the third stage varies considerably and depends on the original speed. A look up table relating time and distance to operating speed is provided in the PIARC guidelines. At 100 km/h, the distance covered before the vehicle can be brought to a complete stop is up to 300 m, allowing for braking distance (note that this may take longer if braking is slow due to a wet road or other circumstances).

The usual ways to avoid this situation in practice are to:

  • eliminate the problem by providing an unobstructed view of the critical point. This is done by attention to road alignment and other design elements;
  • draw attention to the critical point (if the problem cannot be eliminated) by measures such as coloured road surfaces, pavement markings, and other advisory treatments;
  • warn road users and seek to directly influence their behaviour (if attention cannot be drawn to the critical point) by installing traffic control devices such as pedestrian crossing facilities, or road markings or signs to prevent overtaking.

Figure 8.1 The six second requirement - Source: PIARC, 2015.

Note that the times for the driving actions on the left of the diagram are approximate and may overlap.

Requirement No 2: the road must provide a safe field of view

Movement through the environment results in a constantly changing field of view. Driving a motor vehicle can result in a rapidly changing field of view. Perception of space and distance is complex and relies on a number of different cues, such as the relative movement of different objects in the visual field and the apparent size of familiar objects (people, cars, etc.), but we are not consciously aware of these cues. The complexity of the field of view and the speed at which it changes can give rise to errors in space perception that can lead to high-risk driving actions, such as approaching at too high a speed or taking the wrong line through a curve. Factors that detract from a safe field of view include:

  • monotonous approaches to a critical point e.g. vegetation, buildings, tunnels;
  • unbalanced features in the visual field, e.g. asymmetrical structures over the road, highly skewed objects and non-vertical roadside objects;
  • eye-catching objects;
  • guidance cues that are not parallel with the travel lanes, such as safety barriers, bicycle paths or markings (especially old markings that have not been completely erased);
  • framing of the outside of curves (gaps breaking continuity in the vegetation, visual obstructions, including from the inside of the curve);
  • partial obstruction of the course of the road (e.g. when overtaking, crests or dips).

These factors can be addressed by:

  • eliminating the problem by redesigning the approach to remove the misleading cues. If possible, the roadside should be redesigned in a holistic manner to complement the road design;
  • correcting the field of view (if the problem cannot be eliminated), e.g. remedying gaps in the framing of the outside of curves, removing vegetation to improve visibility across the inside of curves;
  • warning road users (if satisfactory correction cannot be achieved) and seeking to directly influence their behaviour by installing traffic control devices such as advisory speeds, speed limits, overtaking prohibitions or pedestrian crossing facilities.

Requirement No 3: the road environment must correspond with the road users’ perception logic

It is essential that the road matches the road users’ expectations; otherwise they are likely to make mistakes that can lead to crashes. As they drive along a road, drivers become used to characteristics such as width and curvature, and to a particular standard of sign provision and line marking. When any one of these characteristics change, a change in driving behaviour is called for (e.g. changes in speed, lateral placement or readiness to respond to emergency situations). If the driver does not respond to the changed road environment, a hazardous situation may occur. The PIARC HFPSP guide gives examples of how road environments fail to meet drivers’ expectations. These include:

  • no visual cues to reinforce a change in function, e.g. at the transition from a rural road to a town;
  • change in a road’s direction that is in conflict with eye-catching landscape features leading in another direction;
  • sudden changes in road characteristics, e.g. change in alignment or width with no transition zone or warning;
  • sudden increase in driver workload, e.g. when many critical points are encountered in quick succession;
  • deficiencies in traffic control devices, e.g. poor visibility, device no longer consistent with changed road environment;
  • deficiencies in direction signage; typically too much information combined with small sign size;
  • wrong relationship between road and a settlement, e.g. busy road dividing a settlement instead of by-passing it.

Remedies suggested in the PIARC HFPSP guide include:

  • eliminating the problem by ensuring consistent design of road sections with the same function, and ensuring good visibility of critical points where the road function changes;
  • correcting inconsistent road sections (if the problem cannot be eliminated) and attracting attention to critical points by coloured surfacing, changes in surfacing material (e.g. brick pavers in areas which are shared by pedestrians and motor vehicles), or changes in travel path (e.g. traffic island);
  • warning road users (if satisfactory correction cannot be achieved) and seeking to directly influence their behaviour by installing traffic control devices such as speed limits, overtaking prohibitions or pedestrian crossing facilities.

Figure 8.2 provides an example where the road direction is in conflict with the landscape, potentially leading the road user to expect that the road goes straight ahead rather than to the right (left images). Improvements can be made by providing clearer guidance through changes in the road environment (right images).

Figure 8.2 False cues regarding lane direction (left) and corrected visual cues (right) - Source: Birth, Pflaumbaum & Sieber, 2006, cited in PIARC (2012b).

Figure 8.2 False cues regarding lane direction and corrected visual cues - Source:Birth, Pflaumbaum & Sieber, 2006, cited in PIARC (2012b)

In addition to these points raised in the PIARC HFPSP guide, it is particularly important to realise that pedestrians and other vulnerable road users also develop expectations about using roads. It is important that road designers and managers understand these expectations and the needs for mobility, access and social interaction that underpin them. Designers and managers should then make adequate provision in the form of footpaths, trading areas, and paths for bicycles or motorcycles. In LMICs, it should not be assumed that the community will automatically understand what activities are appropriate for these different facilities, and it may be necessary to plan for a period of community engagement before and after the introduction of new facilities to ensure their appropriate use.

CASE STUDY – Portugal:  Perceptual treatment - Low Cost Engineering Measures on a dangerous trunk road

The case study describes the approach used in 1998 by the Circulation and Safety Division of the Portuguese Road Administration (JAE) to improve road safety on the interchanges of a single carriageway, two lane, trunk road that connected the Portuguese coastal area and Spain (route IP 5). In a first phase, Low Cost Engineering Measures (LCEM) were applied, to improve the road characteristics; in a second step, exceptionally intense and severe law enforcement actions were employed by the police, to improve driver behaviour. Read more (485 kb). 


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