CHAPTER 1
Why Have Human Factors Guidelines for Road Systems?

The Role of the Human Driver in Roadway Safety

Basic crash statistics in the United States highlight the importance of human factors to road system design. In 2021, there were more than 6 million police-reported crashes in the United States, with attendant loss of life, property, and productivity (National Center for Statistics and Analysis, 2023c). While crashes are complex and it is generally interactions between road users, vehicles, and the environment that lead to crashes, numerous studies have found that some form of driver error (e.g., recognition errors, decision errors, performance errors, inattention, impairment) is a contributing factor in most crashes (approximately 94%; Treat et al., 1979; Wierwille et al., 2002; Singh, 2015; Dong and Wood, 2023).

In particular, “error” means that drivers do not always perform their tasks optimally. While many driver errors are due to deliberate and aberrant behaviors (e.g., alcohol- or drug-impaired driving, speeding, texting while driving), many are due to roadway design and traffic operation features that unknowingly place demands on road users that exceed their capabilities, contributing to misperceptions, slow reactions, and poor decisions. These are the human factors issues that a more driver-centered approach to highway design and operation will address to promote continued improvements in highway safety performance. Addressing human factors issues on our roadways is also entirely consistent with the Safe System Approach (SSA); a key principle of which is that “Drivers make mistakes.” SSA therefore calls for the planning, design, and operation of road systems in a way that recognizes humans make mistakes, have limited physiological abilities to safely negotiate complex situations, and have a limited tolerance of kinetic energy forces (see also Finkel et al., 2020; Signor et al., 2018; Welle et al., 2018). Applying the SSA to design and operations (as well as diagnostics) will involve consideration of all road usersʼ needs, as well as specific topics such as perception-response time, driver expectations, visibility of roadway elements, and the demands of the roadway environment relative to the capabilities of the user.

The study of human factors applies knowledge from domains such as psychology, physiology, sociology, biomechanics, and kinesiology to the design of systems, tasks, and environments to promote and support their safe and effective use. The goal of understanding the effects of human factors is to reduce the likelihood and consequences of human error by designing systems, tasks, and environments that consider inherent, relatively stable human capabilities and recognize usersʼ limitations.

These human capabilities and limitations are relevant to roadway design, driver performance, and the general safety performance and crash potential of a roadway. Road safety outcomes reflect complex interactions between different road users (e.g., drivers, pedestrians, bicyclists), vehicle types (e.g., cars, motorcycles, trucks, and trains), and elements in the environment (e.g., roadways, markings, signs, and weather conditions). The design of the roadway system impacts numerous safety-relevant behaviors on the part of drivers, such as where they are looking as they

drive, maintaining lane position, and their speed selection. These behaviors have been studied by human factors researchers and practitioners for many years. Drivers make mistakes because of their physical, perceptual, and cognitive limitations; some of these mistakes may lead to conflicts between road users or even near misses, but others may lead to crashes that result in injuries or deaths. Critical to understanding the role of human factors in roadway safety is to consider the relationships and interactions between roadways and roadway users. In essence, roadways are communication devices, and at all times are sending messages to the road user. The job of the roadway designer is to design the road in such a way that the right messages are sent at the right time. Application of the Human Factors Guidelines (HFG) and related tools to the roadway design and operations process can thus be seen as a means to improve how, where, and when the roadway communicates with the road user.

Critically, HFG users should recognize the distinction between human factors issues and aberrant driver behavior issues, as they reflect different contributing factors to crashes with corresponding implications for selecting targeted and effective countermeasures. Human factors issues include contributing factors to crashes that most often reflect mismatches between the demands placed on the road user by the roadway infrastructure and the inherent physical, perceptual, and cognitive capabilities and limitations of road users. On the other hand, aberrant driver behavior issues include contributing factors to crashes that generally reflect deliberate violations of law or unsafe driving practices, such as texting while driving, inattention, or driving while impaired by alcohol. Roadway safety professionals and practitioners sometimes conflate these topics and believe “human factors” only include what are more properly viewed as “aberrant driver behavior” issues. A key aim of the HFG is to provide practitioners with a better understanding of human factors and of the capabilities and limitations of road users to help them design roadways that reduce and mitigate fatal and severe injury crashes.

The Purpose of the Human Factors Guidelines for Road Systems

The purpose of the Human Factors Guidelines for Road Systems (HFG) is to provide the best factual information and insight on the characteristics of road users to facilitate roadway designs and countermeasure selections that reduce crash potential and support operational decisions.

Traffic engineers apply technology, science, and human factors to the planning, design, operations, and management of roads, streets, bikeways, highways, their networks, terminals, and abutting lands (Seyfried, 2013). Thus, the discipline of human factors is recognized as an integral contributor to traffic planning and traffic engineering practice. Many transportation planners, highway designers, and traffic engineers, however, do not have a clear understanding of what the study of human factors is and how its principles are relevant to their work. The HFG equips highway designers, traffic engineers, and other road safety professionals with human factors principles, syntheses of key literature, and design solutions supported by scientific evidence. It may also be useful to others beyond the traffic safety community, such as urban planners and rural economic development specialists. It will allow the non-expert in human factors to more effectively bring consideration of the road userʼs capabilities and limitations into the practice of design, operations, and safety through better understanding of the nature of crashes, improved assessments of crash potential within roadway facilities, and the selection and implementation of targeted countermeasures.

A number of existing guides, standards, policies, and references are available to facilitate roadway designs that reduce crash potential and support operational decisions, including A Policy on Geometric Design of Highways and Streets (AASHTO, 2018a), the Manual on Uniform Traffic Control Devices (MUTCD) (FHWA, 2023b), and the Highway Safety Manual (HSM) (AASHTO,

2010). However, these materials often lack a substantive presentation and discussion of human factors principles and concepts that could be used by highway designers and traffic engineers to improve roadway design, traffic safety, and road user behaviors. Despite widespread acknowledgement that traffic safety reflects the consideration and integration of three components—the roadway, the vehicle, and the road user—the information needs, limitations, and capabilities of road users are often neglected in traditional resources used by practitioners. While many roadway design practices are based on extensive, well-documented, and fully appropriate data, this is not always the case. The HFG is intended to supplement existing resources by providing a comprehensive approach to human factors by:

• Providing guidance supported by the best available empirical studies relevant to road user performance.
• Prioritizing recent data to present guidance that is representative of current road user behaviors.
• Presenting clear information about road usersʼ capabilities and limitations.
• Incorporating recent changes in communications technology, vehicle features, roadway features, roadside environment, traffic control devices, and traffic operational characteristics.
• Reflecting the special needs of older and challenged drivers and vulnerable road users, including bicyclists, visually impaired pedestrians, and pedestrians with mobility limitations.
• Addressing the design trade-offs between conflicting demands that are related to important road user characteristics.
• Addressing specific combinations of roadway design features that can have an impact on road user behavior and subsequent crash potential.

The HFG reflects both an immediate need for better information about human factors and the availability of a great number of high-quality research studies examining driver behavior and performance. It provides guidance and countermeasures for roadway location elements (e.g., curves, grades, intersections, construction/work zones, rail-highway grade crossings) and traffic engineering elements (e.g., signing, changeable message signs, markings, traffic signals, lighting); it also provides tools for incorporating human factors into the broader process of assessing safety performance. Overall, the HFG is intended to support and improve the decision-making processes associated with roadway planning, design, and operations. Specific ways that the HFG can be used include:

• Enhancing initial roadway planning and design activities.
• Conducting diagnostic assessments of road and intersection factors leading to increased crash potential.
• Supporting road safety audits.
• Identifying and selecting safety countermeasures.
• Educating planners, traffic engineers, and designers on user needs, capabilities, and limitations.

Roadway design and traffic operations activities are complex endeavors that are guided by a number of design resources (e.g., national standards and local design specifications) and considerations. The HFG complements other primary design references and standards—it does not duplicate or replace them. It is an additional tool the traffic safety professional can use to improve the safety performance of roadways; it is up to the HFG user to adapt the guidance presented here to their individual situations and associated constraints. For example, the HFG and the HSM are both intended to provide practitioners with practical information about quantitative crash reductions and human factors. Using them together can provide complementary information for finding countermeasures to address the design and operational factors leading to crashes and can help improve decisions that reduce crash potential (Campbell, Hull, and Maistros, 2018).

Also, users of the HFG should exercise engineering judgment and identify trade-offs (e.g., multimodal considerations, land use, context, costs, and maintenance) to assess the applicability of the potential countermeasures provided in the HFG to individual sites of interest. The

potential countermeasures identified in the HFG may not be applicable to all sites of interest, and the HFG does not necessarily provide an exhaustive list of countermeasures that could be applied to all sites of interest.

Applying Human Factors to Road System Design and Operations

As noted above, human factors is the scientific discipline concerned with understanding the interactions between people, the products and machines we use, and the environments in which we work and live. In the context of road system design and operations, human factors researchers study the contributions of road user factors (e.g., age, impairment, inattention), vehicle factors (e.g., height, safety features, location of A-pillars), and environmental/roadway factors (e.g., road functional class, geometrics, signs, markings, lighting) to road safety performance. Critically, human factors are not the same thing as human behavior as it relates to roadway safety and engineering. In this regard, some might consider “human factors” to merely address overt, specific behaviors that increase the likelihood of crashes, such as speeding,¹ impaired driving, road rage, or intentionally engaging in distracting behaviors. This is correct—all of those behaviors are rightly considered “human factors” issues. But a more comprehensive view of human factors goes deeper than that and includes the relationships and compatibilities between the demands of the driving task at a particular roadway location and under a specific set of circumstances and the capabilities and limitations of the road user.

The dynamic relationships between roadway demands and user capabilities might include perceptual demands (e.g., can drivers see the roadway alignment under all operating situations?), cognitive/decision-making demands (e.g., does the placement of a wayfinding sign provide enough time for drivers to change lanes ahead of a complex interchange?), and physical/motor demands (e.g., does the excessive skew at an intersection prevent drivers from turning their heads to check for oncoming traffic or limit their ability to quickly detect oncoming traffic?). It also includes complex relationships between roadway elements and driver expectations. For example, typical speed limits on urban freeways of 55 mph are often disregarded by motorists as unrealistic, which raises questions about where and how our roadway designs may be mismatched with driver intuitions and expectations.

In this regard, the importance of supporting adequate visibility of key roadway design elements cannot be overstated. Vision is the primary source of information when operating a motor vehicle. Some researchers have estimated that as much as 90% of information for the driving task is captured through the eyes (Dewar and Olson, 2007). In general, a driverʼs ability to respond quickly to an object in the roadway also depends on the distance available to perceive the object (i.e., sight distance; see Chapter 5). Sight distance depends on scene illuminance and visibility, especially during nighttime conditions, as well as roadway design features such as vertical curvature.

Pedestrian visibility is influenced by several factors, including lighting conditions, the size of the pedestrian, and the color and reflectivity of the clothing worn by the pedestrian. Sight distance is directly related to the time drivers have to respond to an obstacle or situation ahead. The more visible an object or situation is, the more time drivers have to perceive, decide, and respond. Conversely, the less visible the object or situation is ahead, the less time drivers have.

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¹The word “speeding,” as used in this report, can be defined and interpreted in at least two ways: (1) exceeding the posted regulatory speed limit and (2) traveling at a speed that is too fast for roadway and traffic conditions, usually above the 85th to 90th percentile value. Individual uses of the word “speeding” in this report may be unclear as to which definition should be used (e.g., when crash data indicate that speeding is a contributing factor). We leave it to the reader to decide which definition applies to individual uses in this report.

Overall, we can see that human factors encompass much more than simply the “bad behaviors” of drivers. Ideally, standards and guidelines to support roadway planning, design, and traffic operations will be consistent with user expectations, capabilities, and limitations. In addition, safety analyses and diagnostic assessments will include consideration of these capabilities and limitations when assessing contributions to crashes and associated countermeasures.

Addressing human factors considerations is especially valuable for state transportation agencies that have shifted away from a nominal approach to safety, which relies on compliance with well-accepted design criteria, towards a substantive approach that quantifies the safety performance of a facility in terms of crashes using data-driven methods. While the substantive safety approach offers flexibility and the opportunity to consider the broader context of a facility when making design decisions, it requires a robust assessment of the various cost and performance trade-offs that are at the core of state agenciesʼ decision-making processes. As noted above, highway safety professionals frequently face unexplained gaps in commonly used design handbooks and guidelines. These gaps create uncertainties or “gray areas” when planning and designing a roadway, and when conducting regular safety audits, maintenance, and upgrades of the facility. These ‘gray areas’ require science-based information that includes human factors inputs so that, for a given driving context, safety professionals can better determine the acceptable safety performance of a facility in terms of crash and severity potentials. Critically, there is no such thing as a ‘safe facility’ (i.e., one without crashes or injuries); road safety is not a binary concept, but rather a continuous system of interactions, many of which involve road users. For example:

• On a rural road, how will the installation of shoulder rumble strips (SRS) impact bicyclist safety, given the available shoulder width? Bicyclists need separation from traffic, as well as sufficient gaps to cross rumble strips in advance of intersections and to avoid objects in the roadway. What are the expected safety outcomes of adding SRSs, including both the potential benefits of alerting drivers who stray onto the shoulder, the safety implications for bicyclists, and the possible impacts on the vehicle fleet given the increasing number of vehicles with features that assist drivers with lane keeping?
• At a complex freeway interchange, where can guide signs be located, and how should lane designations and destinations be displayed on these signs? Drivers should be able to quickly associate sign information with their current and desired lanes to safely position themselves well in advance of the roadway split. How well do different signing options (e.g., sign locations, layout of sign information, organization of multiple destinations on a sign) support driver expectations and performance, and what are the crash potentials of these options?
• In a busy urban environment, should a bus stop be located midblock or on the near or far side of an intersection? Trade-offs include concerns about congestion and delays from stopped buses, impacts to sight lines, pedestrian behaviors with crash potential, such as midblock crossings, and the potential for rear-end crashes and conflicts with turning vehicles. What are the safety outcomes associated with the bus stop location options, and how can these outcomes be mitigated by the addition of striping, special signing, or bus turnouts?

These are the types of everyday decisions and questions faced by road safety professionals; providing answers to these and similar questions is the aim of the HFG.

Scope and Limitations of the HFG

The HFG is intended to provide:

• An introduction to the field of human factors as it is applied to highway design and traffic engineering.
• Guidance for the more optimal design of highways and traffic control devices.

• Information linking human factors data and analysis with related published guidance in other key highway design and traffic engineering reference documents.
• Help in solving problems related to road user considerations, including the identification of probable human factors causes or countermeasures.
• Objective, defensible information that can be used to support and justify design decisions.

In addition, the HFG has some limitations. Specifically, the HFG is not:

• A design standard or a source of design mandates.
• An alternative to primary design references in highway design and traffic engineering. It is intended to complement and amplify, not replace, other published references, such as the MUTCD (FHWA, 2023b), A Policy on Geometric Design of Highways and Streets (AASHTO, 2018a), the Traffic Control Devices Handbook (Seyfried, 2013), the Highway Safety Manual (AASHTO, 2010), and other guidance.
• A source for comprehensive design specifications or a redundant treatment of other documents. The HFG is meant to add to and refine existing guidance and concepts.
• A textbook or tutorial on human factors or a comprehensive source of human factors literature.
• A guide to crash investigation or a comprehensive reference for safety diagnosis.