Human Factors Guidelines for Road Systems 2021 Update, Volume 1: Updated and New Chapters (2022)

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HFG WHY HAVE HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS? VERSION 2.1 1-1 CHAPTER 1 WHY HAVE HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS? 1.1 The Role of Human Factors in Crashes Basic crash statistics in the United States highlight the importance of human factors to road system design. In 2015, there were more than 6 million police-reported crashes in the United States, with attendant loss of life, property, and productivity (NHTSA, 2016). While crashes are complex and it is generally interactions between road users, vehicles, and the environment that lead to crashes, some form of driver error (e.g., recognition errors, decision errors, performance errors, and non- performance errors) is a contributing factor in most crashes (HSM, 2010). “Error” means the driver did not perform his or her task optimally. Misperceptions, slow reactions, and poor decisions are the products of a poor match between the needs and capabilities of drivers and the task demands that they face on the roadway. A more driver-centered approach to highway design and operation will promote continued improvements in highway safety performance. 1.2 The Purpose of the Human Factors Guidelines for Road Systems The purpose of Human Factors Guidelines for Road Systems (HFG) is to provide the best factual information and insight on the characteristics of road users to facilitate safe roadway design and 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 (Pline, 1999). Thus, the discipline of human factors is recognized as an integral contributor to traffic engineering practice. Many highway designers and traffic engineers, however, do not have a clear understanding of what 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 will allow the non-expert in human factors to more effectively bring consideration of the road user’s capabilities and limitations into the practice of design, operations, and safety. A number of existing guides, standards, and references are available to facilitate safe roadway design and operational decisions, including A Policy on Geometric Design of Highways and Streets (AASHTO, 2018), the Manual on Uniform Traffic Control Devices (MUTCD; FHWA, 2012), and the Highway Safety Manual (HSM) (AASHTO, 2010). However, these materials often lack a substantive presentation and discussion of human factor principles and concepts that could be used by highway designers and traffic engineers to improve roadway design and traffic safety. Despite a widespread acknowledgment that traffic safety reflects the consideration and integration of three components—the roadway, the vehicle, and the roadway user—the information needs, limitations, and capabilities of roadway users are often neglected in traditional resources used by practitioners. While many roadway design practices are based on extensive, well-documented, and fully appropriate behavioral data, this is not always the case. Some design practices recommended by existing standards and guidance can include the following limitations:

HFG WHY HAVE HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS? VERSION 2.1 1-2  They do not have any empirical basis and/or have not been formally evaluated for adequacy for road users.  They are based on outdated data that may no longer be representative of current driver behaviors.  They are based on overly simple models of what road users see or do.  They are based on incorrect assumptions about road users’ capabilities and limitations.  They do not reflect recent changes in communications technology, vehicle features, roadway features, roadside environment, traffic control devices, or traffic operational characteristics.  They do not reflect the special needs of some road users, such as older drivers, visually impaired pedestrians, pedestrians with mobility limitations, heavy truck operators, and users of lower-speed alternative transportation devices.  They do not adequately address trade-offs between conflicting demands that are related to important road user characteristics.  They may not address specific combinations of roadway design features that can have an impact on road user behavior and subsequent safety. In short, existing references applicable to road system design do not provide highway designers and traffic engineers with adequate guidance for incorporating road user needs, limitations, and capabilities when dealing with roadway design, operational decisions, and diagnostic assessments. The HFG was developed to fill such gaps. It reflects both an immediate need for better information about human factors and the availability (especially in the last 15 years) 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, and 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 traffic engineers and designers on user needs, capabilities, and limitations. The HFG is intended to complement other primary design references and standards - it does not duplicate or replace them. It is an additional tool the engineer can use to improve the safety performance of roadways. For example, the HFG and the HSM are both intended to provide practitioners with practical information about quantitative crash reductions and human factors.

HFG WHY HAVE HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS? VERSION 2.1 1-3 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 (see Campbell, Hull, and Maistros, 2018). 1.3 Applying Human Factors to Road System Design and Operations 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., functional class, geometrics, signs, markings, lighting) to road safety performance. Critically, human factors is 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 speeding1, 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: (1) the requirements of the driving task at a particular roadway location and under a specific set of circumstances and (2) the capabilities and limitations of the road user. These might include perceptual requirements (e.g., can drivers see the roadway alignment under all operating situations?), cognitive/decision-making requirements (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 requirements (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 uniformly ignored by motorist as being unrealistic: where and how might our roadway designs be misaligned with driver intuitions or expectations? In this context, we can see that ‘human factors’ encompasses much more than simply ‘bad behaviors’ by drivers. Ideally, standards and guidelines to support roadway planning, design, and traffic operations will be consistent with user 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 to state transportation agencies that have shifted away from a nominal approach to safety that relies on compliance with well-accepted design criteria and towards a substantive approach that relies on the data-driven quantification of the safety performance of a facility in terms of crashes. While the substantive safety approach offers flexibility and the opportunity to consider the broader context of a facility when making design decisions, it requires robust assessment of the various cost and performance trade-offs that are at the 1 The 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.

HFG WHY HAVE HUMAN FACTORS GUIDELINES FOR ROAD SYSTEMS? VERSION 2.1 1-4 core of state agencies’ decision-making process. As noted above, highway safety professionals frequently face 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 binary, but rather a continuous system of interactions, many involving 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 hazards. What are the expected safety outcomes of adding SRSs, including both the potential benefits of alerting cars who stray onto the shoulder and the potential risks to bicyclists?  At a complex freeway interchange, where should 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 safety impacts of these options?  In a busy urban environment, should a bus stop be located mid-block, 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, unsafe pedestrian behaviors like mid-block 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 by 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.