Performance-Based Analysis of Geometric Design of Highways and Streets (2014)

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16 C H A P T E R 3 This chapter provides an overview of possible project outcomes and considerations in identi- fying those outcomes. It also defines the concepts of project performance and geometric design performance, and describes the relationship between these two concepts. 3.1 Audience and Goals Chapter 1 began by describing the role of performance-based analysis in transportation activities and, specifically, the role and value of performance-based analysis in geometric design of highways and streets. Paramount to performance-based analyses is the fundamental model depicted in Exhibit 1-1 that focuses on first identifying intended project outcomes. With out- comes determined, geometric design solutions may be assessed by how well their performance relates to intended outcomes. Section 3.2 describes considerations in defining project perfor- mance. Project goals or performance measures can range from strategic goals of the USDOT to performance categories of MAP-21. Stakeholders or community members may share or express project goals that have connections to federal goals and policies. Livability, community cohesion, economic development, or congestion reduction objectives are common intended outcomes. Understanding whom a project is intended to serve and the ultimate purpose or goal of a project is critical to determining the appropriate performance measures for evaluating both the effectiveness of individual design decisions as well as the collective design of a street or highway. Gaining this understanding is also critical to identifying the design elements and decisions most likely to positively or negatively impact the ability to serve different users and other stakeholders and achieve the desired project outcomes. This section will highlight how to consider the following fundamental questions: • Whom are we serving? • What are we trying to achieve? The question “Whom are we serving?” focuses on identifying the key road users and stake- holders for a given project and project context. The question “What are we trying to achieve?” focuses on identifying and articulating a project’s core desired outcomes. Understanding “whom we are serving” is integral to understanding and defining the intended project outcomes. Defining the intended project outcomes and considering the specific users and other stakeholders helps professionals determine the geometric design elements and options that are more likely to achieve the intended outcomes. Considering the expected performance effects of geometric design decisions for highways and streets, defined as geometric design performance Understanding whom a project is intended to serve and the ultimate purpose or goal of a project is critical to determining the appropriate performance measures for evaluating both the effectiveness of individual design decisions as well as the collective design of a street or highway. Project Outcomes

Project Outcomes 17 in this document, allows designers to assess the level at which their individual decisions and the culmination of their decisions will support the intended project outcomes. 3.1.1 Whom are We Serving? Road users and other project stakeholders tend to be the two fundamental groups that com- pose the primary audience served by a specific project. Facility owners and operators typically strive to best meet the groups’ needs. Different road user types can be identified and considered by mode: bicyclists, pedestrians, motorists, motorcyclists, drivers of large commercial/freight vehicles, drivers of agricultural/ logging/mining equipment/vehicles, and drivers and users of transit vehicles. Road users can also be defined by a target demographic (e.g., younger road users, older road users, and/or transit- dependent populations) and/or a geographic sub-population (e.g., rural town center, central business district, suburban community, and/or industrial area). Other factors can influence the characteristics of road user types, including special events, recreational uses, seasonal variations, or weather patterns and events that influence how users operate. Project stakeholders can encompass a wide range of individuals, groups, and organizations. They can be agency stakeholders who are facility owners and operators or cooperating partners [e.g., city, county, state, or metropolitan planning organization (MPO)] with full or partial own- ership of the project. The cooperating partners may also be involved at just a cursory level because of the project’s influence on or proximity to their jurisdiction. Stakeholders can also be local busi- ness owners whose economic livelihood (perceived or actual) is directly or indirectly influenced by the project. Residents who live, work, and/or recreate within the influence area of the project can also be stakeholders. There also may be interest groups with specific concerns they would like to have considered and addressed within the project (e.g., environmental concerns, safety for a specific group or demographic). Given the wide range of potential road users and other project stakeholders, the key is to iden- tify the core audience the project is intended to serve. This is often directly tied to understanding intended project outcomes (defined and discussed in the following section). Key questions that help to isolate the core audience of a project might include the following: • What is the purpose and function of the existing or planned highway or street? • What are the existing and planned land uses adjacent to and in the vicinity of the highway or street? • What road users will likely desire to use the highway or street given the existing and planned land uses? • What are the existing and anticipated future socio-demographic characteristics of the popula- tions adjacent to and in the vicinity of the existing or planned highway or street? • What are the perceived or actual shortcomings of the existing highway or street? • Who has jurisdiction over the facility? • Where is capital funding for the project originating (or expected to originate)? • Who will operate and maintain the facilities? Answering the previous questions can help frame a project’s target audience, consisting of potential users and other stakeholders. A brief example demonstrates the general approach. • What is the purpose and function of the existing or planned highway or street? – A desire to construct a new street and upgrade existing intersections to improve access to an existing industrial area. – Considered a critical new street to attract additional businesses and associated jobs to a city. A sporting event may create special peaking, and a place with high tourism such as Florida may notice many “new” users. In addition, road user needs in a place like Minnesota may be different than those of users in Arizona. Project Example 5 in Chapter 6 expands upon the example discussed here.

18 Performance-Based Analysis of Geometric Design of Highways and Streets • What are the existing and planned land uses adjacent to and in the vicinity of the highway or street? – An industrial area with existing manufacturing facilities, warehouses, and distribution centers. – The industrial area is located between the downtown business district/residential neighbor- hoods and a popular regional park attracting recreational bicyclists. • What road users will likely desire to use the highway or street given the existing and planned land uses? – Heavy vehicles transporting raw materials and finished products to and from manufacturing facilities, warehouses, and distribution centers. – Bicyclists and motorists traveling to and from the regional park and downtown districts. • What are the existing and anticipated future socio-demographic characteristics of the popula- tions adjacent to and in the vicinity of the existing or planned highway or street? – Existing primary demographics are those associated with employees working within the indus- trial area. Secondary demographics are made up of a wide range of individuals traveling to/ from the regional park. • What are the existing perceived or actual shortcomings of the existing highway or street? – Insufficient connectivity within the existing industrially zoned area to enable its additional development. – Limited access from the industrial area to key regional facilities (e.g., an Interstate). – A roadway and intersection configuration that limits service to large trucks and anticipated truck volumes. – A lack of bicycle facilities to serve the bicyclists traveling to/from the adjacent regional park. • Who has jurisdiction over the facility? – The city will have jurisdiction of the facility. • Where is capital funding for the project originating (or expected to originate)? – The city plans to seek federal funding for part of the project. – A local improvement district (LID) and traffic impact fees from current and anticipated land owners will address other project costs. • Who will operate and maintain the facilities? – The city will operate and maintain the primary facility (roadway and traffic control devices). A local development agency will maintain ornamental streetlights and special landscape features. Based on the answers to these questions, three groups of road users and other stakeholders influence whether this particular project is ultimately successful: • Primary—heavy-vehicle operators accessing the industrial businesses and the associated industrial-oriented businesses • Important Secondary Audience—bicyclists and motorists traveling to and from the regional park and downtown districts • Other Participating Audience—business owners and local development agency funding light- ing and landscaping features Heavy-vehicle operators and the associated industrial-oriented businesses are the primary audience or group the project is intended to target. Their needs for access by heavy vehicles to existing and future planned industrial land uses within the subarea as well as their access to regional higher mobility facilities (e.g., an Interstate, freight rail line) should directly influence the performance measures used to evaluate design decisions. Bicyclists and motorists traveling to and from the regional park and downtown districts are the secondary audience or group the project will influence. While they are not the targeted

Project Outcomes 19 users of the new facility, the proximity of the new street to their desired origins and destina- tions will attract them to use it. Multimodal quality of service can be influenced by roadway and intersection geometric design elements. Within the project context, a decision could be made to design and construct a completely separate facility that addresses bicyclist needs with them as the primary audience (e.g., a separated multiuse path for bicyclists). The project could also move forward as one shared use facility. Evaluating both possible alternatives should include performance measures that address the transportation outcomes for bicyclists, motorists, and heavy vehicles. Finally, the city plans to seek federal funding for a portion of the project; therefore, some of the project performance measures used to evaluate design decisions may need to reflect unique requirements of that funding source (e.g., a project’s impact to wetlands). Similarly, the develop- ment district is contributing to the operations and maintenance of the facility. Their practical funding limits and ability to support future maintenance costs will need to be considered and factored into project decision making. 3.1.2 What Are We Trying to Achieve? Being able to identify and articulate the intended project outcomes will help clarify the key project performance measures, including transportation performance measures, and the associ- ated design elements and decisions most likely to influence whether a project will fulfill those desired outcomes. The intended project outcomes are often closely linked to who the project is intended to serve (see discussion in previous section). The motivation for a project often originates from a planning activity or a community’s expressed desire highlighting a perceived or actual need for an improvement. A project could originate for many reasons, including crash history, traffic operations (existing or forecasted), lack of pedestrian/ bicycle/transit facilities, and/or a desire to attract employers to an area. In the prior example of an industrial area, the physical ability to serve freight vehicles (including vehicle swept paths and forecasted freight volume) is a motivator for advancing complementary design elements that address existing freight movement limitations while facilitating expansion of the industrial area. The way in which a project originates often sets the framework for under- standing what the project is intended to achieve. Continuing to build on the industrial area example, there may not be project performance mea- sures to assess the degree to which a design will attract new employers. However, there are per- formance measures that can assess how well a design provides access and connectivity for heavy vehicles and potential employees as well as the degree to which a design balances the quality of ser- vice provided to other road users (e.g., bicyclists, transit riders). These performance characteristics of the design would, in turn, influence key elements an employer would consider in determining whether to establish a presence in the industrial area. Performance-based analysis of geometric design can help inform discrete design decisions so that a preferred alternative design is identified and better aligned with the purpose and function of the roadway. Understanding, at the broadest level, what the project is intended to achieve sets the stage for identifying the performance categories, specific performance measures, and associ- ated design characteristics that are critical for aligning a project to achieve the original intended outcomes. The following section discusses how project performance can be defined once a practitioner has established and articulated who the project is intended to serve and what the project is intended to achieve.

20 Performance-Based Analysis of Geometric Design of Highways and Streets 3.2 Project Performance 3.2.1 Overview Section 1.5 provided an overview of overall project performance and how it may influence and be influenced by geometric design per- formance. Overall project performance and respective performance measures depend on the nature or catalyst for the project. Section 3.1 considered “whom are we serving?” and “what are we trying to achieve?” with the intent of guiding geometric design solutions to meet user needs and achieve stakeholder objectives. Understanding whom a project is serving and the ultimate purpose or goal of a project helps identify appro- priate performance measures for evaluating the effectiveness of individ- ual design decisions. Understanding how the design elements and decisions positively or negatively impact the project performance can help assess how to best achieve desired project outcomes. Once users and objectives are understood, the performance criteria for assessing the effective- ness of design alternatives (whether at the conceptual or more detailed design phase) can begin to be defined. Section 3.3 discusses geometric design performance, while Section 3.3.2 presents geometric design performance categories from which geometric design performance can be evaluated. The transportation engineering and planning profession is continually evolving to a more holistic approach in how the need for improvement projects is understood and identified. It is increasingly common for transportation planning activities to include considerations such as sustainability, livability, economic vitality, societal health impacts, and environmental health impacts—and to use these considerations to identify the need for projects as well as evaluate the merit of potential projects based on their estimated impact to those broader performance categories ultimately con- nected to quality of life. Measuring the effectiveness of overall project performance depends on the goal, intended outcome, nature, or catalyst for the project. Overall project performance may influence and may be influenced by geometric design decisions and their resultant performance. Geometric design choices or geometric design alternatives will have an influence on the outcomes. However, the ultimate measure of the project’s success may not hinge upon the specific geometric element or value of a specific treatment, solution, or mitigation. 3.2.2 Project Performance Goals and Measures The holistic approach evolving in the profession is consistent with values and project catalysts at the community level and is generally consistent with goals of the USDOT. The USDOT’s Strategic Plan for 2012–2016 includes six strategic goals and, with one exception that is organi- zational, the goals generally apply to community values or agency objectives (1). These USDOT goals are briefly described in Exhibit 3-1, along with comments on the parallels of the goals with common community values and agency objectives. Opportunities exist to connect the themes of the USDOT and the community with project performance measures consistent with emerging trends and national policies. On July 6, 2012, President Obama signed into law P.L. 112-141, the Moving Ahead for Prog- ress in the 21st Century Act (MAP-21) (2). MAP-21 funds surface transportation programs at over $105 billion for fiscal years 2013 and 2014. Of significance to performance-based analysis of geometric design and overall project performance is how MAP-21 transitions the Federal Aid program to a performance-based and outcome-based program. States and metropolitan areas

Project Outcomes 21 will explicitly show how program and project selection will help achieve a set of performance targets related to the following categories: • Congestion reduction • Infrastructure condition • Environmental sustainability • Freight movement and economic vitality • Reduced project delivery delays • Safety • System reliability These categories have common elements and themes to USDOT goals and to the way com- munities and stakeholders increasingly measure the success of projects. With MAP-21, agencies are required to formally establish performance measures. Many agencies have incorporated per- formance outcomes and goals into their strategic planning for some time. These project goals Area Focus Comment Economic Competitiveness Achieve maximum economic returns on policies and investments by implementing strategies such as developing intercity, high-speed passenger rail and a competitive air transportation system; increasing travel-time reliability in freight- significant highway corridors; improving the performance of freight rail and maritime networks; advancing transportation interests in targeted markets around the world; and expanding opportunities in the transportation sector for small businesses. Project catalysts or objectives commonly include desires of economic development or economic vitality. This can include providing employment opportunities, supporting trade and enterprise, or providing vigor or support to local community retail, commercial, and residential areas. Environmental Sustainability Address the challenges associated with the environmental impacts of transportation through strategies such as fuel economy standards for cars and trucks; more environmentally sound construction and operational practices; and expanding opportunities for shifting freight from less fuel-efficient modes to more fuel-efficient modes. In addition to efficient designs that improve capacity and mobility, air quality, noise levels, and water quality treatments and features continue to become increasingly important outcomes to communities, stakeholders, and agencies. Livable Communities Pursue coordinated, place-based policies and investments (e.g., coordinated transportation, housing, and commercial development policies and decisions) that increase transportation choices and access to public transportation services for all Americans. Common project objectives are “quality of life” measures that promote balanced communities serving residential and commercial areas while preserving the nature, character, and historical significance of the community. Organizational Excellence Make the USDOT a high-performance, outcome- driven agency. While not necessarily a direct project catalyst or project performance measure, there is a general interest in efficient and responsive government activities in managing and executing projects and processes. Safety Reduce transportation-related fatalities and injuries. In addition to reducing crashes and resulting injuries for all users, there is an increasing awareness about the quality of experience upon a project’s completion and the importance of comfort and security in using transportation facilities. State of Good Repair Improve the condition of transportation infrastructure by making optimal use of existing capacity, minimizing life cycle costs, and applying sound asset management principles. Whether it is for pedestrians, cyclists, transit users, or vehicle drivers, each user values good conditions and these good conditions support some of these other strategic goals. Exhibit 3-1. USDOT’s strategic goals.

22 Performance-Based Analysis of Geometric Design of Highways and Streets and performance targets have common elements and themes desired by the public and stakeholders as part of successful projects. Geometric design choices and considerations directly influence many of these topic areas. Conversely, the desired project performance and project outcomes can directly influence geometric design decisions. Being able to assess “geometric design performance”— the performance effects of geometric design decisions and outcomes—becomes instrumental in guiding decisions that lead to successful projects. 3.3 Geometric Design Performance 3.3.1 Overview This chapter began by asking the fundamental questions of “whom are we serving?” and “what are we trying to achieve?” Within the context of those questions, Section 3.2 presented a discussion on the broad aspects of defining project performance by way of the project goals and themes of project performance considerations. Project goals and performance considerations provide the means of assessing how well project solutions attained desired objectives. Chapter 2 included an overview of geometric design decisions and discussion about the relation- ship between intended project outcomes and corresponding performance measures. A resonant theme in these discussions is how project-level needs influence geometric design decisions and how geometric design decisions influence project outcomes. This section of the process framework focuses on geometric design performance and the considerations of how geometric design deci- sions influence and guide overall project performance. Geometric design performance is defined as those aspects of performance that are influenced by the roadway and roadside geometrics. Geometric design performance can greatly influence the project outcomes and overall proj- ect performance. Specific design choices may result in certain types of speeds, operating envi- ronments, driver expectations, and safety performance. A desired overall project performance measure may be to retain the local community culture and character while improving the safety performance of a transportation facility in anticipation of increased volumes on a roadway seg- ment or intersection. The choices made by the designer can directly influence the character of the solutions, and therefore, the ability of potential solutions to meet overall project objectives. Discrete design choices—such as median type, shoulder width, or intersection form—can directly influence the long-term expected safety and operational performance of a facility. Overall project performance may be directly linked to the specific design choices—and the specific performance of the design alternatives considered. This section helps the user consider specific performance categories and the how design choices might influence performance mea- surements. The focus is on the multimodal transportation performance effects of geometric design decisions for highways and streets. 3.3.2 Geometric Design Performance Categories Geometric design decisions for highways and streets affect overall project performance in dis- crete ways that ultimately may affect broader concepts such as sustainability or livability. Within the context of conducting performance-based analysis to inform geometric design in this docu- ment, the critical transportation performance categories that influence and are influenced by geo- metric design elements and their characteristics are of interest. These transportation performance categories are as follows and are described in the supporting subsections: • Accessibility • Mobility

Project Outcomes 23 • Quality of service • Reliability • Safety Project performance can include other elements that may not be specific transportation out- comes of accessibility, mobility, quality of service, reliability, and safety. As described in Section 2.2, concepts such as environmental stewardship, livable communities, or economic development may be project performance measures that are fully or partially sensitive to geometric design deci- sions. The example presented in Section 3.1 focused on better serving trucks on existing facilities while attracting more freight users. It included recreational users and the need to appropriately serve non-auto users such as bicyclists and pedestrians. Geometric design performance will be influenced by discrete design choices. Considering the target project needs in terms of the five transportation performance categories allows the transportation-related results of design element choices and dimensional values to be more easily evaluated. Design elements or choices may directly or indirectly influence project performance by how they affect the five transportation performance categories. Transportation terms can be used in many forums and venues. Terminology can be interpreted or used to support a variety of purposes. The following terms are used in this report to convey their specific application to performance-based analysis of geometric design of highways and streets. 3.3.2.1 Accessibility Accessibility is defined as the ability to approach a desired destination or potential oppor- tunity for activity using highways and streets (including the sidewalks and/or bicycle lanes provided within those rights-of-way). In this definition of accessibility, the ability to approach a desired destination or potential opportunity for activity is interpreted as encompassing three concepts: (1) access by a specific user type or vehicle type to use a facility, (2) the opportunities for activity near the facility, and (3) the convenience of reaching the activity destinations from different trip origins. Candidate accessibility performance measures with geometric design sen- sitivity are discussed in the Supplemental Research Materials Report (3) associated with these guidelines. They include “access to a facility by a road user type,” “cumulative opportunity,” and “travel impedance.” As noted in the supplemental research report, these performance measures have not traditionally been considered during geometric design stages of project development, and they tend to require performance prediction tools that are typically not used by designers. Additional research in this area is needed. In this report, accessibility is captured using sur- rogates for accessibility performance measures that are characteristics of the infrastructure, including driveway density, transit stop spacing, and presence of pedestrian and/or bicycle facilities. 3.3.2.2 Mobility Mobility is defined as the ability to move various users efficiently from one place to another using highways and streets. The term “mobility” can sometimes be associated with motorized vehicular movement and capacity. For the purposes of this report, “mobility” is meant to be independent of any particular travel mode. Performance measures for mobility that are sensi- tive to geometric design include speed and measures that involve speed (e.g., delay, travel time). As noted, these measures can be equally applied to any travel mode; however, non-motorized movement performance may be more meaningfully quantified using measures of accessibil- ity and quality of service. Queue characteristics (e.g., queue length, queue storage ratio) and volume-to-capacity ratios also give some insights into expected levels of mobility for different travel movements. Chapter 4 also utilizes one surrogate for mobility that is a measure of the infrastructure design—inferred design speed—with the idea that inferred design speed is associ- ated with free-flow speeds and therefore with mobility.

24 Performance-Based Analysis of Geometric Design of Highways and Streets 3.3.2.3 Quality of Service Quality of service is defined as the perceived quality of travel by a road user. It is used in the Highway Capacity Manual 2010 (HCM2010; 4) to assess multimodal level of service (MMLOS) for motorists, pedestrians, bicyclists, and transit riders. The TRB Highway Capacity and Qual- ity of Service Committee has taken a leadership role in identifying performance measures most related to user perception of quality of service, expressed as a level of service (LOS). These measures include average travel speed, control delay, density, percent time-spent-following, driveway density, separation between motorized and non-motorized modes, amount of space provided for pedestrians and bicyclists, frequency of transit service, transit service amenities, and frequency of opportunities for pedestrians to cross a street. The latter measures are examples of those that capture infrastructure and operational characteristics that affect the quality of service experienced by non-motorized users. HCM2010 (4) served as the primary reference for both the primary and additional quality-of-service measures. Quality of service may also include the perceived quality of travel by design vehicle users such as truck or bus drivers. The quality of service may differ between a geometric solution configured to regularly serve a design vehicle and one configured to accommodate the vehicle, if necessary. Quality-of-service measures may also capture user security, defined in this document as users’ perceptions of safety. 3.3.2.4 Reliability Reliability is defined as the consistency of performance over a series of time periods (e.g., hour-to-hour, day-to-day, year-to-year). Reliability has become a critical transportation perfor- mance measure over the last decade, as evidenced by its role as a theme in the second Strategic Highway Research Program (SHRP 2) and in performance-based decision-making aspects of MAP-21 (2). Reliability of transportation service is commonly linked to travel-time variability, but the basic concept applies to any other travel-time-based metric (e.g., average speed, delay). Reliability is sensitive to geometric design, because the geometric design may affect the ability of a highway or street to “absorb” random, additional traffic demand as well as capacity reduc- tions due to incidents (e.g., crashes, vehicle breakdowns), weather, and maintenance operations, among others. Reliability also is indirectly related to geometry inasmuch as the geometry affects the frequency and severity of random events that impact travel time (e.g., crashes). A more detailed discussion of the expected relationships between reliability and the geometric design of highways and streets is provided in the Supplemental Research Materials Report associated with these guidelines (3). 3.3.2.5 Safety Safety is defined as the expected frequency and severity of crashes occurring on highways and streets. Expected crash frequencies are often disaggregated by level of crash severity and crash type, including whether or not a crash involves a non-motorized user or a specific vehicle type (e.g., heavy vehicle, transit vehicle, motorcycle). Measures that combine crash frequencies and severities into a common unit (e.g., crash cost, equivalent property damage only, relative severity index) are sometimes used when comparing design alternatives. 3.3.3 Role/Influence of Geometric Design Features The role or influence of geometric design on transportation performance is relatively well documented for some performance categories compared to others. In some cases, the role of geometric design has a clear relationship to specific performance category outcomes. For exam- ple, there is relatively extensive documentation related to safety performance functions or crash modification factors of various geometric forms or elements (e.g., roundabouts compared to

Project Outcomes 25 signalized intersections, paved shoulders compared to unpaved shoulders, left-turn-lane pres- ence at intersections). However, there is comparatively less information about the role of geometric elements on reliability and accessibility. In the case of reliability, the presence of shoulders or shoulder width and construction type may improve reliability by allowing incidents to be removed from the traveled way more efficiently, or by allowing through traffic to use the shoulder when one or more travel lanes are blocked. Full-width, hard shoulders are sometimes used as travel lanes in managed motorway facilities during peak periods. The presence of sidewalks or magnitude of roadway grade may influence pedestrian accessibility and quality of service, but there may not be a way to predict a related performance metric to differentiate between design choices. Another example is the difference in performance of a 4-ft-wide sidewalk versus a 6-ft-wide one. If a local jurisdiction requires a minimum sidewalk width of 4 ft, providing that width in the project may meet code compliance but might not necessarily provide the optimal per- formance level for users. In these cases, combining information from various sources could help inform performance-based decisions to a level that is currently practical. In this side- walk-width example, applying the HCM2010 (4) MMLOS evaluation procedures could help a designer assess the relative expected quality of service of alternative sidewalk widths inde- pendently of whether the width complies with an agency’s design criteria. Accessibility per- formance may not be quantifiable in this case, and subjective judgments may still be needed (e.g., the magnitude of roadway grade likely increases pedestrian travel impedance, reducing pedestrian accessibility). Exhibit 3-2 presents performance categories and identifies how well the defined role and influence of geometric design features have been documented. The exhibit also highlights some of the national reference documents that include geometric design elements as inputs or con- tributors to performance prediction procedures. Chapter 4 will summarize current information on the relationships between geometric design elements, design decisions, and their specific performance effects. The information in Chapter 4 will be used to identify key geometric charac- teristics for supporting the desired project outcomes. This will be useful information in develop- ing potential solutions that make progress towards the intended project outcomes as measured within the transportation performance categories. 3.3.4 Geometric Design Decisions Geometric design decisions should consider overall intended project outcomes, project perfor- mance, and transportation performance. Specifically, geometric design decisions should be made considering how the features or qualities of the features may influence performance measures related to accessibility, mobility, quality of service, reliability, and safety. Designers make these choices considering intended project outcomes and understanding how the performance catego- ries or specific performance measures may influence geometric characteristics and decisions. By understanding a process framework for considering performance-based evaluations of geometric design of highways and streets (outlined in Chapter 5), professionals will have a systematic, flexible, and adaptable range of activities to inform design choices. Geometric design decisions for highways and streets may have incremental and cumulative effects. Discrete choices may ultimately impact broader concepts such as sustainability, economic competitiveness, or livability. Within the context of conducting performance-based analysis to inform geometric design decisions, one must first consider the identified project needs and set forth the appropriate design controls consistent with an overall project or specific design con- text. Design controls help establish a baseline from which to measure design performance. For example, 10-ft-wide vehicle travel lanes may be desired to provide bike or parking lanes or to

26 Performance-Based Analysis of Geometric Design of Highways and Streets maximize shoulder width within the paved right-of-way. This may be adequate in serving tractor trailer vehicles (e.g., WB-62) on a collector roadway in a tangent section, but the turning paths of this design vehicle may dictate wider lanes on curved portions of the roadway to accommodate off-tracking on turning roadways. Identifying project design controls (intended operating speeds, design vehicle type, driver performance, and human factors) leads to appropriate design criteria to meet those design control needs. Understanding the intended project outcomes helps define the design controls and allows a designer to customize the design elements to each project’s contextual design environment. This applies whether the context is a complex urban freeway or right-turning movements at an at-grade intersection. Geometric design decisions are influenced by the proj- ect considerations and specifically by the choices needed to define the elements of segments and nodes. Segments define the character of the corridor or design element. Nodes define the qualities and attributes of the intersecting roadways and can include intersections or inter- changes. The design decisions of an interchange can include investigating appropriate ramp terminal intersection forms. Understanding the intended project outcomes helps define the design controls and allows a designer to customize the design elements to each project’s contextual design environment. Performance Category Defined Role/Influence of Geometric Design Features Well Documented Moderate Documentation Limited Documentation Reference Documents Accessibility X • Fundamental access concepts/definitions [from FHWA’s Functional Classification Guidelines, TRB’s Access Management Manual (5 )] • Transit Capacity and Quality of Service Manual (6 ) • HCM2010 (4 ) • Published literature on accessibility [references in Supplemental Research Materials Report (3 )] Mobility X • HCM2010 (4 ) • FHWA Speed Concepts: Informational Guide (7 ) • NCHRP Report 672: Roundabouts: An Informational Guide (8 ) Reliability X • Reports from SHRP 2 Projects L07 and L08 (9, 10) • Published literature on reliability [references in Supplemental Research Materials Report (3 )] Safety X • Highway Safety Manual (11) • Final report of NCHRP Project 17-45/ Enhanced Interchange Safety Analysis Tool (ISATe) (12) • Crash Modification Factors Clearinghouse (13) • NCHRP Report 687: Guidelines for Ramp and Interchange Spacing (14 ) Quality of Service X • HCM2010 (4 ) Exhibit 3-2. Documentation levels of the defined role/influence of geometric design features.

Project Outcomes 27 3.3.5 Project Design Controls and Influences The street or highway function fundamentally influences geometric design decisions. In many cases, functional classification and hierarchies of movements help define the characteristics of facility features for roadway segments and nodes. However, there are frequent cases with no clear definition of or agreement on the facility type or, sometimes, even the facility function. A state transportation agency has the responsibility of managing the National Highway System (NHS). These specific, designated facilities consist of roadways important to the nation’s economy, defense, and mobility. As such, transportation agencies are responsible for managing, maintain- ing, and operating these facilities to meet those needs. However, NHS facilities run through com- munities and, while passing through communities, may have additional purposes, for example, being the city’s main street or other key arterial network component serving community needs that differ from national defense or pure mobility. In these cases, design controls and the selected elements of design must be evaluated with a broader lens, that of meeting national objectives while considering and (to the extent possible) adapting to local community needs. 3.3.5.1 Speed Concepts and Design Decisions Understanding the project context helps establish project limits, modal connection and inte- gration, node type (intersections versus interchanges), capacity targets, access management strategies, and other features influencing geometric design decisions. The jurisdiction(s) engaged with the segments or nodes help establish applicable design standards that can be augmented with national and state guidelines and customary practices. One of the key elements influencing geometric design decisions is that of “speed.” While often considered as “design speed,” there are numerous other speed-related considerations with the ability to significantly influence geo- metric design choices. The designated design speed is used explicitly for determining minimum, maximum, and ranges of values for highway design such as minimum horizontal curve radius, minimum sight distance, and maximum grade. The designated design speed influences a num- ber of geometric design elements and, therefore, the ability to consider design speeds in combi- nation with posted and intended operating speeds could help refine geometric design decisions by establishing “target” operating speeds for each unique project context. Actual operating speeds (i.e., 85th percentile free-flow speed) may be different than both the design speed and the originally intended posted speed. The differences become more substan- tial for “intermediate” and “lower” speed facilities. The FHWA’s Speed Concepts: Informational Guide (7) introduces the concept of “inferred design speed.” Inferred design speed is applicable to features and elements that have a criterion based on (designated) design speed (e.g., vertical curvature, sight distance, superelevation). The inferred design speed of a feature may be different than the designated design speed and provides designers with the ability to consider side friction, superelevation, and lateral acceleration to potentially tailor vertical and horizontal alignment decisions with intended speed-related outcomes. 3.3.5.2 Sight Distance Concepts Chapter 3 of the 2011 A Policy on Geometric Design of Highways and Streets (AASHTO Green Book) (15) provides a complete summary of sight distance concepts and their associated design elements. Sight distance concepts are summarized for stopping, decision, and passing conditions. Stopping sight distance values result in geometric configurations allowing drivers to perceive, react, and stop to avoid collision. Decision sight distance provides additional time to interpret possible choices and react to complex conditions such as at an interchange or intersection. Passing sight dis- tance supports complete passing maneuvers. Intersection sight distance concepts are well presented in NCHRP Report 383: Intersection Sight Distance (16), which provides a comprehensive overview of the concept. Intersection sight distance is intended to provide drivers at or approaching intersec- tions with an unobstructed view of the entire intersection and of sufficient lengths of the intersecting highways to permit the approaching drivers to anticipate and avoid collisions.

28 Performance-Based Analysis of Geometric Design of Highways and Streets The methods of determining the criterion for a particular sight distance type are related to speed. Fundamentally, higher speeds mean a driver travels a greater distance during the per- ception and reaction time compared to a lower speed. Therefore, higher speeds can have an effect on requisite sight distance values, which, in turn, can increase the nominal dimensions of design elements. Therefore, selecting an appropriate target speed and understanding the poten- tial inferred speeds of a facility can provide designers with more flexibility and precision in selecting design values for those geometric features that are directly influenced by design speed. Ultimately, this approach could provide designers with more flexibility to meet desired perfor- mance targets through their informed geometric design decisions. 3.3.5.3 Design Choices for Segments and Nodes While geometric design is presented in final plan sets consisting of basic plan, profile, and cross section, the number of design choices possible within these three-dimensional categories is vast. The resulting performance for these design elements is not necessarily documented for each and every element nor is the interactions between them fully documented or known. Exhibit 3-3 lists example design choices for segments and nodes. The sheer number of elements provides Segments Nodes • Access points and density • Design speed and target speed • Horizontal alignment • Number of travel lanes • Sidewalk and pedestrian facilities • Bicycle accommodation features • Transit accommodation features • Design vehicle accommodation • Median provisions • Travel lane widths • Auxiliary lane widths • Type and location of auxiliary lanes Shoulder width • • Shoulder type • Lane and shoulder cross slopes • • • • • • • • • • • • • • • • • • • • • • • • • • Superelevation • Roadside design features • Roadside barrier • Minimum horizontal clearance • Minimum sight distance • Maximum grade • Minimum vertical clearance • Vertical alignment • Bridge cross section • Bridge length/termini • Rumble strips Intersection form, control type, and features Interchange form and features Design speed and target speed Number and types of lanes Sidewalk and pedestrian facilities Bicycle accommodations facilities Transit accommodations facilities Special/vulnerable user treatments Design vehicle accommodations Traffic islands Lane widths Auxiliary lane lengths Shoulder width and composition Approach or ramp cross section Horizontal alignment of approaches or ramp Mainline ramp gores and terminals Cross road ramp terminals Vertical alignment of approaches or ramp Auxiliary lane terminals and transitions Pavement cross slope and superelevation Intersection sight distance Median opening configuration Curve tapers & radii Ramp roadside Ramp barriers Exhibit 3-3. Example design choices for segments and nodes (intersections and interchanges).

Project Outcomes 29 designers with many degrees of freedom in creating geometric designs to meet the wide array of project contexts. From this exhibit, it is easy to see how design choices for segments and nodes (intersections and interchanges) include similar broad categories of plan, profile, and cross sec- tion. The design of a ramp proper will closely mimic the design elements and process of roadway segments. 3.4 Summary This chapter began by first considering, fundamentally, who is being served and what a project is trying to achieve. It continued with a discussion of how to consider and define project perfor- mance. That led to defining geometric performance, a subset of which included transportation performance—the focus of this document. The transportation performance was divided into performance categories that could help support overall project objectives. This chapter con- cluded by considering the role and influence of geometric design features on achieving project outcomes and desired project performance. Chapter 4 presents a series of tables and other information to help identify which geomet- ric features may influence performance measures related to accessibility, mobility, quality of service, reliability, and safety—and, in turn, which performance measures may influence geometric characteristics and decisions. Chapter 5 presents a performance-based analysis application framework for incorporating performance-based evaluations into the geometric design of highways and streets. Chapter 6 includes project examples intended to reinforce the principles and approach outlined in this report while guiding the user through the application framework. 3.5 References 1. USDOT Strategic Plan 2012–2016. Washington, D.C.: U.S. Department of Transportation. http://www.dot. gov/dot-strategic-plan. Accessed July 1, 2013. 2. Moving Ahead for Progress in the 21st Century. Washington, D.C.: Federal Highway Administration, U.S. Department of Transportation. http://www.fhwa.dot.gov/map21/. Accessed July 1, 2013. 3. Kittelson & Associates, Inc., and University of Utah. Supplemental Research Materials Report. NCHRP Project 15-34A. Portland, Oregon: Kittelson & Associates, Inc. http://apps.trb.org/cmsfeed/TRBNetProject Display.asp?ProjectID=3322. 4. Transportation Research Board. Highway Capacity Manual. Washington, D.C.: Transportation Research Board of the National Academies, 2010. 5. Transportation Research Board. Access Management Manual. Washington, D.C.: Transportation Research Board of the National Academies, 2003. 6. Transportation Research Board (TRB). Transit Capacity and Quality of Service Manual, Second Edition. Washington, D.C.: Transportation Research Board of the National Academies, 2003. 7. Federal Highway Administration. Speed Concepts: Informational Guide. Washington, D.C.: 2009. 8. Rodegerdts, L., J. Bansen, C. Tiesler, J. Knudsen, E. Myers, M. Johnson, M. Moule, B. Persaud, C. Lyon, S. Hallmark, H. Isebrands, et al. NCHRP Report 672: Roundabouts: An Informational Guide, Second Edition. Washington, D.C.: Transportation Research Board of the National Academies, 2010. 9. Potts, I. B., D. W. Harwood, J. M. Hutton, C. A. Fees, K. M. Bauer, C. S. Kinzel, and R. J. Frazier. Iden- tification and Evaluation of Cost-Effectiveness of Highway Design Features to Reduce Nonrecurrent Congestion. Prepublication draft, SHRP 2 Reliability Project L07. Washington, D.C.: Transportation Re- search Board of the National Academies, 2013. http://www.trb.org/Main/Blurbs/169767.aspx. Accessed August 28, 2013. 10. Kittelson, W., M. Vandehey, Cambridge Systematics, ITRE, and Texas A&M Research Foundation. Incor- poration of Travel-Time Reliability into the HCM. Prepublication draft, SHRP 2 Reliability Project L08. Washington, D.C.: Transportation Research Board of the National Academies. http://www.trb.org/Main/ Blurbs/169594.aspx. Accessed August 28, 2013.

30 Performance-Based Analysis of Geometric Design of Highways and Streets 11. American Association of State Highway and Transportation Officials. Highway Safety Manual. Washington, D.C.: 2010. 12. Bonneson, J. A., S. Geedipally, M. P. Pratt, and D. Lord. Safety Prediction Methodology and Analysis Tool for Freeways and Interchanges. Final Report, NCHRP Project 17-45. College Station, Texas: Texas Transportation Institute, 2012. 13. Federal Highway Administration. Crash Modification Factors Clearinghouse. http://www.cmfclearinghouse.org/. 14. Ray, B. L., J. Schoen, P. Jenior, J. Knudsen, R. J. Porter, J. P. Leisch, J. Mason, R. Roess, and Traffic Research & Analysis, Inc. NCHRP Report 687: Guidance for Ramp and Interchange Spacing. Washington, D.C.: Transpor- tation Research Board of the National Academies, 2011. 15. American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets. Washington, D.C.: 2011. 16. Harwood, D. W., J. M. Mason, R. E. Brydia, M. T. Pietrucha, and G. L. Gittings. NCHRP Report 383: Intersection Sight Distance. Washington, D.C.: TRB, National Research Council, 1996.