This chapter outlines a five-stage process for designing thoroughfares in walkable urban contexts where the community has determined that the character of the thoroughfare and its integration with its surroundings are a high priority. It also presents an approach to designing thoroughfares within constrained rights of way and discusses the flexibility the designer has in applying design parameters. While the focus of this report's approach to design is on walkable thoroughfares in mixed-use areas, the design process presented in this chapter is applicable to all types of areas and thoroughfares, regardless of their modal emphasis.
This chapter presents design criteria that form the basis for the design guidance presented in subsequent chapters. As with the design process, the fundamental design criteria and the flexibility inherent in the interpretation and application of the criteria are applicable to all types of thoroughfares in all types of contexts.
Applicability of Design Criteria
The guidance presented in this report focuses on the design of urban thoroughfares in walkable contexts. As with the design process, the fundamental design criteria and the flexibility inherent in the interpretation and application of the criteria is applicable to all types of thoroughfares in all types of contexts. However, most of the guidance is also applicable to thoroughfares in other contexts where vehicle travel may be a priority. When designing a thoroughfare in a walkable area or a vehicle mobility priority thoroughfare, the practitioner can use this report to identify the sections with relevant and applicable considerations and guidance. If not identiied in the report, the guidance provided in the AASHTO A Policy for the Geometric Design of Highways and Streets, otherwise known as the "Green Book" (2004), is recommended.
1. Describes the various components of the thoroughfare and describes fundamental features of CSS in thoroughfare design;
2. Defines terms that are used in the thoroughfare design process;
3. Provides an overview and describes the five stages of the thoroughfare design process; and
4. Outlines a process for designing thoroughfares in constrained rights of way.
Walkable urban thoroughfare design requires attention to many elements of the public right of way and how these elements integrate with adjoining properties. To assist the designer in successfully assembling the elements of the thoroughfare, this report organizes definitions, design principles and criteria into four sections corresponding to the components of a thoroughfare. The three components that comprise the cross-section of the thoroughfare are illustrated in Figure 5.1 (context, streetside and traveled way), while the fourth component, intersections, is discussed below.
Figure 5.2 illustrates many of the fundamental elements of walkable thoroughfare design, including elements in the traveled way and streetside, and as part of the context.
Each of the components can be described as follows: • Context—encompasses a broad spectrum of environmental, social, economic and historical aspects of a community and its people. All of these aspects are important in applying CSS principles to thoroughfare design. Thus, context can be the built environment or part of the natural environment. The built environment consists of properties and activities within and adjacent to the public right of way and the thoroughfare itself, with surroundings that contribute to characteristics that define the context zone.
Figure 5.1 Components of an urban thoroughfare. Source: Community, Design + Architecture.
Figure 5.2 An illustration of the elements of a context sensitive thoroughfare. Source: Concept by Community, Design + Architecture, illustration by Digital Media Productions.
Buildings, landscaping, land use mix, site access and public and semipublic open spaces are the primary shaping elements of the built context. The natural environment includes features such as water or topography. In both environments, context can reflect historic or other protected resources. An urban thoroughfare will often change as the context changes from one zone to another. The thoroughfare itself and the activity it handles become part of the context after it is completed. Finally, all contexts whether built or natural, include the equally important elements of economics, time, community perspective, political positions, trade-offs and a multitude of other factors that will directly or indirectly influence the shaping of the context and thoroughfare design.
Table 5.1 Definition of Terms and Concepts in Chapter 5
Term or Concept
|Frontage Zone||One of the zones comprising the streetside, the frontage zone is the space between the pedestrian travel way and building faces or private property. At a minimum it provides a buffer distance from vertical surfaces or walls and allows people to window shop or enter/exit buildings without interfering with moving pedestrians. The frontage zone provides width for overhanging elements of adjacent buildings such as awnings, store signage, bay windows and so forth. If appropriate width is provided, the frontage zone may accommodate a variety of activities associated with adjacent uses, such as outdoor seating or merchant displays.|
|Throughway Zone||The streetside zone in which pedestrians travel. The throughway must provide a minimum horizontal and vertical clear area in compliance with PROWAG accessible route requirements.|
|Furnishings Zone||The furnishings zone is a multipurpose area of the streetside. It serves as a buffer between the pedestrian travel way and the vehicular area of the thoroughfare within the curbs, and it provides space for streetside appurtenances such as street trees, planting strips, street furniture, utility poles, sidewalk cafes, sign poles, signal and electrical cabinets, phone booths, fire hydrants, bicycle racks and bus shelters.|
|Edge Zone||The edge zone, sometimes also referred to as the "curb zone," is the transition area between the thoroughfare traveled way and the furnishings zone of the streetside and provides space for the door swing from vehicles in the parking lane, for parking meters and for the overhang of diagonally parked vehicles.|
|Right of way||Right of way is the publicly owned land within which a thoroughfare can be constructed. Outside of the right of way, the land is privately owned and cannot be assumed to be available for thoroughfare construction without acquiring the land through dedication or purchase.|
(See Chapters 8 and 9 for further definitions and design guidelines for the components of the streetside and the traveled way.)
This chapter uses terms that are commonly used in transportation planning and engineering and introduces new terms and concepts that require deinition.
Overview of the Design Process
The context sensitive thoroughfare design process presented in this report encompasses the project development steps from developing project concepts to final design. Briefly introduced in Chapter 2, the design process is composed of the ive stages shown in Figure 5.3. While this report presents the process in ive discrete stages for simplicity, the thoroughfare design process is an iterative process that requires collaboration with the public, stakeholders and a mul-tidisciplinary team of professionals. As stated earlier, this process is applicable for the design of all thoroughfare types under any context.
Figure 5.3 Thoroughfare design stages. Source: Kimley-Horn and Associates, Inc.
(Extended text description: Flow chart of thoroughfare design stages. At the top it reads Area Transportation Plan with arrow to Community Vision with arrow to Context Zone and Thoroughfare Type Identification with arrow to Initial Design and Testing with arrow to Thoroughfare Design. The Initial Design and Testing box also points back to Area Transportation Plan and Context Zone and Thoroughfare Type Identification.)
Stage 1: Review or develop an area transportation plan.
An area transportation plan is a long-range plan based on a public/stakeholder process that establishes goals and objectives for the area, town, or region. The plan results in the pattern of the thoroughfare network, the initial sizing of individual thoroughfares and prioritization of transportation improvements.
The area transportation plan entails development of land use and travel demand forecasts and testing of network alternatives in considering context and area objectives. Often this stage is already available and serves as a direction or resource for the thoroughfare designer. This irst stage provides the overall basis for thoroughfare design. The transportation plan establishes guiding principles and policies for the broader community and region. It develops and evaluates the network to ensure the transportation system accommodates projected land use growth.
The plan should identify performance measures for each mode of transportation at the intersection, corridor and network level and should identify how the network supports the community's key goals.
The plan should identify and prioritize discrete thoroughfare projects from which the project development process begins. If an area transportation plan has not been prepared, one should be prepared as part of the thoroughfare design process. Area transportation plans can be in the form of regional transportation plans, comprehensive or general plans, or focused district, area, or speciic plans. Chapter 3 provides background and guidance on network systems and design.
Stage 2: Understand community vision for context and thoroughfare.
Understanding the vision, goals and objectives of the place a thoroughfare serves is a critical step. This includes understanding the context as well as the thoroughfare's role in the transportation system. Context sensitive thoroughfare design considers today's conditions but also reflects plans for the future.
In this stage, the designer collaborates with the public, stakeholders and a multidisciplinary team to develop goals and objectives for the project.
If the community in which the project is located has developed a vision and established goals and objectives, this stage entails a thorough knowledge and understanding to ensure that the project achieves the vision. This stage requires review of planning documents, transportation and circulation plans, and land use and zoning codes. Through the community vision, a multidisciplinary team can determine both the existing and future context for the area served by the thoroughfare. It is the future context that deines the long-term transportation and place-making function of the thoroughfare.
If the community lacks a vision, desires a change, or requires further detail in the project area, this is an opportunity to use a public and/or stakeholder process to answer questions that will form the basis of a vision: What do we want the community to be? What do we want the community to look like? How do we want the community to function? Frequently, it is desirable to use a participatory process to develop concepts and alternatives, even if a vision exists. This establishes public ownership in the project and helps meet the requirements of the National Environmental Policy Act (NEPA), where applicable.
The process for working with the public and stakeholders to develop a vision is outside the scope of this report. However, there are resources available to explain the process such as Public Involvement Techniques for Transportation Decision-Making by the U.S. DOT Federal Transit Administration.
Stage 3: Identify compatible thoroughfare types and context zones.
Stage 3 determines the compatibility between the existing and future context and the appropriate thoroughfare type. It considers land use and transportation integration, modal requirements, place-making objectives and the functional roles of the adjacent land use and street.
This report provides the tools for this stage in Chapter 4—a framework for urban thoroughfare design. Stage 3 relies on an understanding of the existing and future context identiied in Stage 2. Stages will result in the identification of opportunities, design controls and constraints that will dictate thoroughfare design elements and project phasing.
Chapter 4 guides the thoroughfare design team through the process of identifying context and alternative thoroughfare types best suited for the identi-ied context zone. The initial relationship between the context zone and the thoroughfare is tentative, leading to stage 4 of the process.
Stage 3 entails close examination of modal requirements (such as transit, bicycle, pedestrian and freight needs) and establishes design controls such as trafic volumes, speed, corridorwide operations, right-of-way constraints and other fundamental engineering controls (Chapter 7 provides additional information). This stage might be an iterative process that compares needs with constraints, identiies trade-offs and establishes priorities. Speciic steps in this stage include:
1. Determining the context zone(s) within which each segment of the thoroughfare is located.The context zones, whether existing or projected, are determined from a community or regional comprehensive plan if one is available. In the absence of such a plan, the context zones can be derived from the description of the function and configuration, the type of the buildings fronting the thoroughfare and whether the context is predominantly residential or commercial. Note that the context zone will likely vary throughout the length of a corridor, requiring the thoroughfare to be divided into segments that may have varying design parameters and elements. Table 4.1 in Chapter 4 can assist in identifying context zones; and
2. Selecting the appropriate thoroughfare type based on context zone and purpose of the thoroughfare as determined from the area plan, including its functional classification designation.
Tables 4.2, 4.3 and 4.4 in the previous chapter assist a multidisciplinary team in developing the character and general design parameters of the thoroughfare. The thoroughfare's functional classiication establishes the role of the thoroughfare in the transportation network. The thoroughfare type helps determine certain design controls such as target speed, the physical design of the thoroughfare and the design elements that serve the activities of the adjacent uses. For urban thoroughfares in walkable communities, the combination of thoroughfare type, functional classiication and context zone is used to select the appropriate general design parameters presented in Chapter 6 and the streetside, traveled way and intersection design guidelines presented in Chapters 8 through 10, respectively.
Stage 4: Develop and test the initial thoroughfare concept.
Understanding the balance between the regional functions and local needs of the thoroughfare is a key factor in selecting the appropriate design criteria and preparing the initial thoroughfare concept. Stage 4 determines whether the boulevard, avenue, or street concept of initial width is appropriate. This step in the process feeds back into the previous stages if the evaluation of the concept results in the need to change the initial thoroughfare type or modify the system design. In this stage a multidisciplinary team uses the design parameters identiied by the context zone/thoroughfare type combination selected in stage 3 (Tables 6.1 through 6.4 in Chapter 6) to determine the basic elements of the thoroughfare that affect its width, including on-street parking, bicycle facilities, number and width of travel lanes, median and general coniguration of the streetside.
In stage 4, initial thoroughfare concepts are developed by establishing vital parameters such as speed, number of lanes, travel way and streetside widths, right of way and other design parameters. In this stage, the thoroughfare's function beyond the limits of the project are considered along with its multimodal and place-making functions to ensure both the community vision and the overall network operate as planned.
The team then tests and validates the initial concept at the corridor and network level of performance. A successful urban thoroughfare concept is one that, when viewed as part of an overall system, maintains acceptable systemwide performance even though the individual thoroughfare intersections may experience congestion. Network performance should include multimodal performance measures. Chapter 3 describes the role of the thoroughfare in the network and references network-connectivity guidelines.
Evaluation of the thoroughfare at the corridor and network level will either validate the initial concept or indicate the need to revisit the context zone/thoroughfare type relationship or modify the design parameters. The evaluation might even indicate the need to revise regional or subregional land use and circulation plans.
Stage 5: Develop a detailed thoroughfare design.
Once a successful initial concept has been developed and validated, the process leads to the inal step of detailing the thoroughfare design. Stage 5 involves using the guidance to integrate the design of the street components, context, streetside, travelway and intersections. As with any design process, this stage is iterative, resulting in a thoroughfare plan and cross-sections. This stage then leads into preliminary and inal engineering. Speciic steps in this stage include:
The evaluation and initial designs in the previous stages lead to stage 5—refinements and development of a detailed thoroughfare design that reflects the project objectives. This step culminates in final engineering design and environmental approvals.
1. Identifying available right of way and other constraints.
In new developments, this step establishes the necessary right of way to accommodate the thoroughfare type and its desirable elements. In existing built areas, this step identiies the available right of way as an input to the thoroughfare design process. It is important to identify any other constraints that will affect the design, such as utility placement.
In existing areas, an initial cross-section of the desirable streetside and traveled way elements is prepared (see design examples in Chapter 6) and compared with the available right of way. If the total width of the desirable design elements exceeds the right of way, determine the feasibility of acquiring the necessary right of way or eliminating or reducing nonvital elements.
2. Design the traveled way elements.
First identify and select the design controls appropriate for the thoroughfare type and context zone identiied in stage 3. These controls include target speed (affects sight distance and alignment), control/design vehicle (affects lane width and intersection design) and modal requirements, such as level of pedestrian activity, parking, bike routes, primary freight routes, or transit corridor and so forth. A trade-offs evaluation may be necessary if right of way is constrained. The design controls and context, along with the available right of way, assist in the selection of the appropriate dimensions for each design element.
3. Design the streetside elements.
The design of the streetside elements requires understanding the characteristics and activity of the adjacent existing or future context. For example, does or will the context include ground floor retail or restaurants that require a wider frontage zone to accommodate street cafes? Does or will the thoroughfare include a transit corridor that requires a wider furnishings zone to accommodate waiting areas and shelters? This report provides general guidance on the optimal and constrained streetside width used initially, but the actual design might require more analysis of existing and future activity levels.
4. Assemble the thoroughfare components.
This is an iterative process, particularly in constrained rights of way. This process entails identifying trade-offs to accommodate the streetside and traveled way elements within the right of way. It is important to refer back to the community vision stage to understand and evaluate the trade-offs. The last section of this chapter provides an approach to design thoroughfares in constrained conditions.
Flexibility in Application of Design Criteria
Flexibility in the application of design criteria requires an understanding of the functional basis for the criteria and the ramiications of changing dimensions or adding/eliminating design elements. Dimensions, whether for elements in the streetside, traveled way, or intersection, should not be applied arbitrarily but should be based on a speciic rationale. The concept of design flexibility is not limited to thoroughfares in walkable areas but is a concept that recognizes the unique circumstances of every project under every setting. The challenge that this concept presents is aptly summarized in the Federal Highway Administration's Flexibility in Highway Design (1997):
For each potential project, designers are faced with the task of balancing the need for the highway improvement with the need to safely integrate the design into the surrounding natural and human environments.
To correctly apply flexibility, the thoroughfare designer should understand the relationship between a recommended criterion and its role in safety and mobility for all users. The American Association of State Highway and Transportation Oficials (AASHTO) emphasizes this requirement in the following quote from A Guide for Achieving Flexibility in Highway Design (2004c):
Only by understanding the actual functional basis of the criteria and design values can designers and transportation agencies recognize where, to what extent and under what conditions a design value outside the typical range can be accepted as reasonably safe and appropriate for the site-specific context.
Flexibility is related to the design controls used in the selection of criteria. Design controls recognized by AASHTO include functional classiication, location (urban versus rural), traffic volumes and level of service, design vehicle and driver and target speed. All of these design controls are important, regardless of whether the designer believes the thoroughfare design is context sensitive or not.
Design Process in Constrained Right of Way
The nature of thoroughfare design is balancing the desired design elements of the thoroughfare with right-of-way constraints. The thoroughfare designs presented in this report illustrate the desired elements within the cross-section, but actual conditions frequently limit the width of the street. Designing thoroughfares in constrained rights of way requires prioritizing the design elements and emphasizing the higher-priority elements in constrained conditions. Higher-priority design elements are those that help the thoroughfare meet the vision and context sensitive objectives of the community (the objectives established in stage 2). Lower-priority elements have less influence on achieving the objectives and can be relinquished in cases of insuficient right of way.
Often the width of the public right of way varies along the thoroughfare, making the job of the designer even more challenging. When the width of the right of way varies, it is useful to prioritize design elements and develop a series of varying cross-sections representing:
1. Optimal conditions—sections without right-of-way constraints that can accommodate all desirable elements;
2. Predominant—representing sections of the predominant right-of-way width in the corridor that accommodate all of the higher-priority elements;
3. Functional minimum—representing a typically constrained section where most of the higher-priority elements can be accommodated; and
4. Absolute minimum—representing severely constrained sections where only the highest-priority design elements can be accommodated without changing the type of thoroughfare.
Below the absolute minimum, or if the predominant right of way is equal to or less than the absolute minimum, consider changing the thoroughfare to a different type while attempting to maintain basic function, or consider converting the thoroughfare to a pair of one-way thoroughfares (couplet)—or, further still, consider other solutions that achieve the community vision. This requires recycling through the steps of the design process, potentially requiring a review of the community vision for the thoroughfare and the area transportation plan and/or identifying a new context zone/thoroughfare relationship. If the vision for the corridor is long range, then the necessary right of way should be acquired over time as the adjacent property redevelops. Under these circumstances the optimal (or the predominant) thoroughfare width can be phased in over time, beginning with the functional or absolute minimum design in the initial phase.
In constrained conditions it might be tempting to minimize the streetside width and only provide the minimum pedestrian throughway (5 feet). In urban areas, however, even under constrained conditions, it is critical to provide at least a minimum width furnishing zone to accommodate street trees, utility poles and other appurtenances. Without the furnishings zone, trees, utilities, benches and shelters and other street paraphernalia might encroach into the throughway for pedestrians or result in an inadequate width streetside when the community's vision for the context zone is ultimately achieved.
Table 5.2 provides minimum recommended dimensions for the streetside in constrained conditions, which vary by the predominant land use. In residential areas, the furnishings zone can be a minimum of 3 feet. This width continues to provide a buffer between pedestrians and the traveled way and also allows a minimal width for plantings and utilities. The clear throughway for pedestrians should be a minimum of 5 feet. The frontage zone should be a minimum of 1 foot adjacent to buildings or eliminated adjacent to landscaping. These dimensions result in a minimum residential streetside width of 9 feet.
In predominantly commercial areas with ground floor retail, the furnishings zone minimum width is 4 feet to allow for street trees, utilities and so forth. The clear throughway for pedestrians is a minimum of 6 feet to allow for a higher level of pedestrian activity, and the frontage zone minimum is 2 feet to provide a buffer between moving pedestrians and buildings, resulting in a 12-foot streetside width. When a wider frontage zone is needed (for street cafes and so forth), consider requiring the adjacent property to provide an easement to effectively expand the streetside width.
American Association of State Highway and Transportation Officials. 1997. Highway Safety Design and Operations Guide. Washington, DC: AASHTO.
American Association of State Highway and Transportation Oficials. 1999. Guide for the Development of Bicycle Facilities. Washington, DC: AASHTO.
American Association of State Highway and Transportation Officials. 2002. Roadside Design Guide. Washington, DC: AASHTO.
American Association of State Highway and Transportation Oficials. 2004a. Policy on Geometric Design of Streets and Highways, Fifth Edition. Washington, DC: AASHTO.
American Association of State Highway and Transportation Oficials. 2004b. Guide for the Planning, Design and Operation of Pedestrian Facilities. Washington, DC: AASHTO.
Table 5.2 Minimum Recommended Streetside Dimensions for Thoroughfares in Walkable Areas Under Constrained Conditions
|Residential (All Context Zones)|
|Edge and Furnishing Zone (Planting Strip, utilities, etc.)||3 feet|
|Clear Pedestrian Travel Way||5 feet|
|Frontage Zone||1 foot|
|Total Minimum Streetside Width:||9 feet|
|Commercial with Ground Floor Retail (All Context Zones)|
|Edge and Furnishing Zone (Treewell1, utilities, bus stops, etc.)||4 feet|
|Clear Pedestrian Travel Way||6 feet|
|Frontage Zone||2 feet|
|Total Minimum Streetside Width:||12 feet|
1 Plant only small caliper trees (4" diameter when mature) in 4-foot treewells.
The minimum recommended streetside dimensions for thoroughfares in other areas (such as vehicle-oriented areas) should be based on the designer's understanding of the community's objectives, the future desired traversability of the area, the future potential redevelopment of the adjacent property and the need to accommodate all users.
American Association of State Highway and Transportation Oficials. 2004c. A Guide for Achieving Flexibility in Highway Design. Washington, DC: AASHTO.
Federal Highway Administration, Federal Transit Administration. 1996. Public Involvement Techniques for Transportation Decision-Making. Washington DC, FHWA-PD-96-031, September 1996. Accessible via www.fhwa.dot.gov/reports/pittd/cover.htm.
Transportation Research Board, 2004, Highway Capacity Manual, Special Report 209, Washington, DC: TRB.
Transportation Research Board. 2008. Dowling, R., et al, Multimodal Level of Service Analysis for Urban Streets, NCHRP Report 616. Washington, DC: TRB. Accessible via http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_616.pdf .