Design Guide for Low-Speed Multimodal Roadways (2018)

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10 This chapter addresses a wide range of design considerations for low- and intermediate-speed (45 mph or less) roadways and streets that serve a mix of user modes, particularly the vulnerable- user modes of pedestrians and bicyclists. 2.1 Design Controls, Criteria and Elements Controls are physical and operational characteristics that guide the selection of criteria in the design of thoroughfares. Some design controls are fixed, such as terrain, climate and certain driver-performance characteristics, but most controls can be influenced in some way through design and are determined by the designer. The Green Book (AASHTO 2011a) and A Guide for Achieving Flexibility in Highway Design (AASHTO 2004a) identify location as a design control and establish different design criteria for rural and urban settings. AASHTO recognizes the influence context has on driver characteristics and performance. The Green Book defines the environment as “the totality of humankind’s surroundings: social, physical, natural and synthetic” and states that full consideration to envi- ronmental factors should be used in the selection of design controls. The Guide focuses on design controls and critical design elements for all users in low- and intermediate-speed roadway environments, particularly urban and suburban contexts where a wide range of users can be expected. The Green Book identifies functional classification and design speed as primary factors in determining highway design controls and criteria. The Green Book generally separates controls and criteria by both roadway functional classification and two levels of context: rural and urban. The primary differences between rural and urban contexts are • The typical speed ranges at which the facilities operate, • The mix and characteristics of the users, and • The opportunities and constraints of the surrounding context. In addition to functional classification, speed and context, AASHTO presents other design controls and criteria that form the basis of its recommended design guidance. The basic con- trols are: • Design vehicle; • Vehicle performance (acceleration and deceleration); • Driver performance (e.g., age, reaction time, driving task, guidance); • Traffic characteristics (volume and composition); • Capacity and vehicular LOS; • Access control and management; C H A P T E R 2 Design Considerations for All Users in Low- and Intermediate-Speed Environments

Design Considerations for All Users in Low- and Intermediate-Speed Environments 11 • Pedestrians and bicyclists; • Safety; • Environment; and • Economic analysis. The Green Book also addresses the alignment and cross section of roadways and notes its impact on users, communities and the environment. A roadway’s alignment is defined as a variety of design elements that combine to create a facility that serves users safely and efficiently, consistent with the intended function of the facility. AASHTO recommends that each alignment element should complement others to achieve a consistent, safe, and efficient design and lists several principal elements of design that are common to all classes of highways and streets. These elements include sight distance, horizontal and vertical alignments, superelevation, traveled way widening, grades, and all other cross-section elements addressed in the geometric design process. The Green Book presents the pedestrian needs as a factor in highway design and recognizes the pedestrian as the “lifeblood of our urban areas.” Pedestrian characteristics that serve as design controls include walking speed, walkway capacity and the needs of persons with disabilities. The Green Book also states that the bicycle is an important element to consider in the design process and that while it notes that much of the roadway system itself allows for bicycle use, a number of special accommodations may be needed for specially designated bikeway facilities within the right-of-way (AASHTO 2011a). The Guide for the Planning, Design and Operation of Pedestrian Facilities (Pedestrian Facilities Guide), and the Guide for the Development of Bicycle Facilities (Bicycle Guide) expand significantly on the limited design policy and guidance content in the Green Book, presenting detailed factors, criteria and design controls (AASHTO 2004b, AASHTO 2014b). This Guide emphasizes pedestrians and bicyclists as a design control in all contexts, but focuses on facilities operating in the low- and intermediate-speed ranges that exist primarily in urban, suburban and rural town/village contexts. 2.2 Conventional Versus Evolving Roadway Design Practice The Guide presents design guidance that is generally consistent with the AASHTO Green Book, AASHTO’s supplemental publications and conventional engineering practice. There are, however, four design controls in the application of non-motorized user, context-sensitive design principles that are used somewhat differently than in the conventional highway design process. These controls are: • Functional classification; • Design Speed; • Design vehicle; and • Land Use Context. 2.2.1 Functional Classification In the geometric design process, it is important to identify and understand a roadway’s role in the local, regional and statewide functional classification network. The Green Book recog- nizes, however, that relying entirely on functional classification to design a roadway can result in roadway facilities that do not consider the local context, and that design has impacts beyond just traffic service (AASHTO 2011a): A highway has wide-ranging effects in addition to providing traffic service to users. It is essential that the highway be considered as an element of the total environment. The term ‘environment,’ as used here refers to the totality of humankind’s surroundings: social, physical, natural, and synthetic.

12 Design Guide for Low-Speed Multimodal Roadways In addition, FHWA’s Highway Functional Classification: Concepts, Criteria and Procedures recognizes the challenges in balancing the goals of a facility’s functional classification with the land use context and mix of users that may be present (FHWA 2013b): After a functional classification has been assigned to a roadway, however, there is still a degree of flex- ibility in the major controlling factor of design speed. There are no ‘cookie-cutter’ designs for roadways. Instead, there is a range of geometric design options available. Although the functional classification of a facility may remain constant along a route, it is suggested that the design of the facility change as needed to recognize the context, mix of users and community goals of the areas through which the roadway travels. 2.2.2 Design Speed The most influential design control, and the design control that provides significant flexibility in urban areas, is speed. Roadway design in areas with existing or planned multimodal activity normally are designed with the intent of achieving a “target speed” for the facility that best balances safety, accessibility and mobility for all users. Design speed is used as the primary design control in the AASHTO Green Book. Convention- ally, design speed has been encouraged to be as high as is practical. This Guide alters the focus from design speed to target speed, which is driven by the functional classification, roadway type and context, including variations in the type of land use context within the project limits. FHWA’s guidance memorandum on the relationship between design speed and posted speed states that “[in] urban areas, the design of the street should generally be such that it limits the maximum speed at which drivers can operate comfortably, as needed to balance the needs of all users” (FHWA 2015c). The design guidance in the Guide has been developed based on the concept of determining and designing to a selected target speed not only in urban contexts, but in any low- and intermediate-speed facility context where multimodal users are present or expected to be present in the future. Target speed typically is defined as the highest operating speed at which vehicles should ideally operate on a roadway in a specific context. The target speed should complement the level of multimodal activity generated by adjacent land uses to provide both mobility for motorized vehicles and a safe environment for pedestrians and bicyclists. The target speed is intended to become the posted speed limit. In some jurisdictions, the speed limit must be established based on measured speeds. In these cases, it is important that the design consider the context of the roadway, encouraging a desired operating speed that helps ensure actual operating speeds will match the speed limit. Target speed then becomes the primary control for determining the values of the following geometric design elements: • Minimum intersection sight distance; • Minimum sight distance on horizontal and vertical curves; and • Horizontal and vertical curvature. Target speeds typically range from 25 mph to 35 mph for roadway types that are considered walkable and bikeable by today’s practices. These lower target speeds are a crucial characteris- tic of roadways in mixed-use areas, traditional urban areas, some suburban areas, developing rural areas, and small towns and villages on rural roadways. On urban and suburban roadways with higher volumes of vehicular traffic and planned operating speeds ranging from 40 mph to 45 mph, providing a safe and accessible design to accommodate non-motorized users is even more important. In these cases of higher volumes and speeds, additional treatments and safe- guards for non-motorized users usually are warranted.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 13 2.2.3 Design Factors that Influence Target Operating Speed Establishing a target speed that is artificially low relative to the design of the roadway will only result in operating speeds that are higher than desirable and difficult to enforce. Consistent with AASHTO policy, the Guide urges sound judgment in the selection of an appropriate target operating speed based on a number of factors and reasonable driver expectations. Factors in urban areas include transitions between higher- and lower-speed roadways, terrain, intersection spacing, frequency of access to adjacent land, type of roadway median, presence of curb parking and level of pedestrian and/or bicycle activity. AASHTO’s guide to flexibility in highway design aptly summarizes the selection of a target operating speed in urban areas (AASHTO 2004a): Context-sensitive solutions for the urban environment often involve creating a safe roadway environ- ment in which the driver is encouraged by the roadway’s features and the surrounding area to operate at lower speeds. Roadway design for a facility that is intended to accommodate walking and bicycling should start with the selection of a target speed. The target speed should be applied to those geometric design elements where speed is critical to safety, such as horizontal and vertical curvature and intersection sight distance. The target speed is not set arbitrarily; rather, it is achieved through a combination of design and operating measures that normally includes many of the following design elements: • Using narrower travel lanes that encourage motorists to naturally slow their speeds; • Using physical measures such as curb extensions and medians to narrow the traveled way; • Using design elements such as on-street parking to create side friction; • Providing minimal or no horizontal offset between the inside travel lane and median curbs; • Eliminating superelevation; • Eliminating shoulders in urban and suburban applications, except for bicycle lanes; • Incorporating smaller curb-return radii at intersections and eliminating or reconfiguring high-speed channelized right turns; • Introducing roundabouts or traffic circles to produce slow points; • Paving materials with texture (e.g., crosswalks, intersection operating areas) detectable by drivers as a notification of the possible presence of pedestrians; • Setting signal timing for moderate progressive speeds from intersection to intersection; and • Ensuring proper use of speed limit, warning and advisory signs, as well as other appropriate devices to gradually transition speeds when approaching and traveling through areas with multimodal activity. Other factors widely believed among roadway design practitioners to influence speed include a canopy of street trees, the psychological enclosure of a roadway formed by the proximity of a wall of multistory buildings, the striping of edge lines or bicycle lanes and the presence or absence of parking lanes. These features are all elements of urban and some suburban contexts, but they should not be relied upon as speed reduction measures until further research provides more definitive guidance regarding their impacts to all users. The design practitioner is advised to be careful not to relate speed to capacity in urban and suburban areas at low and intermediate speeds, and to avoid the assumption that a high-capacity street requires a higher target speed. Under interrupted-flow conditions, such as on roadways in urban/suburban areas, intersection operations and delay normally have a greater influence on capacity than does speed. 2.2.4 Design Vehicles The vehicle chosen for geometric design greatly influences the selection of design criteria such as lane width and curb-return radii. Some agencies and practitioners will conservatively

14 Design Guide for Low-Speed Multimodal Roadways require the design to accommodate the largest design vehicle that could use a roadway (e.g., WB 50 to WB 67), regardless of the frequency. Consistent with AASHTO guidance, context-sensitive design and consideration of all users emphasizes an analytical approach in the selection of a design vehicle, including evaluation of the trade-offs involved in selecting one design vehicle over another. In urban areas, it is not always practical or desirable to choose the largest possible design vehicle, particularly if such a vehicle will use the facility only occasionally. The impacts of this design choice—on pedestrian crossing distances, speed of turning vehicles and other aspects of the roadway—may be inconsistent with the community’s vision, goals and objectives for the roadway. At the same time, selection of a smaller design vehicle in the design of a facility that is regularly used by large vehicles can invite frequent operational problems. To balance these two concerns, practitioners are encouraged to select two types of vehicles for use in the design process: (1) a design vehicle representative of the vehicles that will use the facility frequently (e.g., a bus on bus routes, a semi-tractor trailer on primary freight routes or routes accessing loading docks) and (2) a control vehicle representative of the type of vehicle that will use the facility infrequently. The design vehicle must be regularly accommodated without encroachment into the oppos- ing traffic lanes. The control vehicle’s infrequent use of a facility must be accommodated, but encroachment into the opposing traffic lanes, multiple-point turns, or minor encroachment into the street side is deemed acceptable. A condition that uses the design vehicle concept arises when large vehicles regularly turn at an intersection with high volumes of opposing traffic (such as a bus route). A condition that uses the control vehicle concept arises when occasional large vehicles turn at an intersection with low opposing traffic volumes (e.g., a moving van in a residential neighborhood or once-per-week delivery at a business) or when large vehicles rarely turn at an intersection with moderate to high opposing traffic volumes (e.g., emergency vehicles). The inside apron of a roundabout also serves the same function allowing the occasional longer wheelbase vehicle to negotiate the design. The practitioner generally can obtain classification counts to determine the mix of traffic and frequency of large vehicles in order to estimate how this mix will change as context changes, keeping the design consistent with the community’s long-range vision. If there are no specific expectations, the practitioner may consider the use of a single-unit truck or local transit vehicle as an appropriate design vehicle. State highways have traditionally served through traffic and heavy/large vehicle traffic, but modern roadway system planning tries to accommodate movements using network and context considerations. Large, heavy and unusually demanding vehicles need to be accommodated with reasonable convenience; however, in some cases, routes other than state highways may be more appropriate or more easily accommodating to these vehicles. 2.2.5 Land Use Context Conventional roadway design is controlled by location to the extent that it is rural or urban (and sometimes suburban) as defined in the AASHTO Green Book. The Guide broadens the choices for context using an expanded context range as outlined in Stamatiadis et al (2017). This recent research provides the following context areas: • Rural; • Rural village/town; • Suburban; • Urban; and • Urban core.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 15 This Guide also expands the variation in design elements controlled by location to include predominant ground floor land uses such as residential, commercial, school, park and so forth. Land uses (existing and planned/future) will generally govern the level of multimodal activity, which in turn influences the design of the roadway. These influences include, but are not limited to, pedestrians and bicyclists, transit, economic activity of adjacent uses and right-of- way constraints. The context-sensitive approach also considers planned land uses that represent changes in existing development patterns and special design districts that seek to protect scenic, environmental, historic, cultural, or other resources. 2.3 Roadway User Definitions and Characteristics The Guide addresses three general categories of users of the roadway right-of-way. In addi- tion, within each user category, generally accepted “classes” are important to consider in the design process. The categories of users are: • Motorized vehicles; • Pedestrians; and • Bicyclists. Because of their vulnerability in any crash situation with a motorized vehicle, pedestrians and bicyclists often are called “non-motorized” or “vulnerable” road users. These terms are used interchangeably throughout the Guide. AASHTO’s Green Book guidance recognizes that addressing the needs of all users within the right-of-way is a necessary part of the geometric design process. The foreword to the Green Book includes the following statement (AASHTO 2011a): Emphasis is placed on the joint use of transportation corridors by pedestrians, cyclists and public tran- sit vehicles. Designers should recognize the implications of this sharing of the transportation corridors and are encouraged to consider not only vehicular movement, but also movement of people, distribution of goods, and provision of essential services. A more comprehensive transportation program is thereby emphasized. 2.3.1 Motorized Vehicles The Green Book establishes four general classes of motorized design vehicles: (1) passenger cars, (2) buses, (3) trucks and (4) recreational vehicles. The passenger-car class includes pas- senger cars of all sizes, sport/utility vehicles, minivans, vans, and pick-up trucks. Buses include intercity (motor coach), city transit, school, and articulated buses. The truck class includes single-unit trucks, truck tractor-semitrailer combinations, and truck tractors with semitrailers in combination with full trailers. Recreational vehicles include motor homes, cars with camper trailers, cars with boat trailers, motor homes with boat trailers, and motor homes pulling cars. The Green Book also notes that if bicycle use is allowed on a roadway, the bicycle should also be considered as a design vehicle (AASHTO 2011a). Motorcycles are not mentioned specifi- cally as a design class, but depending on their volumes, special design considerations may be warranted. 2.3.2 Pedestrians A roadway that is designed to accommodate pedestrians must consider not only their volumes and travel needs, but also the wide range of needs and physical capabilities possible among different pedestrian groups. An agile, able-bodied person can frequently overcome accessibility challenges and pedestrian facility design deficiencies. When age or functional disabilities reduce

16 Design Guide for Low-Speed Multimodal Roadways a person’s mobility, judgment, sight, or hearing, however, providing proper design solutions becomes much more important. The pedestrian user category includes four generally accepted and distinct classes: • Able-bodied pedestrians with average or better agility; • Pedestrians with mobility, vision or hearing disabilities; • Older pedestrians with limited functions and/or mobility; and • Younger pedestrians with more erratic behavior and generally smaller stature. The interactions of all pedestrian classes with other users are a major consideration in roadway design. Pedestrians typically are a part of every roadway environment other than access-controlled roadways, and the Green Book recommends that their presence and needs be considered in both rural and urban areas. The higher volumes of pedestrians in urban, sub urban and town/village areas means that they influence roadway design features more frequently than do the lower volumes of pedestrians in purely rural settings; however, the accommodation, safety and convenience of every type of user must be understood and fully considered in the design process. Given the levels of vehicular traffic in urban areas, it is often difficult to balance the needs of pedestrians with the needs of vehicles (and sometimes bicycles). Pedestrians and motorized vehicles can often safely co-exist in low-speed, low-volume local roadway environments, such as residential neighborhoods. Pedestrian facilities typically are much more critical on roadways with higher speeds and volumes. Necessary accommodations can include sidewalks and their intersection/driveway interactions, crosswalks, traffic control features, and curb cuts (depressed curbs and ramped sidewalks) and ramps for older walkers and persons with mobility impairments. Pedestrian facilities also include transit stops or other loading areas, sidewalks on grade sepa- rations and the stairs, escalators or elevators related to these facilities. The PROWAG must be considered when designing pedestrian facilities in the roadway right-of-way (U.S. Access Board 2011). 2.3.3 Bicycles The bicycle is an important element for consideration in the highway design process. In many urban and suburban areas, the existing street and highway system provides much of the network needed for bicycle travel. Motorized vehicles and bicycles often can safely co-exist in low-speed, low-volume roadway environments such as residential local street networks. As roadway speeds and volumes increase, accommodations for bicycles become a much more critical element of the design process. The bicycle category includes three or four classes of user depending on the source consulted. For the purposes of the Guide, three classes will be used: • “A” class: Advanced bicyclists with considerable experience and confidence; • “B” class: Bicyclists of average skills and confidence; and • “C” class: All other bicyclists (primarily children). The interactions of all bicycle user classes with other users are another major consideration in roadway design. Bicyclists are an increasing element in every roadway environment, especially in urban and suburban areas. The Green Book recommends that bicyclists’ presence and needs be considered in both rural and urban areas (AASHTO 2011a). Balancing a roadway design to meet the needs of bicyclists, pedestrians and motorized vehicles can be a challenging process, especially in con- strained roadways in urban and suburban areas.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 17 Bicycle facilities take many forms, including signed routes, shared lanes with motorized vehicles, on-street striped bike lanes, on-street buffered bike lanes, separated bike lanes and bike paths/trails that are separated from motorized vehicle lanes or from the roadway alto- gether. Other bicycle facilities in the right-of-way may include intersection and non-intersection crossing locations, special traffic control signals, special roadway markings and bicycle parking accommodations. 2.4 Functional System Considerations 2.4.1 Roadway, Bicycle, Pedestrian and Transit Networks Ideally, roadway design is based on a combination of local needs and the role of the facility in the area or region’s transportation network. The functional classifications of roadways in a typi- cal transportation network consist of principal and minor arterials, major and minor collectors and local roads and streets. Planning for a roadway network should anticipate the needs generated by planned land uses (including intensity) while maintaining and promoting compatibility with the resulting neighborhoods and community. Community and neighborhood areas may have widely varying characteristics, needs, features and activity levels. The community’s goals for the specific neigh- borhoods, areas, or corridors supported by the roadway network also may vary. The roadway network typically develops over time in accordance with state and/or regional transportation plans (depending on the agency maintaining the facility) in coordination with a local community’s comprehensive plan (or transportation plan). The density (spacing) of the network, the capacity (vehicle lanes, walkway, bicycle, transit), the space for landscaping, ameni- ties and other components of the right-of-way should encourage and support the development pattern, land use type and level of development intensity in accordance with applicable plans. For many low-volume, low-speed local roadways, the various modes often can be accommodated in one travel space without providing separate facilities for each mode. As speeds, volumes and functional classification levels increase, the need increases to separate modal facilities to address safety and reduce potential conflicts. The transportation network in any urban or suburban region should function as a system of corridors containing rights-of-way for vehicular, pedestrian, bicycle and transit networks that together meet and support the communities’ desired urban or suburban form and growth. The level of roadway capacity provided for each mode will depend on the current and future projected local demand, the degree of interaction between the facilities and local land uses, the amount of multimodal activity generated and the amount of through travel using the roadway network. The design of any individual roadway needs to be responsive to the varied development and activities associated with changes in context zones throughout the project. 2.4.2 Network Planning Principles for Multimodal Roadways Principles outlined in ITE’s Designing Walkable Urban Thoroughfares: A Context Sensitive Approach outline an approach for planning and designing roadway networks that are sensitive to community objectives and context (ITE 2010a). Considering all of these elements in the design process will help create a balanced, multimodal environment for each roadway facility in the network. Chapter 3 of the ITE publication lists the following principles for creating effective, connected multimodal networks in urban and suburban areas for all types of users (ITE 2010a): • Multimodal network planning should be integrated into long-range comprehensive plans that simulta- neously address land use, transportation and urban form.

18 Design Guide for Low-Speed Multimodal Roadways • Network planning should address multimodal mobility and access needs along with goods movement, utilities placement and emergency services. • Reserving right-of-way for the ultimate width of roadways should be based on long-term needs of all users defined by objectives for both community character and mobility. Ideally, network and corridor operating plans will serve all modes and all users, with uses varying on some roadways according to context, needs, objectives and priorities while consider- ing overall network needs. Network rights-of-way that form a grid-like pattern of continuous roadways can better distribute traffic loading, and roadways can be interconnected with specified distances between intersections to: • Provide choices of routes; • Reduce travel distances; • Promote use of transit, bicycles and walking; • Accommodate utility needs; and • Provide traffic control systems that support and encourage use of existing and planned walking, bicycling and transit modes. 2.5 Functional Classification and Urban Roadway Terminology The roadway design process in the Guide refers to both functional classification and the urban “thoroughfare” type typically used to classify streets in urban areas. Conventional federal functional classification is intended to define a roadway’s function and role in the network and govern the selection of certain design controls. Designers typically use the functional class to determine: • Continuity of the roadway through a region and the types of places it connects (such as major activity centers); • Purpose and lengths of vehicle trips accommodated by the roadway; • Range of typical design speeds; • Level of land access and level of access management; • Type of freight service; and • Types of public transit services (e.g., bus, bus rapid transit, light rail). In urban and suburban areas, additional factors are typically considered to inform the designer’s decisions about both the physical design and the operations of the roadway, con- sidering all users. These factors include: • Categories, types and volumes of non-motorized users, both current and anticipated; • The surrounding context, which is used to guide the most appropriate physical configuration (e.g., elements, criteria, dimensions) of the: – Traveled way (e.g., lanes, medians, on-street parking, on-street bicycle lanes); – Side of street (e.g., sidewalks, off-street bike facilities, landscaping, public space, user amenities); – Intersections; and • A target speed appropriate for the context and mix of users; and • Sight distance. Exhibit 2-1 shows specific thoroughfare types that are commonly used in urban area planning in the United States and gives a general description of each type of facility. As this Guide focuses on roadways with design speeds at or below 45 mph, only four of the six types fall into this

Design Considerations for All Users in Low- and Intermediate-Speed Environments 19 category: boulevards, avenues, streets and rural roads. These roadway thoroughfare types typi- cally serve a mix of modes, including pedestrians, bicyclists, motorized vehicles (for passenger and freight) and possibly transit. Exhibit 2-1 does not address a fairly typical urban or suburban roadway type that has four to six through lanes, intermediate operating speeds (40 mph to 45 mph), and either no median, a two-way left-turn lane (TWLTL), or possibly a raised or flush pavement median. Inherent modal conflicts exist on these motorized vehicle-focused roadways that present significant design challenges to accommodating all modes. Exhibit 2-2 shows the typical relationship between thoroughfare types used in community planning and roadway functional classification. In general, boulevards serve an arterial function, avenues may serve arterial or collector functions and streets typically serve a collector or local function in the network. Thoroughfare Type Functional Definition Freeway/ Expressway/ Parkway High-speed (50 mph +), controlled-access thoroughfare with grade-separated interchanges and no pedestrian access. Includes tollways, expressways and parkways that are high- or medium-speed (45 mph +), limited-access thoroughfares with some at-grade intersections. Parkways generally include landscaping on each side and a landscaped median. Truck access may be limited on parkways. Rural Highway High-speed (50 mph +) thoroughfare designed both to carry traffic and to provide access to abutting property in rural areas. Intersections are generally at grade. Boulevard Low- to intermediate-speed (30 mph to 45 mph), divided and undivided arterial thoroughfare in an urban environment and designed to carry both through and local traffic, pedestrians and bicyclists. May be a long corridor; typically four lanes but sometimes wider, serves longer trips and provides pedestrian access to land. May be a high-ridership transit corridor. Boulevards are primary goods movement and emergency response routes and use vehicular and pedestrian access management techniques. Boulevards may have raised medians or two-way left-turn lanes (TWLTLs), and curb parking may be used on some boulevards in low-speed settings. Multiway Boulevard Low- to intermediate-speed (30 mph to 45 mph) boulevard in an urban environment characterized by a central roadway for through traffic and parallel access lanes accessing abutting property, parking and pedestrian and bicycle facilities. Parallel access lanes typically are separated from the through lanes by curbed islands with landscaping; these islands may provide transit stops and pedestrian facilities. Avenue Low- to medium-speed (25 mph to 35 mph) urban arterial or collector thoroughfare, generally shorter in length than a boulevard and primarily serving access to abutting land. Serves as a primary pedestrian and bicycle route and may serve local transit routes. Avenues generally do not exceed four lanes, and access to land is a primary function. Goods movement is typically limited to local routes and deliveries. Some avenues feature a raised landscaped median. Avenues may serve commercial or mixed-use sectors and usually provide curb parking. Street Low-speed (25 mph to 30 mph) thoroughfare in an urban area that primarily serves abutting property. Streets are designed to (1) connect residential neighborhoods with each other, (2) connect neighborhoods with commercial and other districts and (3) connect local streets to arterials. A street also may serve as the “main street” of commercial or mixed-use sectors and emphasize curb parking. Goods movement is restricted to local deliveries only. Rural Road Low- to medium-speed (25 mph to 35 mph) thoroughfare in a low-density suburban area that primarily serves abutting property. Source: Adapted from ITE (2010a) Exhibit 2-1. Urban area thoroughfare type descriptions.

20 Design Guide for Low-Speed Multimodal Roadways 2.6 Modes: Separation, Integration and Conflict Reduction When multiple user modes (e.g., pedestrians, bicyclists, transit, and motorized vehicles) oper- ate in the same right-of-way, conflicts can and do occur. Producing designs that reduce conflicts is critical for vulnerable road users such as pedestrians and bicyclists. Vulnerable road users are at a higher risk of injury or death when involved in a crash with a motorized vehicle. Similarly, pedestrians and bicyclists alike are at a higher risk of injury or fatality when a crash occurs between them. The design guidance in this Guide provides practitioners with tools to better understand and reduce the potential for conflicts between modes using various processes, poli- cies and design strategies. Some design and operations treatments can eliminate most conflict potential between modes (e.g., grade-separated bike/pedestrian overpasses, all-red pedestrian phases at signalized inter- sections), but most roadways require designs that manage conflicts rather than eliminate them. Reducing conflicts between modes often involves evaluating performance trade-offs across modes with the goal of achieving the best balance of safety and other performance measures among all users. FHWA’s Achieving Multimodal Networks: Applying Design Flexibility and Reducing Conflicts provides the following guiding principles to minimize and manage conflicts where user modes cross or meet in the right-of-way 1. Safety Do the design, operations, and maintenance decrease the severity and likelihood of crashes? Where modes come together, the design should eliminate conflicts to the greatest extent possible. If it is not feasible to eliminate the conflict, designers should minimize the speed differential between modes to ensure that if a crash occurs, the severity of the injury is likely to be lower. Safety considerations are also incorporated and implied in all other principles. 2. Accommodation and comfort Does the design serve all modes and provide a sense of comfort? Designs should accommodate people of all ages and abilities. Solving conflicts by eliminating access for pedestrians, bicyclists or transit users is not an acceptable solution. 3. Coherence and predictability Are the facilities for each mode recognizable and consistent? Where different modes come together, the design should provide clear right-of-way assignments, visibility of all users, and a clear path of travel for all modes, whether they are intended to operate in shared or separated spaces. This encourages predictable and safer behaviors for all users. 4. Context sensitivity Does the design incorporate and support the natural environment and adjacent land use, such as transit stations, employment centers, and other destinations. Does it support community health, economic, and livability goals? Thoroughfare Types Federal Functional Classification Freeway/ Expressway/ Parkway Rural Highway Boulevard Avenue Street Rural Road Principal Arterial Minor Arterial Major/Minor Collector Local Exhibit 2-2. Relationship between functional classification and thoroughfare type.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 21 The management of conflict points should consider and incorporate access to current and future adjacent land uses. Designs should minimize barriers to walking, bicycling, and transit use, and promote improved economic, social and public health. 5. Experimentation Are there innovative and creative solutions that can be tested to reduce conflicts? Experimenting with new treatments to resolve multimodal conflict points should be considered to expand the tools available to improve multimodal accommodations and reduce the likelihood and severity of conflicts (FHWA 2016a). In addition, ITE’s Recommended Design Guidelines to Accommodate Pedestrians and Bicycles in Interchanges provides design guidelines for reducing conflicts and improving safety and acces- sibility for pedestrians and bicyclists within and across roadway interchanges (ITE 2016). The guidelines identify specific dimensions, safety features, signing, pavement markings, design geometries, and other treatments to address pedestrian and bicycle conflicts. The best practices in the report are intended to provide insight into future updates of statewide or federal highway design manuals. 2.7 Understanding and Assessing Context The context of a roadway is a critical factor to consider in developing a project’s purpose and need, making fundamental design decisions such as cross-section determination, and selecting detailed design elements such as roadside amenities, street light fixtures or construction materials. Development of a roadway design that is sensitive to and respectful of the surrounding context is ultimately important for project success. Context-sensitive design refers to both the process and its results. An open stakeholder and community process that begins early in project development is desirable to ensure development of consensus about a project’s purpose and need. This process should continue through the design phase so that the features of the project are assembled to produce an overall solution that satisfies the project’s purpose and need, respects surrounding resources, and is consistent with the community goals and values. Historically, the roadway design process has focused on a project’s transportation elements, particularly those associated with motorized vehicle travel. A context-sensitive design should begin with analysis of the contextual elements, such as the environmental and community resources of the area through which a roadway passes. Beginning with a contextual analysis is critical in urban and suburban contexts as well as rural towns and villages. Once the designer understands the area surrounding the road and the road’s users, the designer can then consider the transportation elements of the roadway, its function within the local and regional transpor- tation system, and the appropriate level of accessibility of all users between the roadway and adjacent properties. 2.7.1 Context Definition The context of a roadway begins with its environmental context, which includes nearby natu- ral resources, terrain, and the manmade environment (development patterns, historic, cultural, and recreational assets). The environmental context can be a determinant of the desired type of accommodation for different users. This context often establishes the physical constraints of the roadway alignment and cross section, and it influences the selection of motorized vehicle design speed. A roadway corridor frequently traverses a variety of changing environments. Additionally, the volume and character of pedestrian, bicycle, public transit, freight and motorized vehicle activity can change considerably along its route. Land use is the fundamental determinant in the

22 Design Guide for Low-Speed Multimodal Roadways function of a road; as land use changes along a road, the road’s functions also change. Roadways should be designed to serve the existing land use while supporting the corridor and the com- munity’s future land use goals. 2.7.2 Context Types It is important to recognize that a roadway’s formal classification as urban or rural—which is determined from census data using criteria adopted (and periodically adjusted) by the United States Office of Management and Budget—may differ from actual site circumstances or pre- vailing conditions. For example, a rural arterial route passing through a small town may be classified as rural, but where the road passes through the town there may be a significant length over which the surrounding land use, prevailing speeds and transportation functions are more urban or suburban than rural. For this reason, it is important for the designer, working with the community and project stakeholders, to determine a project’s appropriate context type—or types—early in the planning process. Context types illustrate the broad range of environments that the designer may encounter throughout a corridor. The designer should also identify unique or project-specific contextual elements that will influence the design beyond those generalized area types. These elements might include, but are not limited to, schools, churches, pedestrian or bicycle-focused areas, parks and recreation areas, economic/retail areas and transit service hubs. 2.7.3 Transect Many urban planners use a concept called the “transect” to generally define land use context across a range of possible conditions from rural to urban areas. As shown in Exhibit 2-3, the transect is generally divided into six zones: core (T6), center (T5), general urban (T4), suburban (T3), rural (T2) and natural (T1). A remaining zone or category, special district, can be applied to unique parts of the urban environment that have specialty uses that do not fit into neighborhoods. Special districts could include power plants, airports, college campuses, and large retail-center power centers. Source: DPZ SmartCode Exhibit 2-3. The transect—an organizing system for land use.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 23 Stamatiadis et al. (2017) considered the transect in developing land use context categories that expand on AASHTO’s definitions of rural and urban areas. Exhibit 2-4 presents the latest evolu- tion of those expanded context definitions and their typical land use type, density and expected building setbacks from the right-of-way. Essentially, the guidance from Stamatiadis et al. suggests two rural categories, one suburban category and two urban categories as described in the table. Because this Guide focuses on multimodal activity within roadways operating in low- and intermediate-speed ranges (45 mph and lower), design guidance is provided for the rural town, suburban, urban and urban core categories. Rural streets and highways will often include areas with design and operating speeds above 45 mph as well as much lower levels of non-motorized user activity, but these contexts are not specifically addressed in the Guide. If a low-speed rural roadway facility is being designed to serve a mix of motorized and non-motorized users, however, the Guide will also be beneficial to the designer of that facility. 2.8 Context-Sensitive Design Principles Context is an important consideration in designing roadways to accommodate all users because context generally drives the presence, levels and activities of non-motorized users. The principles of context-sensitive design promote a collaborative, multidisciplinary process that involves all stakeholders in planning and designing transportation facilities that: • Meet the needs of users and stakeholders; • Are compatible with their setting and preserve scenic, aesthetic, historic and environmental resources; • Respect design objectives for safety, efficiency, multimodal mobility, capacity and maintenance; and • Integrate community objectives and values relating to compatibility, livability, sense of place, urban design, cost and environmental impacts. Context Category Density Land Use Setback Rural Lowest (few houses or other structures) Agricultural natural resource preservation and outdoor recreation uses with some isolated residential and commercial Usually large setbacks Rural Town Low to medium (single family houses and other single purpose structures) Primarily commercial uses along a main street (some adjacent single family residential) On-street parking and sidewalks with predominately small setbacks Suburban Low to medium (single and multifamily structures and multistory commercial) Mixed residential neighborhood and commercial clusters (includes town centers, commercial corridors, big box commercial and light industrial) Varied setbacks with some sidewalks and mostly off-street parking Urban High (multistory, low rise structures with designated off-street parking) Mixed residential and commercial uses, with some institutional and industrial and prominent destinations Minimum on-street parking and sidewalks with closely mixed setbacks Urban Core Highest (multistory and high rise structures) Mixed commercial, residential and institutional uses within and among predominately high rise structures Small setbacks with sidewalks and pedestrian plazas Exhibit 2-4. Land use context zones in pre-publication draft of NCHRP Research Report 855 (Stamatiadis et al. 2017).

24 Design Guide for Low-Speed Multimodal Roadways Applying the principles of context-sensitive design enhances the geometric design process by addressing objectives and considerations not only for the transportation facility but also for the surrounding area and its land uses, developments, economic and other activities and environmental conditions. With a thorough understanding of the context-sensitive principles and design process, the practitioner designing a roadway can integrate community objectives, accommodate all users and make decisions based on an understanding of the trade-offs that frequently accompany multiple or conflicting needs. The Guide provides guidance in how context-sensitive principles may be considered and applied in the process of developing street and roadway improvement designs for multimodal use. As documented in the Context-Sensitive Solutions Primer (FHWA 2009a), TRB E-Circular E-C067 (TRB 2004), NCHRP Report 480: A Guide to Best Practices for Achieving Context Sensitive Solutions (Neuman et al. 2002) and other sources, the principles of context-sensitive design are successfully used in towns and cities as well as in rural areas. Integrating context-sensitive prin- ciples into the project design process results in the consideration of a broad range of objectives and an attempt to balance these objectives based on the needs and conditions specific to each project, all users and the project’s context. 2.9 Relationship of Design Elements to Context Designing streets and roadways for all users requires paying attention to many elements of the public right-of-way, including how these elements integrate with each other and with adjoin- ing properties. Along a roadway, three basic elements must be designed to work together to provide a facility that effectively serves all users: (1) the traveled way, (2) the roadside and (3) the “context” (as defined by the adjacent property). These elements are illustrated in Exhibit 2-5. A fourth design element, intersections, is a unique component of traveled way design given the conflict potential within and across all user modes in the shared space of the intersection’s functional area. 2.9.1 Context as a Design Element 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 context- sensitive principles to street and road design. Exhibit 2-5. Design elements of a typical urban street.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 25 Broadly speaking, context can consist of an urbanized built environment, part of the natural environment, or both. The built environment consists of properties and activities within and adjacent to the public right-of-way—and the roadway itself—with surroundings whose charac- teristics help to define the context within that zone. Buildings, parking facilities, landscaping, land use mix, site access and public spaces are the primary elements of the built context. The natural environment generally includes undeveloped lands that may include open space or farming activities. In both environments, context can reflect historic or other protected resources. A roadway design will often change as its context changes from one zone to another. The roadway itself and the activity it supports become part of the context after it is completed. 2.9.2. The Roadside as a Design Element In urban, suburban and rural town context zones, the public right-of-way typically includes planting areas and pedestrian facilities (sidewalks) between the back of the curb (or edge of the shoulder) and the front property line of adjoining parcels. In urban and urban core areas, the roadside is further divided into a series of zones that emphasize different functions, including frontage, throughway, furnishings and edge zones. Exhibits 2-6 and 2-7 provide further defini- tion of these areas. In many communities the roadside also may include separate bicycle facilities (e.g., separated bicycle tracks, multiuse paths or exclusive cycle tracks). The function of roadside zones and the level of pedestrian and bicycle use of the roadside are directly related to the activities generated by the adjacent context and, in the case of bicycles, possible bicycle networks. Exhibit 2-6. Components of an urban roadside.

26 Design Guide for Low-Speed Multimodal Roadways 2.9.3 The Traveled Way as a Design Element The public right-of-way between curbs includes parking lanes and the travel lanes for vehicles, goods movement, transit vehicles and bicycles. The traveled way includes medians, turn lanes, transit stops, exclusive transit lanes, the curb and gutter and loading/unloading zones. Bicycle facilities within the traveled way may include striped bicycle lanes and buffers as well as physi- cally separated bicycle tracks. On-street parking designs may include parallel, forward-angle or reverse-angle back-in parking. 2.9.4 Intersections as a Design Element An intersection is a junction at which two or more public streets meet and where pedestrians and bicycles may share the traveled way. In urban and suburban context areas, intersections often are characterized by high levels of activity and shared use, multimodal conflicts, complex movements and possibly special design treatments that address the accessibility, convenience and safety of pedestrians, bicyclists and transit users. 2.10 Relationship of Design, Operating and Posted Speed to Context The Green Book defines design speed as follows (AASHTO 2011a): Design speed is a selected speed used to determine the various geometric features of the roadway. The assumed design speed should be a logical one with respect to the topography, anticipated operating speed, the adjacent land use, and the functional classification of the highway. Design speed differs from the other controlling criteria in that it is a design control, rather than a specific design element. In other words, the selected design speed establishes the range of design values for many of the other geometric elements of the highway. Because it affects Street/ Roadway Area Definition Frontage Zone One of the zones that make up the roadside, the frontage zone is the space between the pedestrian travel way and the building faces or private property. At a minimum, this zone 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 This is the roadside zone on which pedestrians travel. The throughway must provide a minimum horizontal and vertical clear area in compliance with PROWAG accessible route requirements; it is desirable to design the throughway zone for peak pedestrian demand conditions. Furnishings Zone The furnishings zone is a multipurpose area of the roadside. It serves as a buffer between the pedestrian travel way and the vehicular area of the street within the curbs, providing space for roadside appurtenances such as street trees, planting strips, street furniture, utility poles, sidewalk cafes, sign poles, signal and electrical cabinets, fire hydrants, bicycle racks and bus shelters. Edge Zone The edge zone, sometimes also called the “curb zone,” is the transition area between the thoroughfare traveled way and the furnishings zone of the roadside. This zone provides space for the door swing from vehicles in the parking lane, for parking meters and for the overhang of diagonally parked vehicles. Exhibit 2-7. Roadside design elements on urban streets and roads.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 27 so much of a highway’s design, the choice of a design speed is fundamental and significant. The selected design speed should be high enough so that an appropriate regulatory speed limit will be less than or equal to it. It is desirable for drivers’ comfortable operating speed to be close to the posted speed limit. In recognition of the wide range of site-specific conditions, constraints, and contexts that designers face, this Guide allows a great deal of design flexibility by providing typical ranges of values for design speed (see Exhibit 2-8). For most cases, the ranges provide adequate flexibility for designers to choose an appropriate design speed without the need for a design exception. A Guide for Achieving Flexibility in Highway Design provides additional information on how to select appropriate design speeds for various roadway types and contexts (AASHTO 2004a). NCHRP Report 504: Design Speed, Operating Speed, and Posted Speed Practices examines the relationship between design speed, posted speed, and operating speed. The report acknowledges that strong relationships between design speed, operating speed and posted speed limit are desirable, and that these relationships can be used to design and build roads that could pro- duce the speed desired for a facility. However, the report concludes that “while a relationship between operating speed and posted speed limit can be defined, a relationship of design speed to either operating speed or posted speed cannot be defined with the same level of confidence” (Fitzpatrick et al. 2003). The research finds that design speed appears to have minimal impact on operating speeds unless tight horizontal or vertical curves are employed. The report also concludes that when posted speed exceeds design speed, liability concerns may arise even though drivers can safely exceed the design speed. Apart from the posted speed limit, several variables appear to influ- ence the 85th percentile free-flow operating speed on tangents. These variables include access density, median type, parking presence along the street and pedestrian activity level. Other studies have found that lane width, degree of curve and perception of hazard severity also affect operating speeds. FHWA’s guidance memorandum on the relationship between design speed and posted speed provides the following information (FHWA 2015c): • The context along a roadway should play a significant role in determining the most appropriate operat- ing speed for the facility, considering all users and their safety and accessibility; • Selection of a posted speed is an operational decision for which the owner and operator of the facility is responsible; • Inferred design speeds less than the posted speed limit do not necessarily present an unsafe operating condition; • Operating and posted speeds should be considered in the selection of the design speed, but there is no regulation establishing a more direct relationship; and • In urban areas, the design of the street should generally be such that it limits the maximum speed at which drivers can operate comfortably, as needed to balance the needs of all users. Exhibit 2-8. Typical ranges of design speed by classification. Federal Functional Classification 20 mph and lower 25 mph 30 mph 35 mph 40 mph 45 mph 50 mph and higher Principal Arterial Minor Arterial Major/Minor Collector Local

28 Design Guide for Low-Speed Multimodal Roadways 2.11 Speed Management as a Design Goal Transportation safety is not only about motorized vehicle safety; it also encompasses protect- ing vulnerable users. Setting safe, consistent and reasonable target speed limits is the first step in speed management and is important in order to reasonably accommodate and protect all roadway users. Transportation designers and practitioners employ various strategies to manage speeds on roadways, and speed limits are an integral part of many of those strategies; however, setting a posted speed limit on a street or road does not always yield acceptable vehicle oper- ating speeds for all users or for the area context. Simply lowering the posted speed limit on a particular section of roadway does not ensure that motorists will lower the actual speed at which they travel. Therefore, transportation designers and operations engineers often employ other strategies, such as increased enforcement or physical speed-management countermeasures to encourage motorists to drive at a target speed that is more appropriate to the context and mix of users along that section of roadway. As discussed in the previous section, selection of a posted speed is an operational decision for which the owner and operator of the facility is responsible. Anticipated operating and posted speeds should be considered in the selection of the design speed, but no regulation establishes a more direct relationship. FHWA’s guidance memorandum also states that “In urban areas, the design of the street should generally be such that it limits the maximum speed at which drivers can operate comfortably, as needed to balance the needs of all users” (FHWA 2015c). This speed is typically referred to as the target speed for a facility. In 2016, FHWA published a reference guide addressing speed management: Integrating Speed Management within Roadway Departure, Intersections, and Pedestrian and Bicyclist Safety Focus Areas (FHWA 2016d). This reference guide identifies the common issues regarding speeding- related pedestrian and bicycle crashes that were identified both by public agencies and by national crash data analysis results. Several speed-management strategies also were identified and recommended through agency interviews and published resources. These strategies recog- nize that every situation or location is unique, and that agencies exercise engineering judgment for determining the appropriate solution for specific crash concerns. Strategies suggested in the reference guide include the following techniques and improvements: • Lighting; • Rectangular rapid flash beacons; • In-roadway warning lights; • Public outreach and education; • Raised median or refuge islands; • Pedestrian hybrid beacon; • Barriers to prevent unwanted crossings; • Context-sensitive design; • Road diets; • [Accommodations for] pedestrians and bicyclists; and • Bicycle-friendly rumble strips. Roundabouts and various traffic calming techniques also have been used to assist in managing speeds along a roadway corridor. Other useful speed management resources to assist in design- ing streets and roads that serve a range of motorized and non-motorized users include: • Engineering Speed Management Countermeasures: A Desktop Reference of Potential Effec- tiveness in Reducing Crashes (FHWA 2014a). This desktop reference source summarizes studies about the effectiveness of engineering countermeasures in reducing crashes and man- aging speed. An extensive table presents 52 separate techniques in the categories of vertical deflection, horizontal deflections/road narrowing, surface treatments and markings, vertical

Design Considerations for All Users in Low- and Intermediate-Speed Environments 29 delineation, dynamic signing and access controls. More than 100 references detail the effec- tiveness of these engineering countermeasures. The document also references studies where an increase in crashes was reported, as this information also is relevant in selection of countermeasures. • Speed Management: A Manual for Local Rural Road Owners (FHWA 2012b). This document was developed to provide local road practitioners information on how to address speeding- related crashes through the implementation of a comprehensive speed management program. The document discusses several engineering countermeasures that can be used to influence driver speed choice. These countermeasures are grouped into three categories: traffic control devices, road and street design, and traffic calming: – Traffic control devices. “Installing or upgrading signs and pavement markings on an affected roadway can be a cost-effective measure to reduce speeding. Such improvements include advisory speed signs and pavement markings, speed activated signs, and optical speed bars” (FHWA 2012b). – Road and street design. “There are several modifications to the design of a road or street that can induce speed reductions and have other safety and operational benefits for all road users. These include reduced lane widths, road diets, center islands or medians, and round- abouts. Several of these countermeasures can be implemented on higher-speed roadways as well as lower-speed roads” (FHWA 2012b). – Traffic calming. “Traffic calming is the design or retrofit of a roadway to encourage uniform vehicle speeds and improve conditions for non-motorized users. Traffic calming is gener- ally applied to roads with operating speeds of 30 mph or less. There are numerous identified traffic calming countermeasures that can be applied on different types of roads and streets, and these are identified in ITE’s Traffic Calming: State of the Practice [ITE 1999]. Some of the measures can also be applied in rural town contexts where pedestrian and/or bicycle activity may be a design issue” (FHWA 2012b). • NCHRP Report 737: Design Guidance for High-Speed to Low-Speed Transition Zones for Rural Highways (Torbic et al. 2012). A common speeding-related problem occurs when a driver approaches a rural town or village from a higher-speed rural road. Gateway treatments (gateways) can be used in rural areas to capture the attention of drivers and inform them that the nature of the roadway is changing, and that, as a result, they should reduce their speed. A gateway is a “combination of traditional and nontraditional traffic control treatments, such as enhanced signing, lane reduction, colored pavements, pavement markings, experimental striping, gateway structures and traditional traffic calming techniques or other identifiable features” (Torbic et al. 2012). Differences between the design standards and policies for high-speed and low-speed road- way environments complicate design of the transition zone. Many communities that want to use transition zones as gateways to the community have unrealistic expectations as to the magnitude of speed reduction. Transition zone design must attempt to meet many objectives while maintaining safety for all users. NCHRP Report 737 addresses designing the transition from a high-speed rural highway to a lower-speed section, typically approaching a small town. Providing a methodology for assessing these highway sections and a catalog of potential treatments for addressing problems, this report is useful to geometric designers and traffic engineers responsible for these situations. • NCHRP Report 613: Guidelines for Selection of Speed Reduction Treatments at High-Speed Intersections (Ray et al. 2008). Evaluating the effectiveness of treatments to reduce vehicle speeds at high-speed intersections, this report addresses geometric design features; signage and pavement markings; and stop-controlled, yield-controlled and uncontrolled approaches to signalized and unsignalized intersections. The guidelines apply to intersections with approach speeds of 45 mph or higher that are located primarily on suburban and rural roadways. The report focuses on public roadway intersections, but many of the principles discussed also can

30 Design Guide for Low-Speed Multimodal Roadways be applied to private driveways that have public roadway-like features. The report does not address speeds on roadway segments outside the influence area of an intersection, but it does discuss the relationship between segment speed and speed within the intersection influence area. 2.12 Flexibility in Application of Design Elements and Criteria Applying flexibility in the geometric design process to address multimodal needs requires knowledge of existing standards and guidelines, recognition of the range of options available, and understanding of how deviating from these may impact operations, safety and context. A flexible design approach uses existing geometric design criteria, controls and elements in cre- ative and varied ways to solve unique design challenges. To accomplish this goal, the designer needs to have a broad understanding of variables, thresholds and available alternatives to achieve multiple objectives for all the modes served by the project. Current national guidelines and standards provide significant levels of design flexibility. Flexibility in Highway Design (FHWA 1997) highlights the flexibility available to designers within existing standards and guidelines, and encourages them to apply this flexibility when designing roads to fit into the natural and human environment. A Guide for Achieving Flexibility in Highway Design (AASHTO 2004a) promotes the incorporation of sensitive community and environmen- tal issues into the design of transportation facilities. The AASHTO flexibility guide shows road- way designers how to think flexibly, how to recognize the many available choices and options, and how to arrive at the best solution for the particular situation or context. It also emphasizes that flexible design need not entail a fundamentally new design process; rather, it can be integrated into the existing transportation culture. The guidance in Bicycle and Pedestrian Facility Design Flexibility (FHWA 2013a) clearly states the agency’s support for flexibility in the design of bicycle and pedestrian facilities. More recently, in May 2016 the AASHTO Standing Committee on Highways (SCOH) passed a resolution, titled Direction on Flexibility in Design Standards, which states that AASHTO should provide guidance to state departments of transportation (DOTs) and other users of the Green Book regarding flexibility in design; that this guidance should follow the AASHTO model of being research- based and peer-reviewed; that this guidance should assist in educating engineers and designers on the flexibility inherent in the Green Book, as well as new and additional guidance on specific design issues; and that this guidance should address designing in and for a multimodal trans- portation system. In December 2015, Congress approved the current federal surface transportation funding legislation, titled Fixing America’s Surface Transportation (FAST) Act (Pub. L. No. 114-94). The FAST Act contains several changes to design standards to increase flexibility and provide for greater accommodation of all highway users. Specific provisions address the following: • Design considerations for roadways that are part of the NHS. The FAST Act now requires that designs shall consider (previously “may take into account”): – The constructed and natural environment of the area; – The environmental, scenic, aesthetic, historic, community, and preservation impacts of the activity; – Access for other modes of transportation; and – Cost savings by utilizing flexibility that exists in current design guidance and regulations. • Development of criteria for the NHS. The FAST Act adds two new resources that DOTs must consider when developing criteria to implement the requirements for the NHS. These new resources for consideration are:

Design Considerations for All Users in Low- and Intermediate-Speed Environments 31 – The AASHTO Highway Safety Manual (HSM, AASHTO 2010); and – The National Association of City Transportation Officials (NACTO) Urban Street Design Guide (NACTO 2013). • Design standard flexibility for localities. Under the FAST Act, a locality may, with state approval, use a different roadway design publication than the state if: – The roadway is owned by the locality; – The roadway is not on the Interstate highway system; – The locality is the direct recipient of federal funds for the project; – The publication is recognized by FHWA and adopted by the locality; and – The design complies with all other applicable federal laws. • Accommodation of non-motorized users. The FAST Act requires DOTs to encourage states and metropolitan planning organizations (MPOs) to adopt design standards for federal surface transportation projects that provide for the safe and adequate accommodation (as determined by the state) of all users of the surface transportation network, including motorized and non- motorized users in all stages of project planning, development and operation. Additionally, by 2017 (no later than 2 years after the enactment of the FAST Act), DOTs were required to release reports identifying examples of state laws and policies in this area and examples of best practices. 2.12.1 Flexibility in Existing Design Policy, Standards and Guidelines Roadway designers have the responsibility to understand what flexibility is allowed and encouraged in applying existing standards and guidelines. The documents listed in this section are typically used by agencies to develop design guidance for the selection of geometric design controls and criteria, traffic controls and traffic analysis. • The Green Book (AASHTO 2011a) and AASHTO’s supplemental guides for roadside design, pedestrian facility design, bicycle facility design and others are recognized as the national guidance for the design of roadways and paths. The Green Book has been adopted by FHWA as the standard for the design of projects on the NHS. Some states have adopted these AASHTO guides in their entirety, whereas other states have used them as the basis to create their own design guidance. The Green Book provides the most comprehensive guidance on geometric design and is a key resource used by designers. • The HSM (AASHTO 2010) provides a science-based technical approach for safety analysis of design and operation of streets and highways. The HSM emphasizes the use of analytical methods to quantify the safety effects of decisions in planning, design, operations and main- tenance. The HSM provides tools to conduct quantitative safety analyses, allowing safety to be quantitatively evaluated alongside other transportation performance measures such as traffic operations, environmental impacts and construction costs. The HSM also provides methods for developing an effective roadway safety management program and evaluating its effects, a predictive method to estimate crash frequency and severity, and a catalog of crash modifica- tion factors (CMFs) for a variety of geometric and operational treatment types, backed by robust scientific evidence. The HSM is written for practitioners at the state, county, MPO or local level. • The MUTCD (FHWA 2009b) sets the national standard for traffic control devices, including signing, pavement markings and traffic signals. The MUTCD is included by reference in the Code of Federal Regulations (CFR) and is recognized as the national standard for all traffic control devices installed on any street, highway, bikeway or private road open to public travel. • TRB’s Highway Capacity Manual, Sixth Edition: A Guide for Multimodal Mobility Analysis (HCM) (TRB 2016b) is the national guideline for analyzing traffic operations. The HCM

32 Design Guide for Low-Speed Multimodal Roadways 6th Ed. does not establish a legal standard, but it provides guidance on techniques to analyze various modes of traffic. 2.12.2 Flexibility in the AASHTO Green Book The Green Book emphasizes the need for a comprehensive design approach that requires the consideration of contexts, all modes of travel and the use of engineering judgment. The follow- ing statement from the introduction to the Green Book highlights how the guidelines allow for flexibility (AASHTO 2011a): The intent of this policy is to provide guidance to the designer by referencing a recommended range of values for critical dimensions. Good highway design involves balancing safety, mobility, and preservation of scenic, aesthetic, historic, cultural, and environmental resources. This policy is therefore not intended to be a detailed design manual that could supersede the need for the application of sound principles by the knowledgeable design professional. Sufficient flexibility is permitted to encourage independent designs tailored to particular situations. Throughout, the Green Book also references the need for the designer to understand land use context and the needs of all facility users in the geometric design process. Two excerpts highlight that recurring guidance: Emphasis is placed on the joint use of transportation corridors by pedestrians, cyclists and public transit vehicles. Designers should recognize the implications of this sharing of the transportation corridors and are encouraged to consider not only vehicular movement, but also movement of people, distribution of goods, and provision of essential services. A more comprehensive transportation program is hereby emphasized. and . . . the designer should keep in mind the overall purpose that the street or highway is intended to serve, as well as the context of the project area. Each document stresses the need for flexibility in the design process and encourages the designer to employ engineering judgment and consider context when designing roadways. How- ever, the designer must realize that no publication can address every real-world situation; each project contains unique combinations of community goals, context and mix of users. Keeping this reality in mind, a designer who understands the engineering principles behind the design guidance being used can recognize the degree of flexibility that can be applied safely and effec- tively to a project design. Several additional publications provide information on best practices and innovations in multimodal design. These publications include: • Achieving Multimodal Networks: Applying Design Flexibility and Reducing Conflicts (FHWA 2016a); • Designing Walkable Urban Thoroughfares: A Context Sensitive Approach (ITE 2010a); • Urban Bikeway Design Guide (NACTO 2014); and • Urban Street Design Guide (NACTO 2013). FHWA has expressed support of the use of these additional resources and has emphasized that they can be used to inform the planning and design process. Several government agencies have adopted or endorsed these publications and are using them as resources in their roadway and street design processes. 2.13 Design Exceptions In 1985, FHWA established a policy regarding what were considered “controlling” criteria for design. Thirteen (13) criteria were identified as having substantial importance to the operational and safety performance of any roadway such that special attention should be paid to them in

Design Considerations for All Users in Low- and Intermediate-Speed Environments 33 design decisions. For any roadway on the NHS, FHWA required a formal written design excep- tion if any of these 13 design criteria were not met. Although all of the criteria contained in the adopted standards are important design consid- erations, recent research has shown that they do not equally affect the safety and operations of a roadway, and therefore do not require the same level of administrative control. Based on these findings and on the recommendations of NCHRP Report 783: Evaluation of the 13 Controlling Criteria for Geometric Design (Harwood et al. 2016), in 2016 FHWA reduced the number of controlling criteria from 13 to 10, applicable to projects on the high-speed roadways on the NHS (i.e., Interstate highways, other freeways, and roadways with design speed ≥50 mph). Of the 10 remaining criteria, only two criteria—design loading structural capacity and design speed—now apply to low-speed roadways on the NHS (45 mph or lower). The 10 controlling criteria for projects on the NHS with design speeds at and above 50 mph are: 1. Design speed; 2. Lane width; 3. Shoulder width; 4. Horizontal curve radius; 5. Superelevation rate; 6. Stopping sight distance (SSD); 7. Maximum grade; 8. Cross slope; 9. Vertical clearance; and 10. Design loading structural capacity. Subject to approval by FHWA (or on behalf of FHWA if a state transportation agency has assumed the responsibility through a Stewardship and Oversight Agreement), design exceptions are required for projects on the NHS only when the controlling criteria described above are not met. The level of analysis should be commensurate with the complexity of the project. As docu- mented in the Federal Register, FHWA expects documentation of design exceptions to include (81 Fed. Reg. 87 [5 May 2016]): • Specific design criteria that will not be met. • Existing roadway characteristics. • Alternatives considered. • Comparison of the safety and operational performance of the roadway and other impacts such as right-of-way, community, environmental, cost, and usability by all modes of transportation. • Proposed mitigation measures. • Compatibility with adjacent sections of roadway. Design Speed and Design Loading Structural Capacity are fundamental criteria in the design of a project. Exceptions to these criteria should be extremely rare and FHWA expects the documentation to provide the following additional information: • Design Speed exceptions: – Length of section with reduced design speed compared to overall length of project – Measures used in transitions to adjacent sections with higher or lower design or operating speeds. • Design Loading Structural Capacity exceptions: – Verification of safe load-carrying capacity (load rating) for all State unrestricted legal loads or rou- tine permit loads, and in the case of bridges and tunnels on the Interstate, all Federal legal loads. The FHWA encourages agencies to document all design decisions to demonstrate compliance with accepted engineering principles and the reasons for the decision. Deviations from the criteria contained in the standards for projects on the NHS that are not considered to be controlling criteria should be documented in accordance with state laws, regu- lations, directives and safety standards. Depending on state laws and risk management practices,

34 Design Guide for Low-Speed Multimodal Roadways states can determine their own level of documentation and may adopt policies that are more restrictive than the FHWA policy outlined above. FHWA also encourages agencies to work with stakeholders “to develop context-sensitive solutions that enhance communities and provide multiple transportation options to connect people to work, school, and other critical destinations” (81 Fed. Reg. 87 [5 May 2016], p. 27189). 2.14 Liability Considerations Many designers are concerned about both personal and agency liability when applying flex- ibility in the roadway design process. Because of these concerns, some designers may rigidly apply conservative, vehicle-based design criteria to the detriment of other modes. Without con- sidering the safety, accessibility and convenience of other roadway users, adherence to the most conservative design values may not constitute reasonable care on behalf of the designer. A bal- anced geometric design approach will consider not only vehicles, but also pedestrians, bicyclists and transit users. As noted by FHWA, “. . . a designer who deviates from established design guidance is not neces- sarily negligent, particularly if the designer follows and documents a clear process, using engi- neering judgment, when dealing with design exceptions, and experimentation” (FHWA 2016a). Succeeding with a flexible approach to geometric design typically requires some combination of engineering judgment, documentation and experimentation. 2.14.1 Engineering Judgment Engineering judgment requires an understanding of engineering principles and the assump- tions and relationships inherent in adopted standards and guidelines. It also requires knowl- edge and understanding of community goals, context and site-specific conditions for the subject project. A designer should understand the impacts of applying various design criteria and elements (and their combinations). Using engineering judgment, the designer determines the most appropriate applications of—or changes from—conventional guidance to achieve a design solution that considers all factors. Ideally, designers will consider the safety and comfort of all legal users in current and future design-year conditions along with constraints and needs related to limited space, resources, and funding. Designs also should account for the scenic, historic, aesthetic, and cultural values of the surrounding community, and any goals the community may have for the project areas. As crucial contributors to the design process, stakeholders can and should influence the design engineer’s judgment. The opinions, needs and desires of neighborhoods, property owners, facility user groups and the public at large are all important. Ultimately, it is the designer’s responsibility to consider the opinions of various stakeholder groups while also educating them about the range of possible design options, solutions and outcomes for the project. 2.14.2 Documentation Designers should document their design decisions, especially those that involve the applica- tion of innovative, flexible, and creative approaches. Developing reports, studies and other types of documentation that explain the rationales used to create specific design solutions can be par- ticularly important if those solutions vary from typical approaches or established design guid- ance. Depending on the facility type, design speed and other factors, formal design exceptions

Design Considerations for All Users in Low- and Intermediate-Speed Environments 35 also may be required. Documenting design decisions is a required component of the design exception process. 2.14.3 Experimentation Liability concerns need not prevent designers from applying innovative and unique approaches that, in their engineering judgment, are reasonable solutions to a particular design challenge. In some situations, formal experimentation may be necessary to support the use of design treat- ments and safety countermeasures. This formal approach is typically applied for the design or use of traffic control devices that are not specifically included in, or compliant with, the MUTCD (FHWA 2009b). Section 1A.10 of the MUTCD outlines a formal experimentation process that includes eval- uation and follow-up adjustments to the design (including removal of the design) as needed. The experimentation process helps drive the advancement of the design practice and the adoption of new traffic control devices and their applications in the MUTCD. The experimentation process has resulted in the ability to apply new types of traffic control devices such as pedestrian hybrid beacons, bicycle signals, and colored pavement markings, all of which are important to provid- ing safe and accessible accommodation of pedestrians and bicyclists in certain settings. 2.14.4 Liability Defense Practices NCHRP Legal Research Digest 57: Tort Liability Defense Practices for Design Flexibility (Parker 2012) presents the results of a study of tort liability defense practices and cases involving the exercise of discretion in roadway design practices. This research digest provides the designer with a framework for determining successful strategies to use when defending design decisions made using flexibility driven by multimodal needs, context-sensitive solutions (CSS), practical solutions and other related initiatives. The digest explores the concept of discretion as a defense to government tort liability, and defending flexible design decisions based on the designers’ and policymakers’ discretion may be described by terms such as governmental immunity, official immunity, design immunity, or policy immunity. Existing law is relevant to analysis of tort legal defenses available to protect the decisions inherent in CSS. Many roadway design agencies have adopted flexible design policies and the use of CSS principles or related concepts to encourage flexibility in design decision making. The digest’s suggested processes for documenting design decisions, articulating clearly the various factors considered in making a decision with a focus on decisions that involve design exceptions, should be of great help to designers, their agencies and others responsible for exercising such judgment and decisions. 2.15 Considerations for Users with Disabilities As detailed in a joint document from the U.S. Department of Justice (U.S. DOJ) and U.S.DOT regarding Title II requirements of the Americans with Disabilities Act (ADA), state and local governments must “ensure that persons with disabilities have access to the pedestrian routes in the public right-of-way.” An important part of this requirement is the obligation to provide curb ramps where street level pedestrian walkways cross curbs whenever streets, roadways or highways are altered (U.S. DOJ and U.S.DOT 2013): An alteration is a change that affects or could affect the usability of all or part of a building or facil- ity. Alterations of streets, roads, or highways include activities such as reconstruction, rehabilitation,

36 Design Guide for Low-Speed Multimodal Roadways resurfacing, widening, and projects of similar scale and effect. Without curb ramps, sidewalk travel in urban areas can be dangerous, difficult, or even impossible for people who use wheelchairs, scooters, and other mobility devices. Curb ramps allow people with mobility disabilities to gain access to the sidewalks and to pass through center islands in streets. Otherwise, these individuals are forced to travel in streets and roadways and are put in danger or are prevented from reaching their destination; some people with disabilities may simply choose not to take this risk and will not venture out of their homes or communities. The ADAAG were adopted as the design standards for making facilities accessible to all pedes- trians (U.S. Access Board 2002). These guidelines, and more recently adopted updates, include requirements such as limiting criteria for clear passage, turning spaces, longitudinal and cross slopes, ramps with handrails, curb ramps between sidewalks and street crossings, and handicap parking spaces. However, the ADAAG are more applicable to building and site construction then they are to the longitudinal public right-of-way. Designers and agencies have been left to translate the ADAAG standards to highways and streets and make judgments on issues not fully addressed by the standards. The U.S. Access Board published the PROWAG in 2011 and a supplemental notice with guidance on shared-use paths in 2013 (U.S. Access Board 2011, 2013). The PROWAG will become enforceable as a standard only after the U.S. Access Board publishes a final rule and after the U.S. DOJ and/or the U.S.DOT adopt the final guidelines into their respective ADA and Section 504 regulations. At publication of this Guide, the U.S. Access Board had not issued a final PROWAG rule. Currently the U.S. DOJ’s 2010 ADA Standards for Acces- sible Design (2010 ADA Standards) and U.S.DOT’s 2006 Americans with Disabilities Act (ADA) Standards for Transportation Facilities (2006 ADA/Section 504 Standards) for recipients of federal financial assistance from U.S.DOT provide enforceable standards applicable to the public right-of-way (U.S. DOJ 2010a, U.S.DOT 2006). Where the 2010 ADA Standards or the 2006 ADA/Section 504 Standards do not address a specific issue in the public right-of-way, FHWA encourages public entities to look to the draft PROWAG for best practices. Several jurisdictions have chosen to apply the draft PROWAG as an alternative to, or equivalent facilitation for, the 2010 ADA Standards because they provide more specific coverage of accessibility issues in the public-right-of-way. It is advised that jurisdictions that have adopted the draft PROWAG as their standard consistently apply all provisions of the draft PROWAG. With these new guidelines, early consideration of accessible design alternatives will be more imperative than ever. Requirements for crosswalks will affect horizontal and vertical alignments and drainage at intersections. Curb ramps may require additional rights-of-way at intersection corners. Utilities may need to be removed or relocated to provide proper widths and clearances. When designers leave consideration of accessible pedestrian facilities until later in the design, they may severely limit their opportunities to develop a design that best meets the needs and provides the best functionality for all users. Because these ADA guidelines are based on civil rights law, designers do not have the same latitude to vary from them as they would other design standards and guidelines. Under extra- ordinary conditions with limiting terrain or other severe constraints, designers may determine that meeting the guidelines is impracticable, but they must still design the facilities to conform with accessibility standards to the maximum extent feasible. This Guide cites the draft PROWAG in anticipation of final PROWAG being adopted as the enforceable standard in the near future. Public entities and/or recipients of federal financial assistance are responsible for complying with the current ADA and Section 504 accessibility standards and/or demonstrating equivalent facilitation.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 37 2.16 Considerations for Bridges and Other Structures Bridges are key components of any transportation network, particularly for pedestrian and bicycle networks in urban and suburban areas. They often are the only way for all modes to travel across natural obstacles (rivers, ravines), railroads, freeways and grade-separated crossings. In addition, because the typical lifespan of a bridge is much longer than that of a typical section of road, it is important to address considerations for bicycle and pedestrian connections during bridge construction and reconstruction projects. It is important that bridges safely accommo- date pedestrians and bicyclists; without such accommodation, these vulnerable users may try to use vehicle lanes to cross the structure, which is unsafe for pedestrians and may be unsafe for some bicyclists. A bridge without safe walking and bicycling accommodation also can create lengthy detours that make the entire trip impractical for pedestrians and bicyclists. The United States Code (U.S.C.) addresses accommodations on bridges for non-motorized users. Specifically, 23 U.S.C. 217(e) emphasizes the need to address bicycle accommodations during bridge replacement projects: In any case where a highway bridge deck being replaced or rehabilitated with Federal financial par- ticipation is located on a highway on which bicycles are permitted to operate at each end of such bridge, and the Secretary determines that the safe accommodation of bicycles can be provided at reasonable cost as part of such replacement or rehabilitation, then such bridge shall be so replaced or rehabilitated as to provide such safe accommodations. Bridge projects can be used to make critical new connections in pedestrian and bicycle net- works. In some locations, a truly cohesive network may only exist with bridge connections for non-motorized users. In other areas, a new bridge may provide a more direct route than the ones currently available. For existing bridges, improving the safety and comfort of non-motorized users may require them to be retrofitted with more appropriate, separated facilities. Providing pedestrian and bicycle accommodation during initial construction generally costs less than retrofitting later. Current design guidance generally provides adequate flexibility on how to best accommodate these users in bridge design projects. Safe pedestrian access often can be implemented at the same time as bicycle accommodations and should be provided on bridges whenever possible. To avoid creating barriers, bridges also should accommodate bicycle and pedestrian facilities traveling under them. The U.S.DOT’s 2010 Policy Statement on Bicycle and Pedestrian Accommodation Regula- tions and Recommendations urges transportation agencies and local communities to go beyond minimum design standards and requirements to create safe, attractive, sustainable, accessible, and convenient walking and bicycling networks. With regard to “integrating bicycle and pedes- trian accommodation on new, rehabilitated and limited-access bridges,” the policy statement “encourages bicycle and pedestrian accommodation on bridge projects including facilities on limited-access bridges with connections to streets or paths. . . . It is more effective to plan for increased usage than to retrofit an older facility. Planning projects for the long-term should antici- pate likely future demand for bicycling and walking facilities and not preclude the provision of future improvements” (U.S.DOT 2010). AASHTO also supports the provision of pedestrian and bicycle accommodation on and across bridges as noted in excerpts from published design guides (AASHTO 2014b, AASHTO 2004b): Bridges, viaducts, and tunnels should accommodate bicycles . . . there are numerous examples of limited-access highway bridges that cross major barriers (such as wide waterways) that incorporate a shared-use path for bicyclists and pedestrians. The absence of a bicycle accommodation on the approach roadway should not prevent the accommodation of bicyclists on the bridge or tunnel. Provisions should always be made to include some type of walking facility as a part of vehicular bridges, underpasses, and tunnels, if the facility is intended to be part of a pedestrian access route.

38 Design Guide for Low-Speed Multimodal Roadways 2.17 Coordination with Stormwater and Green Infrastructure FHWA estimates that about 35 percent of U.S. roads are located in urban areas. Urban areas also include many other types of impervious surfaces, such as roofs, sidewalks and parking lots. Although all these surfaces contribute to stormwater runoff, the effects and necessary mitigation of the various types of surfaces can vary significantly. Of these surfaces, streets and roads present significant urban stormwater runoff and generally carry the most potential as sources of pollution. As a result, streets and roadways in urban areas offer one of the greatest opportunities to apply advanced stormwater management techniques. These techniques, which often are applied in roadway border areas, must be coordinated with provisions for pedestrian and bicycle facilities. Stormwater management and other “green” infrastructure applications often are used to increase the separation between the traveled way and other roadside users. Green infrastructure, a cost-effective and resilient approach to managing wet weather impacts, can provide many community benefits. Traditional street design has directed “gray” stormwater runoff from impervious surfaces into storm sewer systems (using gutters, drains and pipes) that move urban stormwater away from the built environment and discharge directly into surface waters, rivers and streams. In contrast, “green” infrastructure is designed to reduce, capture, and treat stormwater at its source (where the rain falls). Green street techniques encourage the interaction of stormwater with soil and vegetation to promote infiltration and retention. Treatments can incorporate a wide variety of design elements such as street trees, permeable pavements, bioretention and swales. Although the design and appearance of green streets will vary, the functional goals are typically the same. In urban and suburban areas, streets and roadways present many opportunities for coordi- nated green infrastructure use. Many agencies have begun to capitalize on the benefits gained by introducing green infrastructure in transportation projects. To assist agencies in applying these techniques, the EPA developed Managing Wet Weather with Green Infrastructure Municipal Handbook: Green Streets (U.S. EPA 2008). Following is a sampling of the stormwater management approaches presented in the Green Streets handbook (U.S. EPA 2008): Alternative Street Designs (Street Widths) A green street design begins before any BMPs [best management practices] are considered. When building a new street or streets, the layout and street network must be planned to respect the existing hydrologic functions of the land (preserve wetlands, buffers, high-permeability soils, etc.) and to mini- mize the impervious area. If retrofitting or redeveloping a street, opportunities to eliminate unnecessary impervious areas [such as using narrow travel and parking lanes] should be explored. Swales Swales are vegetated open channels designed to accept sheet flow runoff and convey it in broad shallow flow. The intent of swales is to reduce stormwater volume through infiltration, improve water quality through vegetative and soil filtration, and reduce flow velocity by increasing channel roughness. In the simple roadside grassed form, they have been a common historical component of road design. Additional benefit can be attained through more complex forms of swales, such as those with amended soils, bio- retention soils, gravel storage areas, underdrains, weirs, and thick diverse vegetation. Bioretention Curb Extensions and Sidewalk Planters Bioretention is a versatile green street strategy. Bioretention features can be tree boxes taking run- off from the street, indistinguishable from conventional tree boxes. Bioretention features can also be attractive attention-grabbing planter boxes or curb extensions. Many natural processes occur within bioretention cells: infiltration and storage reduces runoff volumes and attenuates peak flows; biological and chemical reactions occur in the mulch, soil matrix and root zone; and stormwater is filtered through vegetation and soil.

Design Considerations for All Users in Low- and Intermediate-Speed Environments 39 Permeable Pavement Permeable pavement comes in four forms: permeable concrete, permeable asphalt, permeable inter- locking concrete pavers, and grid pavers. Permeable concrete and asphalt are similar to their impervious counterparts but are open graded or have reduced fines and typically have a special binder added. Sidewalk Trees and Tree Boxes From reducing the urban heat island effect and reducing stormwater runoff to improving the urban aesthetic and improving air quality, much is expected of street trees. Street trees are even good for the economy. Customers spend 12% more in shops on streets lined with trees than on those without trees. Sources of Additional Information This publication supplements the sources listed at the end of Chapter 1. Harwood, D. W., et al. 2016. NCHRP Report 783: Evaluation of the 13 Controlling Criteria for Geometric Design. Transportation Research Board of the National Academies, Washington, D.C.