This chapter discusses the fundamental design controls that govern urban thoroughfare design. This chapter is a prelude to the following chapters that present detailed design guidance for the streetside, traveled way and intersections. This chapter identifies the consistencies and divergences between design controls used where capacity is the dominant consideration and where walkability and the character of the thoroughfare is the dominant consideration.
1. Defines the term "design controls" and identifies the controls used in the conventional design process;
2. Identifies design controls used in the CSS process and explains how they differ from conventional practice;
3. Discusses the concept of a "target speed" for selecting design criteria;
4. Identifies factors that can be used in thoroughfare design to influence speed;
5. Discusses the concept of a "control vehicle" in combination with a design vehicle to select intersection design criteria; and
6. Provides an overview of the design controls recommended.
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 American Association of State Highway and Transportation Officials (AASHTO) Green Book and its supplemental publication, A Guide for Achieving Flexibility in Highway Design (2004b), 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 environmental factors should be used in the selection of design controls. This report focuses on design controls and critical design elements in the urban context.
Design Controls Defined by AASHTO
AASHTO guidelines identify functional classification and design speed as primary factors in determining highway design criteria. The Green Book separates its design criteria by both functional classification and context—rural and urban. The primary differences between contexts are the speed at which the facilities operate, the mix and characteristics of the users and the 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 controls are:
AASHTO's 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. AASHTO's Guide for the Planning, Design and Operation of Pedestrian Facilities (2004c) and Guide for the Development of Bicycle Facilities (1999) expand significantly on the Green Book, presenting factors, criteria and design controls. This report emphasizes pedestrians and bicyclists as a design control in all contexts but particularly in the walkable, mixed-use environments primarily addressed.
Differences from Conventional Practice
This report 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 CSS principles that are used differently than in the conventional design process. These controls are:
The most influential design control, and the design control that provides significant flexibility in urban areas, is speed. Thoroughfare design should be based on target speed.
Target speed is the highest speed at which vehicles should operate on a thoroughfare in a specific context, consistent with the level of multimodal activity generated by adjacent land uses to provide both mobility for motor vehicles and a safe environment for pedestrians and bicyclists. The target speed is designed 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 for the design of the thoroughfare to encourage the desired operating speed to ensure actual speeds will match the target speed.
Conventionally, design speed—the primary design control in the AASHTO Green Book—has been encouraged to be as high as is practical. In this report, design speed is replaced with target speed, which is based on the functional classification, thoroughfare type and context, including whether the ground floor land uses fronting the street are predominantly residential or commercial. Target speed then becomes the primary control for determining the following geometric design values:
Target speed ranges from 25 to 35 mph for the primary thoroughfare types described in this report. A lower target speed is a key characteristic of thoroughfares in walkable, mixed use, traditional urban areas.
Design Factors that Influence Target 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, this report urges sound judgment in the selection of an appropriate target speed based on a number of factors and reasonable driver expectations. Factors in urban areas include transition from higher- to lower-speed roadways, terrain, intersection spacing, frequency of access to adjacent land, type of roadway median, presence of curb parking and level of pedestrian activity. AASHTO's A Guide for Achieving Flexibility in Highway Design (2004c) aptly summarizes the selection of speed in urban areas:
"Context-sensitive solutions for the urban environment often involve creating a safe roadway environment in which the driver is encouraged by the roadway's features and the surrounding area to operate at lower speeds."
Urban thoroughfare design for walkable communities 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 but rather is achieved through a combination of measures that include the following:
Other factors widely believed to influence speed include a canopy of street trees, the enclosure of a thoroughfare formed by the proximity of a wall of buildings, the striping of edge lines or bicycle lanes, or parking lanes. These are all elements of walkable, mixed-use urban areas but should not be relied upon as speed-reduction measures until further research provides a definitive answer.
The practitioner should be careful not to relate speed to capacity in urban areas, avoiding the perception that a high-capacity street requires a higher target speed. Under interrupted flow conditions, such as on thoroughfares in urban areas, intersection operations and delay have a greater influence on capacity than speed.
The Highway Capacity Manual (TRB 2000) classifies urban streets (Class I through IV) based on a range of free-flow speeds. The thoroughfares upon which this report focuses have desired operating speeds in the range of 25 to 35 mph (Class III and IV based on the Highway Capacity Manual). Level of service C or better is designated by average travel speeds ranging from 10 to 30 mph. Therefore, adequate service levels can be maintained in urban areas with lower operating speeds. Capacity issues should be addressed with highly connected networks; sound traffic operations management, such as coordinated signal timing; improved access management; removal of unwarranted signals; and the accommodation of turning traffic at intersections.
Conventional thoroughfare design is controlled by location to the extent that it is rural or urban (sometimes suburban). This report broadens the choices for context using the urban transect, ranging from suburban to high-density urban cores. Additionally, the variation in design elements controlled by location is expanded to include predominant ground floor uses such as residential or commercial. Land uses govern the level of activity, which in turn influences the design of the thoroughfare. These influences include, but are not limited to, pedestrians and bicyclists, transit, economic activity of adjacent uses and right-of-way constraints. The CSS approach may also consider planned land uses that represent a departure from existing development patterns and special design districts that seek to protect scenic, environmental, historic, cultural, or other resources.
The design vehicle influences the selection of design criteria such as lane width and curb-return radii. Some practitioners will conservatively select the largest design vehicle (WB 50 to WB 67) that could use a thoroughfare, regardless of the frequency. Consistent with AASHTO, CSS 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 design vehicle that might occasionally use the facility, because the impacts to pedestrian crossing distances, speed of turning vehicles and so forth may be inconsistent with the community vision and goals and objectives for the thoroughfare. In contrast, selection of a smaller design vehicle in the design of a facility regularly used by large vehicles can invite frequent operational problems. The practitioner should select the design vehicle that will use the facility with considerable frequency (for example, bus on bus routes, semi-tractor trailer on primary freight routes or accessing loading docks and so forth). Two types of vehicle are recommended:
In general, the practitioner should obtain classification counts to determine the mix of traffic and frequency of large vehicles and should estimate how this mix will change as context changes and keep 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 as an appropriate design vehicle.
Although state highways have traditionally served through and heavy/large vehicle traffic, modern thoroughfare system planning tries to accommodate movements where they are best handled from a network and context consideration. 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. Any such diversions from state routes need to be clearly marked.
Multimodal Level of Service Measures
A fundamental goal of CSS is to effectively serve all modes of travel. Although good network planning, access management and innovative street designs can provide significant vehicle capacity while accommodating bicycles and pedestrians, trade-offs among modes can be an issue. Evaluating these trade-offs has historically been hampered by the fact that performance measures were developed primarily to measure vehicle movement. However, the traditional Highway Capacity Manual level of service framework has been adapted to evaluate performance from a transit, pedestrian and bicycle perspective.
These multimodal performance measures focus as much on the quality and convenience of facilities as they do on movement and flow. For example, the adequacy of pedestrian facilities is not determined by how crowded a sidewalk is but by the perception of comfort and safety. For transit services, frequency is an important attribute, but "on-time performance" and the pedestrian environment surrounding bus and rail stations are also critical aspects of the traveler experience. Below are examples of multimodal performance measures.
Bicycle Level of Service Measures
Pedestrian Level of Service Measures
For more information on multimodal level of service, see References for Further Reading at the end of this chapter.
Chapter 10 (Intersection Design Guidelines) provides further guidance on the design of intersections to accommodate large vehicles.
Functional classification describes a thoroughfare's theoretical function and role in the network, as well as governs the selection of certain design parameters, although the actual function is often quite different. As discussed in Chapter 4, functional class may influence some aspects of the thoroughfare such as its continuity through an area, trip purposes and lengths of trips accommodated, level of land access it serves, type of freight service and types of public transit served. These functions are important factors to consider in the design of the thoroughfare, but the physical design of the thoroughfare in CSS is determined by the thoroughfare type designation (as introduced in Chapter 4 and further discussed in Chapter 6).
The Role of Capacity and Vehicular Level of Service in CSS
The conventional design process uses traffic projections for a 20-year design period and strives to provide the highest practical vehicular level of service. CSS takes traffic projections and level of service into account and then balances the needs of all users or emphasizes one user over another depending on the context and circumstances (for example, reduces number of mixed-flow travel lanes to accommodate bicycle lanes or an exclusive busway). While capacity and vehicular level of service play a role in selecting design criteria, they are only two of many factors the practitioner considers and prioritizes in the design of urban thoroughfares. Often in urban areas, thoroughfare capacity is a lower priority than other factors such as economic development or historical preservation, and higher levels of congestion are considered acceptable. The priority of level of service is a community objective; however, variance from the responsible agency's adopted performance standards will require concurrence from that agency. CSS also considers network capacity in determining the necessary capacity of the individual thoroughfare (see Chapter 3).
Thoroughfare Speed Management
Under the conventional design process, many arterial thoroughfares have been designed for high speeds and traffic volumes. As the context of these thoroughfares change over time, such as to walkable compact mixed-use areas, the speed encouraged by the design becomes a matter of concern. Further, municipalities establishing speed limits based on the measured 85th percentile speed are finding they are required to establish higher speed limits than the community desires for the area. In these cases, traffic engineers are tasked with identifying methods to reduce arterial speeds. This section identifies research and the practical experience of agencies in managing arterial speeds.
It is popularly held that higher operating speeds result in higher crash rates and higher severity of crashes. Research on the effect of actual operating speed on crash rate is inconclusive (TRB 1998). However, research does show that higher operating speeds do result in higher crash severity—higher percentages of injury and fatality crashes and more serious property damage. Hence, lower vehicular traffic speeds will be beneficial when collisions occur with other vehicles or pedestrians.
Speed management is an approach to controlling speeds using enforcement, design and technology applications. While "traffic calming" is a type of speed management usually used on local residential streets, speed management can be used on all types of thoroughfares. Speed management methods can use technologies that provide feedback to the motorist about their speed, or designs in which the motorist perceives the need for a lower speed. These techniques include signage, signalization, enforcement, street designs and built environments that encourage slower speeds. Other methods include physical devices that force drivers to slow down, such as roundabouts, raised intersections, or narrowed sections created by curb extensions and raised medians. Physical devices are generally more effective at changing driver behavior but may be more costly to implement and may not be appropriate on all thoroughfares.
Speed management is often a multidisciplinary decision because it requires input from emergency services, engineering, street maintenance departments, law enforcement and transit service providers. The process of implementing a speed management program benefits from public involvement to understand how the community uses thoroughfares and how it perceives various speed management methods. Bicycle and pedestrian advocacy groups should also be involved in the process. Effective speed management requires knowledge of the existing traffic patterns, both quantitative and qualitative. Quantitative measures of traffic counts, intersection turn movements and speeds help to determine the existing condition and the need. Qualitative information, often gathered from the public or through observation, can explain behavioral issues. Implementation of speed management should be examined along corridors and across jurisdictions. It is important for a corridor to have a consistent speed through different jurisdictions if the character and context also remain constant.
The following is a list of speed management techniques or measures commonly used in the United States on thoroughfares designated as arterials or collectors:1
(Note 1: Based on interviews with public agencies and experts in the field of speed management. Source: "Best Practices in Arterial Speed Management," prepared for the City of Pasadena. Kim-ley-Horn and Associates, Inc, and ITE Journal article "Complete Streets: We Can Get There From Here," LaPlante, J. and McCann, B., May 2008.)
Additional Controls to Consider in Thoroughfare Design
In addition to the design controls discussed previously, other critical design controls in the conventional design process remain applicable in the application of CSS principles. Design controls related to roadway geometry—sight distance, horizontal and vertical alignment and access control—continue to be based on conventional design practices.
Pedestrian and Bicyclist Requirements as Design Controls
Pedestrian and bicyclist requirements affect the utilization of a thoroughfare's right of way. Thoroughfares with existing or desired high levels of pedestrian and bicycle usage require appropriate streetside and bicycle facilities to be included in transportation projects. This requirement usually affects the design elements in the traveled way. Therefore, pedestrian and bicycle requirements function as design controls that influence decisions for the utilization and prioritization of the right of way. For example, requirements for bicycle lanes might outweigh the need for additional travel lanes or a median, resulting in a design that reduces the vehicular design elements to provide bicycle design elements. The design of walkable urban thoroughfares emphasizes allocating right of way appropriately to all modes depending on priority and as defined by the surrounding context and community objectives. This process results in a well thought out and rationalized design trade-off—the fundamental basis of context sensitive solutions.
Sight distance is the distance that a driver can see ahead in order to observe and successfully react to a hazard, obstruction, decision point, or maneuver. Adequate sight lines remain a fundamental requirement in the design of walkable urban thoroughfares. The criteria presented in the AASHTO Green Book for stopping and signalized stop- and yield-controlled intersection sight distances based on the target speeds described above should be used in urban thoroughfare design.
Horizontal and Vertical Alignment
The design of horizontal and vertical curves is a controlling feature of a thoroughfare's design. The criteria for curvature is affected by speed and is dependent on the target speed. For urban thoroughfares, careful consideration must be given to the design of alignments to balance safe vehicular travel with a reasonable operating speed. The AASHTO Green Book provides guidance on the design of horizontal and vertical alignments for urban streets.
Access management is defined as the management of the interference with through traffic caused by traffic entering, leaving and crossing thoroughfares. Access management can be a regulatory, policy, or design tool. Access management on urban thoroughfares controls geometric design by establishing criteria for raised medians and median breaks, intersection and driveway spacing, and vehicle movement restrictions through various channelization methods. The AASHTO Green Book and the Transportation Research Board's Access Management Manual (2003) provide extensive guidance on this subject. Chapter 9 (Traveled Way Design Guidelines) provides an overview of access management methods and general guidelines for managing access on urban thoroughfares.
American Association of State Highway and Transportation Officials. 2004a. A Policy on Geometric Design of Highways and Streets. Washington, DC: AASHTO.
American Association of State Highway and Transportation Officials. 2004b. A Guide for Achieving Flexibility in Highway Design. Washington, DC: AASHTO.
American Association of State Highway and Transportation Officials. 1999. Guide for the Development of Bicycle Facilities. Washington, DC: AASHTO.
American Association of State Highway and Transportation Officials. 2004c. Guide for the Planning, Design and Operation of Pedestrian Facilities. Washington, DC: AASHTO.
Transportation Research Board. 2000. Highway Capacity Manual. Washington, DC: TRB.
Transportation Research Board. 2003. Access Management Manual. Washington, DC: TRB.
Transportation Research Board. 1998. Speed Management, Special Report 254. Washington, DC: TRB.
Sources of Additional Information
Parham, A. and Fitzpatrick, K. 1998. Handbook of Speed Management Techniques. Texas Transportation Institute.
Dowling, R. et al. 2008. NCHRP Report 616 Multimodal Level of Service Analysis for Urban Streets. Transportation Research Board, Washington, DC.
Dowling, R. 2008. NCHRP Web Document 128
Multimodal Level of Service Analysis for Urban Streets: Users Guide. Transportation Research Board, Washington, DC.
Florida Department ofTransportation. 2002. Quality/ Level of Service Handbook. Systems Planning Office, Tallahassee, FL.
Steiner, Ruth et al. 2003. Multimodal Trade-off Analysis in Traffic Impact Studies. Florida Department of Transportation Office of Systems Planning.
Williams, Kristine and Karen Seggerman. 2004. Model Regulations and Plan Amendments for Multi-modal Districts. National Center for Transit Research and Florida Department of Transportation Systems Planning Office. Tallahassee, FL.