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Functional Classification


The traditional philosophy towards functional classification is one that designates roadways as arterials, collectors, and local roads, and uses that classification to determine the design and function of the thoroughfare for vehicular travel in the network. The more current philosophy, as described in ITE's Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities, is one that additionally considers thoroughfare type in the planning and design of a roadway. After many years of applying the traditional philosophy of functional classification, it is clear that thoroughfares require more detailed consideration, not only of their role in the overall transportation network, but also of their role as places that must function well for all modes.

The Traditional Approach

The functional classification for a given thoroughfare is determined based on its setting (urban or rural) and whether its main role is providing connectivity, mobility, or accessibility. The number of vehicle miles traveled (VMT), average annual daily traffic (ADT), and abutting land uses of a thoroughfare are also considered. Traditionally, the roadway functional classification system has been used to describe how travel flows through the regional roadway network and to determine project eligibility for inclusion in the Long Range Plan and short-range Transportation Improvement Program (TIP). Roadways with certain functional classes are eligible for certain improvement projects.

East-West Gateway, the metropolitan planning organization (MPO) and council of governments (COG) for St. Louis, is responsible for maintaining and updating the region’s functional classification system.

Urban and rural roadway functional classes
Credit: EW Gateway COG

To maintain the functional classification system, East-West Gateway accepts applications for functional classification revision during the months of May and November each year. A system-wide review is conducted every 3-5 years. The table at right depicts the region’s traditional functional classification system. The functional classes that are most applicable to the St. Louis Great Streets Initiative have been shaded yellow.

A portion of a typical urban/suburban network is shown in the figure below. The arterial streets form the backbone of the network. Local roads feed the collectors, which in turn feed the arterials.

Street network example
Source: FHWA

This example is similar to many of the roadway networks throughout the St. Louis region.

Traditional planning and design standards classify the functionality of highway and street networks based on two major factors:

  • Access to adjacent properties and land uses; and
  • Mobility for vehicles needing to travel through the area without stopping at adjacent developments.

In this traditional approach, the graphic below is often used to illustrate the relationship between access, mobility, and the street network.

The prevalence of this well-known figure and the concepts it illustrates, help explain why some of our thoroughfares are unwelcoming to pedestrians and provide poor access.

Proportion of service: access vs. mobility
Source: FHWA

Arterials are often characterized as facilities designed to transport vehicles and goods, with an emphasis on mobility.

While freeways and expressways within the principal arterial system (such as I-64 or I-170) are intended to move vehicles and goods quickly and efficiently, other principal arterials and minor arterials in the transportation network serve many other functions, which are often not considered in the design standards associated with the functional class.

Minor arterials that are designed as and function more like freeways present a major problem for the St. Louis region. For example, facilities like Watson Road, Olive Road, Manchester Road, and Lindbergh Boulevard carry the majority of the metropolitan area’s traffic volume.

Arterial vs. collector vs. local diagram
Source: FHWA

Balancing the need to provide access and mobility along many of these thoroughfares is one the greatest challenges facing the region in the effort to create great streets. The traditional philosophy towards functional classification does not address that challenge adequately and should only be used as a starting point.

The Current Approach

In Chapter 4 of the ITE publication Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities, streets and highways are classified according to traditional functional classification and what is referred to as “thoroughfare type.” This additional classification acknowledges that functional classification alone does not adequately describe the character and adjacent land uses of a thoroughfare. The categories in the traditional approach towards functional classification are too broad to reflect the true character of a thoroughfare or capture differences between roadway segments.

The ITE publication uses the term “major thoroughfare” to describe major urban or suburban multimodal streets (typically arterials or collectors) that are designed to support and complement adjacent land uses. As the regional bus and light rail systems continue to expand and gain ridership, the design of major thoroughfares will be essential in St. Louis.

Major urban thoroughfares are divided into two main types:

  • Walkable, pedestrian-oriented urban streets serving small, mixed-use developments; and
  • Auto-oriented, mobility-focused streets serving single-use developments.
Functional classification and street type table
ITE Context Sensitive Solutions

The ITE publication predominantly focuses on the first of the two categories noted above; however Chapter 11 discusses some of the key considerations for mobility-priority streets.

The table at right illustrates the relationship between traditional functional classification categories and thoroughfare types. In light of these relationships, the Great Streets Initiative focuses on the following thoroughfare types:

  1. Boulevards: divided arterials in urban and suburban environments; can be high speed (40-45 mph) or low speed (35 mph or lower). These thoroughfares typically serve as primary routes for goods movement and emergency response.
    • High speed boulevards are mobility-priority corridors emphasizing traffic movement over longer distances, with very few access points. Adjacent land uses are typically larger single-use parcels with sizeable, landscaped setbacks.
    • Low speed boulevards are walkable, multimodal corridors providing relatively few access points (but still more than high-speed boulevards). These thoroughfares are often transit corridors with high ridership.
  2. Avenues: low- to medium-speed arterials and collectors; shorter in length than boulevards with more access provided. These thoroughfares are typically walkable, with a heavy emphasis on pedestrian and bicycle travel.
  3. Streets: low-speed minor arterials and collectors that focus on access to adjacent land uses. These thoroughfares are often the “main streets” of commercial or mixed-use areas, with parking provided along the curb.

The table below outlines the primary thoroughfare types applicable to the place types discussed in this guide. These recommendations are intended as general guidelines; every community is unique and there is no one-size-fits-all rule for thoroughfare type selection. In order to provide the most comprehensive, yet precise description of a roadway, one may distinguish the traditional and current approaches to wards functional classification in the following way.

Primary thoroughfare type for each place type
Credit: CH2M HILL

Functional class will be used to define:

  • The thoroughfare’s role in the regional network;
  • Type of freight service provided; and
  • Type of transit service provided.

Thoroughfare type will be used to define:

The ITE guide indicates that a road’s functional classification should dictate its design speed. Because many of the thoroughfares in the St. Louis region have a functional classification that would prescribe an inappropriately high design speed under such an approach, this guide recommends that design speed be determined based on place-specific characteristics and the community’s vision for the particular place. The use of these alternative criteria would likely result in speed reductions for certain segments of the thoroughfare. When determining design speed, the road’s functional classification may be an appropriate starting point, but it should not be applied without considering a number of other important factors.

Functional Classification for Downtown Main Streets

Apply the current ITE Context Sensitive Solutions approach. Downtown main streets are context-rich places that must function safely for all modes, but particularly pedestrians. The traditional functional classification system does not adequately address the impacts of roadway characteristics in downtown main street environments. The ITE guide, particularly the guidance relating the streets thoroughfare type, better addresses the range of needs required by downtown main streets.

Use the traditional approach to wards functional classification to inform decisions regarding transit and freight service. Although traditional functional classification is not adequate for the planning and redevelopment of downtown main streets, elements of it may be applied to better understand the nature of vehicular movement through the particular roadway segment and the role of a thoroughfare in the regional network. Many downtown main streets carry high volumes of traffic (often because they occur along designated routes), which must be done safely.

Recognize that functional classification is one of many factors informing design and planning for downtown main streets. Downtown main streets are complex places and require appropriately complex solutions, which will be informed in part by functional classification but also by aspects such as planning and design for the pedestrian, adjacent land use,and urban design, to name a few.