Functional Classification
Article
Traditional Philosophy
The St. Louis region contains a number of individual streets and street types, each serving a different purpose within the transportation network. A functional classification system is used to group and describe roads according to the type of service they provide and their role in the network.
The functional classification for a given roadway 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 roadway 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).
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. 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.
Table 1, at right, depicts the region’s traditional functional classification system. The 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.
This example is similar to many of the arterial 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 exhibit 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, at least in part, why some of our arterial streets are unwelcoming to pedestrians and provide poor access.
Arterials are often characterized as facilities designed to transport people and goods and provide mobility.
While freeways and expressways within the principal arterial system (such as I-64 or I-170) are intended to move people and goods quickly and efficiently, the other principal arterials and minor arterials in the transportation network are not intended for this purpose.
Minor arterials which are designed or 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.
Balancing the need to provide access and mobility along many of these arterial streets is one the greatest challenges facing municipalities, local agencies, and communities in their efforts to create great streets.
New Philosophy
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 scheme is used with the recognition that functional classification alone does not adequately describe the character and adjacent land uses of an arterial street. The categorizations used in the functional classification system are too broad to reflect the true character of an arterial street 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, major thoroughfare design considerations will be increasingly relevant in St. Louis.
Major urban thoroughfares are divided into two main types:
- Walkable, pedestrian-oriented urban streets serving small, mixed-use developments
- Auto-oriented, mobility-focused streets serving single-use developments
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:
-
Boulevards: divided arterials in urban and suburban environments; can be high speed (40-45 mph) or low speed (35 mph or lower). 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 roadways are often transit corridors with high ridership.
- 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.
- Streets: low-speed minor arterials and collectors that focus on access to adjacent land uses. These roadways are often the “main streets” of commercial or mixed-use areas, with parking provided along the curb.
Table 3 below outlines the primary thoroughfare types applicable to the place types discussed in this guide. These recommendations are intended as general guidelines; because every community is unique, there is no one-size-fits-all rule for thoroughfare type selection.
Given the need to use functional class and thoroughfare type to describe roadway characteristics, this guide will use the following categorizations to define the various elements of the arterial street.
Functional class will be used to define:
- The arterial’s role in the regional network
- Type of freight service provided
- Type of transit service provided
Thoroughfare type will be used to define:
- Roadside characteristics and treatments
- Traveled way elements
- Intersections
The ITE guide indicates that a road’s functional classification should dictate its design speed. Because many of the arterial streets 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 arterial.
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.
Resources
Links:
- ITE Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities (See Chapter 4)
- FHWA Flexibility in Highway Design, Chapter 3 Functional Classification
- Massachusetts Design Guide, Chapter 3 Design Controls
- FHWA Functional Classification Guidelines
- East-West Gateway Council of Governments
Other References:
(not currently available in electronic format)
- AASHTO Green Book, Chapter 1, (2004 Edition)
- ITE Traffic Engineering Handbook, Chapter 4 (1999 Edition)
- Last Updated on Monday, 16 March 2015