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Flexibility in Highway Design

Chapter 3: Functional Classification

One of the first steps in the design process is determining the functional classification of a facility.

Refer to Chapter I of the AASHTO Green Book

aerial photo of a multi lane highway

BACKGROUND

Functional classification is the process by which streets and highways are grouped into classes, or systems, according to the character of traffic service that they are intended to provide. There are three highway functional classifications: arterial, collector, and local roads. All streets and highways are grouped into one of these classes, depending on the character of the traffic (i.e., local or long distance) and the degree of land access that they allow. These classifications are described in Table 3.1.

The Functional Classes:

  • Arterial
  • Collector
  • Local
Each type of road has a specific purpose or function.
Functional System Services Provided
Arterial Provides the highest level of service at the greatest speed for the longest uninterrupted distance, with some degree of access control.
Collector Provides a less highly developed level of service at a lower speed for shorter distances by collecting traffic from local roads and connecting them with arterials.
Local Consists of all roads not defined as arterials or collectors; primarily provides access to land with little or no through movement.

Table 3.1

Functional Classification Systems

Typically, travelers will use a combination of arterial, collector, and local roads for their trips. Each type of road has a specific purpose or function. Some provide land access to serve each end of the trip. Others provide travel mobility at varying levels, which is needed en route.

There is a basic relationship between functionally classified highway systems in serving traffic mobility and land access, as illustrated in Figure 3.1. Arterials provide a high level of mobility and a greater degree of access control, while local facilities provide a high level of access to adjacent properties but a low level of mobility. Collector roadways provide a balance between mobility and land access.

graphic - top right blue, text Mobility, bottom left red, text Land Access

Arterials

  • higher mobility
  • low degree of access

Collectors

  • balance between mobility and access

Locals

  • lower mobility
  • high degree of access

Figure 3.1

Relationship of functionally classified highway systems in serving traffic mobility and land access.

Source: Safety Effectiveness of Highway Design Features, Volume I, Access Control, FHWA, 1992

Route 187 in Connecticut was upgraded to provide minimal stops for through traffic, while allowing easy access to development along the road.

photo of two lane Route 187 in Connecticut

The Role of Functional Classification in the Design Process

The AASHTO Green Book explicitly recognizes the relationship between highway functional classification and design criteria. State, county, and city highway design manuals likewise relate design criteria to highway functional classification. The AASHTO Green Book states:

The first step in the design process is to define the function that the facility is to serve. The level of service required to fulfill this function for the anticipated volume and composition of traffic provides a rational and costeffective basis for the selection of design speed and geometric criteria within the range of values available to the designer (for the specified functional classification). The use of functional classification as a design type should appropriately integrate the highway planning and design process. (p. 17)

Once the functional classification of a particular roadway has been established, so has the allowable range of design speed. With the allowable range of design speed defined, the principal limiting design parameters associated with horizontal and vertical alignment are also defined. Similarly, a determination of functional classification establishes the basic roadway cross section in terms of lane width, shoulder width, type and width of median area, and other major design features (see Figure 3.2).

flow chart: Functional Classiffication-> (top) Design Speed-> Horizontal/Vertical alignment/(bottom)->Cross Section

Figure 3.2

The flexibility available to a highway designer is considerably limited once a particular functional classification has been established.

The importance of the functional classification process as it relates to highway design lies in the fact that functional classification decisions are made well before an individual project is selected to move into the design phase. Moreover, such decisions are made on a systemwide basis by city, county, or State DOTS or MPOs as part of their continuing longrange transportation planning functions. Such systematic reassessments are typically undertaken on a relatively infrequent basis. Thus, the functional classification of a particular section of highway may well represent a decision made 10 or more years ago.

Even after the decision has been made to functionally classify a highway section, there is still a degree of flexibility in the major controlling factor of design speed. Table 3.2, excerpted from the 1995 edition of the Roadway Design Manual of the Virginia DOT, illustrates the manner in which one state has related a range of allowable design speeds to different roadway functional classifications.

Table 3.2

Range of Design Speeds for Various Highway Functional Classes In this example, there are at least three different design speeds for each functional classification.

Functional
Classification
20 mph 30 mph 40 mph 50 mph 60 mph 70 mph
Rural Principal
Arterial
     
X
X
X
Rural Minor Arterial
     
X
X
X
Rural Collector
X
X
X
     
Rural Local Road
X
X
X
     

Source: Roadway Design Manual, Virginia DOT, 1995.
Note: 1 mph = 1.613 km/h

It is important to remember that there are no "cookiecutter" designs for arterial highways or collector streets. Because of the range of geometric design options available, arterials and collectors can vary considerably in appearance, as shown in the following photographs

aerial photo of 1476/US Rt. 1 Interchange, Montgomery County, PA Representative freeway.
(I476/U.S Rt. 1 Interchange, Montgomery County, PA)

Representative urban arterial
(Windsor, CT)

photo of an urban arterial in Windsor, CT
photo of a tree lined rural arterial in Taconic State Park, NY

Representative rural arterial.
(Taconic State Parkway, NY)

Representative rural arterial.

(Rt. 58, CT)
Representative Collectors and Local

photo of rural arterial Rt. 58, CT
photo of a collector in a residential area, Greenbelt, MD

Representative collector in a residential area.
(Greenbelt, MD)

Representative urban collector.
(Lambertville, NJ)

photo of an urban collector in Lambertville, NJ
winter photo of a rural collector in Easton, CT

Representative rural collector.
(Easton, CT)

Representative local street.
(Montgomery Co., MD)

photo of a residential street with a speed bump in Montgomery County, Maryland

Current Highway Functional Classifications

The highway system of the United States consists of slightly over 6.3 million km (3.9 million miles) of road.' Of this total, 5.0 million km (3.1 million miles) are located in rural areas, and the remaining 1.3 million km (800,000 miles) are urban streets. Each of these rural and urban streets has been given a specific functional classification, as illustrated in Table 3.3. In terms of jurisdictional responsibility, about 5 percent of the total is administered by the Federal Government, approximately 16 percent is under State control, and the remaining 79 percent is under the control of county and local governments.

Functional System
Percent of
Total Mileage
Percent of
Total Travel
Interstate
1.2
22.8
Other Freeway/Expressway
.2
6.2
Other Principal Arterial
3.8
24.3
Minor Arterial
5.7
18.4
Major Collector
11.1
7.8
Minor Collector
7.2
2.1
Collections
2.2
5.3
Local
68.6
13.1
 

Total
100
100

Table 3.3 Functional System (Rural and Urban) Mileage and Travel

overhead view of I-95 in Virginia with heavy traffic The Interstate System represents 1.2 percent of the total road mileage but serves 22.8 percent of the Nation's total travel. (I95, VA) 1 Our Nation's Highways: Selected Facts and Figures, FHWA, Washington, DC, 1995, p. 17-18

ISSUES

The Need To Update Highway Functional Classifications

Traffic service patterns on a roadway and the roadway's function can change over time. If the functional classification system for a specific jurisdiction is not updated on a regular basis, roadways may be designed using inappropriate design standards.

Solution

Clearly, there is a need to reevaluate a locality's functional classification system on a relatively frequent and regular basis to ensure that the functional classification of any particular route accurately reflects the traffic function of the route now and in the foreseeable future. This continuing reassessment process can be viewed as an application of design flexibility even before the decision is made to begin designing a particular project. The decision to change the functional classification should be made based on careful review of changed conditions and sound reasoning.

The Functional Classification Process Is Not an Exact Science

Drawing of three overlapping circles: blue (Arterial), yellow (Collector), and purple (Local). The overlap shows the Areas of Uncertainty.
Figure 3.3

One of the difficulties surrounding the relationship between highway functional classification and design guidelines is that the classification process is not an exact science. The predominant traffic service associated with a particular route cannot be definitely determined without exhaustive surveys of traffic origindestination patterns on each link of the road network. Engineering judgment based on experience must play a role in making design decisions.

Solution

As a result of variances within the highway functional classification system, design guidelines established either in the Green Book or in other State and local government design manuals have overlapping ranges of values. This allows the designer greater flexibility in choosing a road design that is most appropriate within the determined functional classification (see Figure 3.3). For example, the 1994 Green Book indicates that the width of the traveled way for twolane rural collector facilities should be between 6.0 m and 7.2 m (20 ft and 24 ft), depending on traffic volumes, while the shoulder widths on each side of the traveled way should be between 0.6 m and 2.4 m (2 ft and 8 ft). Thus, the total width of the roadway can vary from 7.2 m to 12 m (24 ft to 40 ft), which is a considerable amount (see Figure 3.4). Twolane rural arterials can vary from a total roadway width, including shoulders, of 9 m to 12 m (30 ft to 40 ft). This flexibility allows designers to choose more accurately the specific geometric dimensions that are appropriate for that roadway.

Figure 3.4

The total width of a rural collector can vary from 7.2 m to 12 m, depending on traffic volume.

graph comparing a 7.2m collector to a 12m collector

The Impact of Land Use Changes on Road Functions

Land use is an important determinant of the function of an area's roads. As land use changes because of development, especially at the urban fringe, road functions also change. It is not uncommon for roads that once served as rural local access routes to farmland, and now serve suburban residential subdivisions and commercial land uses, to be reclassified as urban collectors or arterials depending on the intensity of development and the type of traffic generated by the development. Design standards or guidelines must change to meet actual or impending change in traffic character and road function.

Actions taken by a local jurisdiction to control or direct the form and location of growth or to preserve the current physical and scenic characteristics of a highway corridor should also reflect the need for a reexamination of existing functional classification and, perhaps, even a change in jurisdictional responsibility (see Figure 3.5). For example, the construction of a new controlledaccess bypass route might allow for a downward reclassification of what had been the existing arterial route through a community to a collectorlevel facility.

Figure 3.5
Road functions can change over time. These views show changing land use along a rural highway. The first view (a) shows a new road through the country. The second view (b) shows the first residences along the road. The third view (c) shows suburbanization and the need for mitigation.
three drawings of rural highways

Solution

One solution to the issue of changing land use is to directly relate the functional classification of the highway system to the "level of development" or, in other words, the design criteria that should be applied. The State of Washington is one jurisdiction that has done this. This relationship is shown in Table 3.4.

Functional Class Design Criteria
Interstate New Construction/Reconstruction Standards
Principal Arterial New Construction/Reconstruction Standards
Minor Arterial 3R Standards
Collector Maintenance of Structural Integrity and Operational Safety.

Table 3.4 Relationship of Functional Classification and Design Criteria

This process allows for improvements to even minor arterialtype routes to be designed using 3R standards, as opposed to applying traditional design criteria for newlocation highway facilities that fall within this functional classification.

Updated: 09/19/2012
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