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Flexibility in Highway Design

Chapter 4: Design Controls

In addition to functional classification, there are a number of design controls that affect the geometry of a highway. (Rt. 71, IL)

Refer to Chapter II of the AASHTO Green Book

photo of Route 71, Illinois

BACKGROUND

In order to design the basic elements of a highwayincluding its alignment and cross sectionthe designer must have an understanding of the basic design controls and criteria associated with the highway. One of the most important, highway functional classification, was discussed in Chapter 3. Other important design controls include, but are not limited to the following:

  • The design speed of the facility
  • The acceptable degree of congestion (i.e., the designyear peakhour level of service) on the facility
  • The physical characteristics of the "design vehicle" (i.e., the largest vehicle that is likely to use the facility with considerable frequency); in virtually all instances, the highway design vehicle is an overtheroad tractortrailer
  • The performance of the design vehicle (particularly important in terms of accommodating heavy trucks in mountainous terrain or buses and recreational vehicles in areas subject to high levels of tourist activity)
  • The capabilities of the typical driver along the facility (i.e., local residents using lowspeed neighborhood streets versus interstate travelers on rural freeways)
  • The existing and designyear traffic demands to be placed on the facility (e.g., daily and peakhour traffic volumes, the mix of passenger cars and trucks on the facility)

Two of the most important of these factors are design speed and peakhour level of service. This chapter will focus on these two important criteria. The peakhour level of service, however, only serves as a controlling factor for a small number of highways. For most highways, after the functional classification and associated design speed for a particular highway facility have been established, the degree of flexibility available to the designer is significantly limited.

Speed limits are not the same as the design speed. They are regulatory limits enforceable by law.
photo of a six lane divided highway and a 45 MPH speed limit sign

Design Speed

Design speed is defined by the AASHTO Green Book as: ...the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern.

Table 4.1

Relationship Between Design Controls and Design Features

 
Design Controls
Design Features Functional Classification Traffic Data Terrain Locale Design Speed
Lane width, rural
X
X
 
X
Lane width, urban
X
 
X
 
Rural shoulder width, type
X
X
 
 
Urban shoulder width, type
X
 
X
 
Guiderail offset
X
X
 
 
Degree of curve
 
 
 
X
Grades
X
 
X
X
Bridge clearances (horizontal & vertical)
X
X
 
 
Stopping sight distance
 
 
 
X
Superelevation
 
 
 
X
Widening on curves
 
 
 
X
Rural design speeds
X
X
X
 
Urban design speeds
X
 
X
 

Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio DOT.

All geometric design elements of the highway are affected in some way by the selected design speed. Some roadway design elements are related directly to and vary appreciably with design speed. These include horizontal curvature, superelevation, sight distance, and gradient (see Table 4.1). Other elements are less related to design speed, such as pavement and shoulder width and clearances to walls and traffic barriers. The design of these features can, however, affect vehicle operating speeds significantly. As a result, more stringent criteria for these features are generally recommended for highways with higher design speeds. Conversely, less stringent criteria for these features may be more appropriate on roadways with lower design speeds.

The selection of a particular design speed is influenced by the following:

  • The functional classification of the highway
  • The character of the terrain
  • The density and character of adjacent land uses
  • The traffic volumes expected to use the highway
  • The economic and environmental considerations.

Typically, an arterial highway warrants a higher design speed than a local road; a highway located in level terrain warrants a higher design speed than one in mountainous terrain; a highway in a rural area warrants a higher design speed than one in an urban area; and a highvolume highway warrants a higher design speed than one carrying low traffic volumes.

(Left) Lower design speeds are appropriate for city streets.

photo of a city street
photo of a rural highway

(Right) Higher volume rural highway facilities should use higher design speeds.

As discussed in Chapter 3, most States and localities have adopted a range of acceptable design speeds for each of the major classes of highways and streets (i.e., freeway, other arterial, collector, and local). Table 4.2 illustrates typical minimum design speeds for various types of highways located in level, rolling, and mountainous terrain.

Freeways Design Speeds
Terrain Rural Urban
Flat 70-80 70
Rolling 60-70 60-70
Mountainous 50-60 50-60
Arterial Highways
Terrain Rural
Urban
Flat 60-70 30-60
Rolling 40-60
30-50
Mountainous 30-50 30-50
Collector and Local Roads
Terrain
Rural
Urban
Flat 30-50 30-40
Rolling 20-40 20-40
Mountainous 20-30 20-30
Source: Traffic Engineering Handbook (Fourth Edition), Institute of Transportation Engineers, Washington, DC, 1992, p. 156. Note: 1 mile/hr = 1.613 km/hr

Table 4.2 Typical Minimum Design Speeds for Various Types of Highways (in mph)

The values presented in Table 4.2 are minimum acceptable design speeds for the various conditions of terrain and traffic volumes associated with new or reconstructed highway facilities. Designers need to balance the advantages of a higher vehicle operating speed gained through the use of a higher design speed against the flexibility lost in design. It may be more important to retain the maximum possible flexibility, so that a contextsensitive roadway that is more in tune with the needs of a community is designed using a lower design speed.

As used here, the term "context sensitive" refers primarily to the land use and environmental conditions adjacent to the highway. For example, for any particular highway other than a freeway or major arterial, as land use density increases, the design speed would typically decrease. The design speed of an urban collector street passing through a residential neighborhood should be appreciably lower than that for a rural highway with the same functional classification. This also recognizes the fact that bicycles and pedestrians would be more likely to use a route located in an urban area.
Similarly, in areas that have significant historic interest or visual quality, a lower design speed may be appropriate in recognition of lower average operating ,speeds and the need to avoid affecting these historic or aesthetic resources.

The Green Book agrees with this philosophy:

Aboveminimum design values should be used where feasible, but in view of the numerous constraints often encountered, practical values should be recognized and used.

Along arterial streets, the controlling factor of design speed applies to a lesser degree than on rural highways or hightype urban facilities, such as freeways or expressways. On many of the arterial streets located in large urban areas, maximum vehicle operating speeds for several hours of the day may be limited to those at which the recurring peak period traffic volumes can be accommodated. Thus, speeds may be governed by the presence of other vehicles traveling en masse both in and across the through travel lanes and by traffic control devices, rather than by the physical characteristics of the street.

During offpeak periods of lowtomoderate traffic demand, vehicle operating speeds are governed by such factors as speed limits, midblock turns, intersection turns, number of driveways and entrances, traffic signal spacing, and signal timing. As a result, when arterial street improvements are being planned, the selection of the appropriate design speed must be balanced against such factors as speed limits, physical and economic constraints, and the probable running speeds that can be attained during offpeak hours.

Although most States have adopted a range of allowable design speeds appropriate for each of the various functional classifications for use in the design of new or reconstructed highway facilities, situations may arise where even the use of the lowest typically acceptable value would result in unacceptably high construction or rightofway costs or unacceptable impact on adjacent properties. In such instances, the design exception process discussed in Chapter 2 can be employed. For example, the reconstruction of a twolane rural arterial route through a relatively flat but environmentally sensitive area might need to employ a design speed of 80 km/h (50 mph) rather than the recommended value for this functional classification of 100 km/h (60 mph) shown in Table 4.2.

PeakHour Level of Service

Once an appropriate design speed has been selected, the other basic defining elements of the highway (i.e., the number of lines and the basic configuration of junctions with other highway facilities) can be determined through application of the concept of acceptable peakhour level of service. Level of service is a grading system for amount of congestion, using the letter A to represent the least amount of congestion and F to refer to the greatest amount. For a comprehensive treatment of this topic, refer to the Highway Capacity Manual'. Table 4.3 presents a brief description of the operating characteristics associated with each level of service.

photo of a congested city street As congestion approaches capacity, speed decreases.
Level of
Service
Description
A
Free flow with low volumes and high speeds.
B
Reasonably free flow, but speeds beginning to be restricted by traffic conditions.
C
In stable flow zone, but most drivers are restricted in the freedom to select their own speeds.
D
Approaching unstable flow; drivers have little freedom to select their own speeds.
E
Unstable flow; may be short stoppages
F
Unacceptable congestion; stop-and-go; forced flow.
Source: Adapted from the AASHTO Green Book. 1 1995 Highway Capacity Manual (Special Report 209), Transportation Research Board, Washington, DC, Third Edition, updated 1994

Table 4.3

Level-of-Service Characteristics

The appropriate degree of congestion (that is, the level of service) to be used in planning and designing highway improvements is determined by considering a variety of factors. These factors include the desires of the motorists, adjacent land use type and development intensity, environmental factors, and aesthetic and historic values. The factors must be weighed against the financial resources available to satisfy the motorists' desires.

Table 4.4 presents the relationship between highway type and location and the level of service appropriate for design, suggested by the AASHTO Green Book. Taking into consideration specific traffic and environmental conditions, the responsible highway agency should attempt to provide a reasonable and costeffective level of service.

Table 4.4
Guide for Selection of Design
Levels of Service
 
Type of Area and Appropriate Level of Service
Highway
Type
Rural
Level
Rural
Rolling
Rural
Mountainous
Urban and
Suburban
Freeway
B
B
C
C
Arterial
B
B
C
C
Collector
C
C
D
D
Local
D
D
D
D
Source: Adapted from the AASHTO Green Book

While the Highway Capacity Manual provides the analytical basis for design calculations and decisions, judgment must be used in the selection of the appropriate level of service for the facility under study. Once a level of service has been selected, all elements of the roadway should be designed consistently to that level.

For example, along recreational routes subject to widely varying traffic demands according to the time of year or in response to environmental or land use considerations, the designer may conclude that the selection of a level of service that is lower than what is usually recommended may be appropriate. The selection of the desired level of service for a facility must be weighed carefully, because the facility's overall adequacy depends on this decision.

ISSUES

Application of Appropriate Design Speed

For some projects, community residents may perceive an imbalance between the scale of improvement deemed appropriate by the highway designers and that viewed as appropriate by the affected community. Much of this conflict can be traced to the design speed for the subject project.

For example, an older twolane rural road with a posted speed limit of 45 mph (72.5 km/h) may be adequate to accommodate current and anticipated future traffic demands, except for a short section containing several sharp curves that has a high incidence of accidents. If this facility were classified as a minor arterial, a State's design criteria might suggest that the reconstruction of the deficient section of roadway utilize a minimum design speed in the range of 60 to 70 mph (96.6 to 112.7 km/h).

If these criteria were followed, the reconstructed section would have a significantly higher design speed (and, hence, a higher operating speed and magnitude of physical impact on its surroundings) than the immediately adjacent sections of highway, resulting in a potentially unsafe condition.

Solution

One approach to avoiding this problem would be to apply a lower uniform design speed over the entire length of the route. This would suggest the application of a design speed of 50 mph (80.5 km/h) to the reconstruction project to preserve the design continuity and character of the route.

A similar approach was taken during the design of State Route 9A in New York City to better integrate the project into its surroundings. Although this facility is classified as a principal urban arterial street with an allowable design speed under New York State DOT design criteria as high as 60 mph (96.6 km/h), a design speed of only 40 mph (64.4 km/h) was used. The roadway's capacity remains unchanged and the roadway is functioning safely and efficiently.

Note that the design speed must be higher than the posted speed and should also be above the operating speed on a facility, regardless of the posted speed.

Updated: 09/19/2012
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