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Flexibility in Highway Design

Chapter 5: Horizontal and Vertical Alignment

One of the best examples of integrating a highway into its surroundings. (Columbia River Highway, Hood Co., OR)

Refer to Chapter III of the AASHTO Green Book

photo: Columbia River Highway, Hood, Colorado, Oregon


One definition of a visually attractive and unobtrusive highway is the degree to which the horizontal and vertical alignments of the route have been integrated into its surrounding natural and human environments. This takes careful planning and design, as noted in the AASHTO Green Book:

Coordination of horizontal alignment and profile should not be left to chance but should begin with preliminary design, during which adjustments can readily be made ...The designer shouldstudy long, continuous stretches of highway in both plan and profile and visualize the whole in three dimensions.

This application of a holistic approach to highway design, where the road is integrated into its surroundings, separates the outstanding project from one that merely satisfies basic engineering design criteria (see Figures 5.1 and 5.2). An excellent description of this holistic design process is contained in the publication Aesthetics in Transportation, from which the following is excerpted: A general rule for designers is to achieve a "flowing" line, with a smooth and natural appearance in the land, and a sensuous, rhythmic continuity for the driver. This effect results from following the natural contours of the land, using graceful and gradual horizontal and vertical transitions, and relating the alignment to permanent features such as rivers or mountains.

photo of Rt. 8, Naugatuck

The alignment of Rt.8 gracefully follows the Naugatuck River.

(Naugatuck, CT)

Figure 5.1

Inappropriate road design does not integrate with the natural surroundings. 1 Aesthetics in Transportation, U.S. DOT, November 1980, p. 89.

drawing of two road designs; left-straight road by curving river, text "Inappropriate Road Design"; right side, road follows curves of river, text "Road Respects Natural River Edge"
drawing: from left to right, houses, trees, road, trees, waterway. Click the image for a text description.

Figure 5.2

The alignment of a new facility plays an important role in minimizing impacts on the surroundings. In this example, the character of the landscape was disrupted.

In this alternative, the valley is undisturbed and the road is partially shielded from view.

drawing; from left to right, houses, trees, valley, waterway, trees, embankment, road. Click the image for a text description.


The greatest opportunities for influencing the horizontal and vertical alignments of a highway occur during the planning and preliminary engineering phases associated with a newlocation facility. The designs of such facilities have the most dramatic effects on the natural and human environments through which they pass.

Preservation of unique rock formations enhances the view along this highway. (State Route 313, Moab, UT)

photo of St. Rt. 313, Moab, UT, and rock formations on either side

The more typical design problem faced by today's highway engineers is the improvement of an existing highway or street. In many instances, the basic alignments may have been established well over 100 years ago. Regardless, the same basic design principles with respect to horizontal and vertical alignments can, however, be applied to both new and existing facilities.

Important points to consider regarding horizontal and vertical alignments are that they should be consistent with the topography, preserve developed properties along the road, and incorporate community values. The superior alignments are ones that follow the natural contours of the land and do not affect aesthetic, scenic, historic, and cultural resources along the way. Construction costs may be reduced in many instances when less earthwork is needed, and resources and development are preserved. It is not always possible, however, to avoid having an impact on both the natural and human environments. That is why the superior alignments incorporate input received by the community through a participatory design process.

When possible, the alignment should be designed to enhance attractive scenic views, such as rivers, rock formations, parks, historic sites, and outstanding buildings. The designation of certain highways as scenic byways recognizes the importance of preserving such features along our Nation's roadways.

Equally important as the consideration of the horizontal alignment is that of the facility's vertical alignment. A number of factors influence the vertical alignment of a highway, including the following:

  • Natural terrain
  • Minimum stopping sight distance for the selected design speed
  • The number of trucks and other heavy vehicles in the traffic stream
  • The basic roadway crosssection; i.e., two lanes versus multiple lanes
  • Natural environmental factors, such as wetlands and historic, cultural, and community resources


The interrelationship of horizontal and vertical alignment is best addressed in the route location and preliminary design phases of a project. At this stage, appropriate tradeoffs and balances between design speed and the character of the roadtraffic volume, topography, and existing developmentcan be made. A mistake often made by inexperienced engineers is designing the horizontal alignment first and then trying to superimpose the design onto a vertical profile. Because they must be complementary, horizontal and vertical geometries must be designed concurrently. Uncoordinated horizontal and vertical geometries can ruin the best parts and accentuate the weak points of each element. Excellence in the combination of their designs increases efficiency, and safety, encourages uniform speed, and improves appearancealmost always without additional cost.

One tool to assist in coordinating horizontal and vertical geometries is the use of computeraided design (CAD). CAD enables highway designers to quickly assess the interrelationships between horizontal and vertical alignment, particularly in areas of difficult terrain.

Proper consideration of these basic design considerations will help to ensure that both newlocation facilities and improvements to existing highways fit harmoniously into their surroundings.

Good integration of horizontal andvertical alignment on a reconstructed twolane rural highway.

(State Route 89 near Lake Tahoe, CA)

photo: State Route 89 near Lake Tahoe, CA


There are numerous examples around this country of excellence in integration of the horizontal and vertical alignments of highways into their surroundings. Unfortunately, there are also examples of new or widened highways that have scarred a rural landscape or disrupted an established community. While these past actions cannot easily or inexpensively be rectified, future problems can be avoided by applying the principles outlined above and the creative approaches detailed below.

Avoiding Impact on Adjacent Natural and Human Environments

Particularly during the era of Interstate construction from the 1950's to the 1980's, a number of instances of new highway construction had a devastating impact on communities and areas of environmental sensitivity. It is readily acknowledged that there will be some degree of physical impact on the surroundings associated with the construction of any new location highway or major reconstruction or widening of an existing highway facility. However, from the perspective of horizontal and vertical alignment, much of this impact can and should be alleviated.


Impact on the surrounding environment can be minimized by careful attention to detail during the route location and preliminary design phases and a willingness of all concerned parties to work together toward a common goal. For example, minor adjustments to the originally proposed horizontal and vertical alignments (combined with the use of short sections of retaining wall) along the Lincoln Beach Parkway (U.S. Route 101) in Oregon eliminated the need to acquire any of the adjacent homes and businesses.

Minor alignment changes can avoid impacts on adjacent properties. (U.S. Route 101, Lincoln Co., OR)
photo: US Rt. 101, Lincoln, Co, two vans driving near the ocean

Similarly, a minor horizontal alignment shift at the beginning of the project allowed for the Hollister Bypass (SR 156) in San Benito County, CA, to avoid affecting a number of historic properties.

photo: a truck on the Hollister Bypass, San Benito, CA, with Mitchell Fruit Farm in the background

Impact on the historic Mitchell Fruit Farm was avoided as a result of a minor shift in the horizontal alignment of the Hollister Bypass.

(San Benito, CA)

The use of a "cutandcover" design, whereby the roadway is placed below the existing ground level and covered over with a park, building, or other public space can help to avoid negative impact. Lake Place Park in Duluth, MN and other public parks were the result of cutandcover tunnels that not only saved historic properties but also gave pedestrians improved access to Lake Superior.

Designers employed several cutandcover tunnels along I35 to avoid impacting an historic district and to reconnect the downtown to the waterfront.

(Duluth, MN)

photo: aerial view of I35, Dulutn, MN, showing cutandcover tunnels

In many cases, there is a potential for designing a divided highway with independent horizontal and vertical alignments for each direction of traffic.

photo: Rt 395, Inyo National Forest, CA

Separate profiles and varied median widths on divided roadways are options for designers to minimize impact on the environment.

(Rt 395, Inyo National Forest, CA)

Each side of the highway can have a different alignment to better integrate the roadway into its surroundings.

(I15, MT)

photo: aerial view of Route 115, Montana

Coordination Between Horizontal and Vertical alignment

When horizontal and vertical alignments are designed separately from one another, unnecessarily large cuts and fills may be required, resulting in very dramatic and often visually undesirable changes to the natural landscape.


One of the ways to ensure the most effective coordination of horizontal and vertical alignment is through the use of a multidisciplinary design team during the planning and engineering phases of a project. On such projects as 166 in Fairfax and Arlington Counties in Virginia, the combined expertise of landscape architects, urban designers, structural engineers, and historic preservationists, in addition to civil engineers and highway designers, has resulted in superior highway improvement projects.

The concept of using a multidisciplinary design team is not new; it was pioneered in the early 1900's during the planning and design of the Bronx River Parkway in Westchester County, NY After a period of use primarily on largescale or controversial projects, this approach has come back into more general application as a way to achieve community consensus.

A multidisciplinary design team can help minimize the impact an urban freeway has on a community.

(I66, VA)

photo: aerial view of I-66, Virginia
Updated: 9/19/2012
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