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Section 3: Responsibilities of Landscape Architects

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TxDOT uses a team approach to project development. Landscape architects and consulting design professionals participate in the design process as defined in this manual and in Chapter 5, Section 4, 5280: Design landscape/aesthetic plans of the Project Development Process Manual.

Landscape architects routinely work in a multi-disciplinary context, exemplified by the TxDOT project development process. They are trained in the earth sciences, construction materials, and technology as well as aesthetics design. The landscape architect’s primary role is to assist in integrating environmental and aesthetic concerns with engineering and safety requirements. In this role landscape architects contribute to a broad range of highway planning and design activities.

Landscape architects in transportation practice have five primary responsibilities:

Each of these responsibilities is explained in a subsection of this section. Other topics include:

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Safety of the Traveling Public

The most important consideration is to ensure the safety of the traveling public and those that maintain transportation facilities. Ensuring safety is a demanding task that requires a broad understanding of vehicle performance, driver capabilities, and design geometry as well as landscape design.

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Stewardship of Land, Air, Water, Scenic, Cultural, and Historic Resources

The conservation of land and water resources is essential to the long-term sustainability of the state’s transportation system. Statutory mandates that reinforce TxDOT’s stewardship responsibilities include:

The landscape architect and other design professionals have a primary responsibility in helping the department meet its stewardship obligations as defined by these acts.

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Mitigation of Adverse Environmental and Cultural Resource Impacts

The landscape architect provides assistance at all levels of the project development process related to environmental and resource stewardship and impact mitigation. Their unique planning and design skills are particularly useful in the mandated process of documenting project need, demonstrating impact avoidance, and preparing strategies for mitigation. In the planning phase of the project, landscape architects assist in identifying and hiring subject matter experts, prepare preliminary reports, and plan for all types of environmental and cultural resource impact mitigation. In the final design process the landscape architects prepare or oversee the preparation of PS&E documents related to environmental agreements as well as any landscape and aesthetics improvements.

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Integration of the Transportation Network into the Adjacent Landscape

Transportation corridors interact with the environment through every conceivable landscape type. Projects that involve opening a new corridor, or improving and expanding an existing corridor, require careful consideration of how the proposed improvements will fit into the existing landscape whether it is urban, suburban, or rural. Fitting a complex modern highway into the adjacent landscape requires close attention to detail, knowledge of environmental constraints and an appreciation for the safety and engineering requirements of the highway structure.

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Enhancement of the Aesthetic Quality of the Transportation Network

Transportation agencies are often asked to enhance the aesthetic quality of the transportation corridor. Enhancement in this context means careful coordination of the architectural details of structures along with the skillful manipulation of the landform and careful planning of clearing, revegetation, reforestation, and erosion control operations in order to blend the highway with its surroundings.

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Visual Perception of Highway and Transportation Corridors

An individual aesthetic experience of a highway or other transportation corridor is a function of what one sees over time and space. This is very different than other types of landscape design, which focus on the design of spaces that are experienced as individual places, for example, a back yard, a civic plaza, or a park. Even a development as large as Disney World is designed and perceived as a unified place.

By contrast, highways and other transportation corridors are large-scale landscapes, revealed as a sequence of visual experiences over time. In this context the aesthetic quality of a corridor is the sum of the visual experience over time and not the quality of any single view. In other words, a highway may have some visually unpleasant elements, yet still have a very favorable overall visual impression.

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Design Considerations for Scale

The scale of the highway landscape is probably the most critical aesthetic design consideration. For example, a typical freeway interchange will occupy a site of 30 acres to as many as 100 acres. A mile of typical urban freeway, which represents approximately 1 minute of travel time, is approximately 36 acres of right-of-way. However, depending on the topography, a single mile of highway may represent a view shed of several square miles.

Designers must be conscious of the basic design implications related to the scale of the transportation system:

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  • Surface and landscape treatments generally have to be limited to areas where they achieve the greatest visual impact.
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  • Supplementing the existing landscape is usually more successful and cost effective than creating a new or different landscape.
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Laws of Perspective Related to Highway and Transportation Design

The viewer’s perception of a roadway corridor follows the laws of visual perspective. The critical relationships of perspective in highway and transportation design practice are the station point, horizon line, vanishing point(s), and the cone of vision. The station point is the location of the viewer in relation to what is being observed. The horizon line is a plane through the viewer’s eye level. The vanishing point(s) is a reference on the horizon line that appears to be the origin of horizontal lines or planes. The cone of vision is the viewing area where all objects will be in focus.

Station Point. The station point of a vehicle operator is constantly moving usually along a path parallel to the centerline of the right-of-way. This means that the perspective view is constantly changing. Traveling at normal speeds, an observer processes the equivalent of about 50 snap shots every second. Therefore a viewers impression of a transportation corridor is the sum of many individual observations.

Horizon Line. The eye level horizon line determines the apparent height of an object. Driver eye level is generally between 44 in and 48 in from the ground. This is about 12 in to 16 in below eye level in a standing position (which is about 60 in). As eye level decreases, the first point on the ground that can be observed moves further from the viewer and the apparent area of the ground plane decreases rapidly. For this reason the ability to perceive patterns on the ground is decreased dramatically. In general, objects that lie between the horizon and the ground are difficult to distinguish. This is because they foreshorten rapidly as the distance from the viewer increases and, depending on the color, they may blend with other objects in or on the ground plane. Therefore, low objects or plant materials that do not break the horizon are not prominent as visual design elements.

Vanishing Point. A corridor is a long narrow space that appears to have a single vanishing point on the horizon line located perpendicular to the horizon line. A corridor type space is one of the few types of space that is perceived as a “one point” perspective.

Cone of Vision. The primary cone of vision, objects that compose the primary field of view, is 60 degrees for a stationary observer. Objects outside this vision cone, 60 degrees to ±180 degrees, are perceived with peripheral vision; they are visible but not in sharp focus. Research has also demonstrated that as an observer begins to move the cone of vision begins to narrow depending on the activity and task load. For most drivers the vision cone is estimated to be 30 degrees. With a 30-degree cone of vision, objects 20 ft from the edge of the pavement will be at least 120 ft from the viewer to be within the 30-degree cone of vision.

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Figure 1-1. Due to the scale of the roadway, only objects that are large or near the viewer are readily seen.

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Design Implications of Visual Perspective

The laws of visual perspective are mathematical relationships that will be constant regardless of the type of transportation facility. They govern how users perceive space and time relationships. While there are no hard rules where aesthetic design is concerned, the laws of perspective govern the viewer’s perception of space and detail. Designers must take this into account and design accordingly.

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  • The elements perceived by a vehicle operator are at considerable distance, usually a minimum of 120 ft to 150 ft.
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  • Details, particularly patterns on the ground, are difficult to see at highway speeds and have little impact on the driver’s sense of space and design.
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  • Objects that have the greatest impact on the driver’s perception are those that break the driver’s horizon line. This means that vertical elements, such as embankments, cut slopes, walls, and trees will have the most significant impact on the driver’s field of view.
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