3.3 Sight Visibility Triangles

Visibility within intersections, also referred to as intersection sight distance, and at approach ramps must be considered during the design process. The sight visibility triangles (see
Figure 3-2
,
Figure 3-3
, and
Figure 3-4
) should allow a full view of traffic for all turning movements at intersections (including driveways and median turns)and merging movements at ramps. The exact dimensions of the sight visibility triangle are a function of highway function and speed. Refer to
RDM
Chapter 4, Section 4.11.4 for additional information regarding intersection sight distance. Materials used should:
  • Maintain and enhance lines of sight through the intersection; and
  • Focus the view on the intersection.
Site visibility triangles – Plants must not be placed where they may obstruct signage.
Site visibility triangles – Visibility within intersections is a primary goal. Plant material must be limited to low-growing varieties.
Site visibility triangles – Approach ramps require long, unobstructed sight lines.
Cone of Vision
The primary cone of vision (objects that compose the primary field of vision) is 60 degrees for a stationary observer. Research has shown that as an object begins to move, this cone begins to narrow. For most drivers, the vision cone is estimated to be 30 degrees. Objects 20 feet from the edge of the pavement and 120 feet from the viewer fall within this 30- degree cone of vision. Outside this vision cone, objects are perceived with peripheral vision; they are visible but not in sharp focus (see
Figure 3-5
).
Site visibility triangles – Approach ramps require long, unobstructed sight lines.

3.3.1 Aesthetics & Maintenance

Plants can be an important addition to the ROW as an aesthetic enhancement that blends highway structures with the surrounding environment. TxDOT is committed to use regionally appropriate vegetation within the roadways and on facility beautification projects. Whether in an urban context or a rural context, well-maintained projects enhance the community character and driver safety.
Roadside vegetation is maintained to accomplish specific goals:
  • Sight-distance;
  • Clear view of obstructions;
  • Erosion control; and
  • Aesthetics.
Design alternatives should be reviewed to be sure that minimum standards are met for each of these issues. Most roadways are kept mowed to a height that accomplishes the needs of these issues and meets with the public’s expectations for the appearance of the roadside. Public standards should be considered when developing aesthetic programs that affect roadside vegetation.