Page 95 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

78 CHAPTER TWO

TABLE 2.1 Relationship between Design Controls and Design Features

Design controls
Functional Traffic Design
Design features classification data Terrain Locale speed
Lane width, rural X 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 X
Grades X X X X
Bridge clearances (horizontal and vertical) X X
Stopping sight distance X
Passing and intersection sight distance X
Decision sight distance X
Superelevation X X
Widening on curves X
Rural design speeds X X X
Urban design speeds X X
Vertical alignment X X X X
Horizontal alignment X X

Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation,
with permission.

Design speed is defined as “a selected speed used to determine the various geometric
design features of the roadway” (Ref. 1). When designing new or reconstructed roadways,
the design speed should always equal or exceed the proposed legal speed of the
roadway.
Table 2.1 (Ref. 7) shows the relationship of the functional classification, traffic data,
terrain, locale, and design speed to the various geometric design features listed on the chart.
It should be noted that there are situations when it will not be possible or reasonable to
meet the design standard for a particular feature in a given project. When this occurs, the
designer must bring this to the attention of the reviewing authority for approval of what is
being proposed, or suggestions on what other course of action to take. A design exception
must be approved by the reviewing authority when a substandard feature is allowed to
remain as part of the design. In this way, it can be documented that this was not an error
or oversight on the part of the designer and that every effort has been made to provide the
best design possible in the given situation.

2.2.2 Sight Distance
A primary feature in the design of any roadway is the availability of adequate sight
distance for the driver to make decisions while driving. In the articles that follow, the
text contains conclusions based on information contained in Ref. 1. Derivation of for-
mulas and references to supporting research are contained in that document and will
not be repeated here. The reader is encouraged to consult that document for more
detailed background information. The following paragraphs discuss various sight
distances and the role they play in the design of highways.