Ingcivil

CHAPTER 3 ELEMENTS OF DESIGN
INTRODUCTION
The alignment of a highway or street produces a great impact on the environment, the fabric of the community, and the highway user. The alignment is comprised of a variety of elements joined together to create a facility that serves the traffic in a safe and efficient manner, consistent with the facility's intended function. Each alignment elementshould complement others to produce a consistent, safe, and efficient design.
The design of highways and streets within particular functional classes is treated separately in later chapters. Common to all classes of highways and streets are several principal elements of design. These include sight distance, superelevation, traveled way widening, grades, horizontal and vertical alignments, and otherelements of geometric design. These alignment elements are discussed in this chapter, and, as appropriate, in the later chapters pertaining to specific highway functional classes.
SIGHT DISTANCE General Considerations
A driver's ability to see ahead is of the utmost importance in the safe and efficient operation of a vehicle on a highway. For example, on a railroad, trains are confined to a fixedpath, yet a block signal system and trained operators are needed for safe operation. In contrast, the path and speed of motor vehicles on highways and streets are subject to the control of drivers whose ability, training, and experience are quite varied. For safety on highways, the designer should provide sight distance of sufficient length that drivers can control the operation of their vehiclesto avoid striking an unexpected object in the traveled way. Certain two-lane highways should also have sufficient sight distance to enable drivers to occupy the opposing traffic lane for passing other vehicles without risk of a crash. Two-lane rural highways should generally provide such passing sight distance at frequent intervals and for substantial portions of their length. On the other hand,it is normally of little practical value to provide passing sight distance on two-lane urban streets or arterials. The proportion of a highway's length with sufficient sight distance to pass another vehicle and interval between passing opportunities should be compatible with the design criteria established in the subsequent chapter pertaining to the functional classification of the specific highwayor street.
Four aspects of sight distance are discussed below: (1) the sight distances needed for stopping, which are applicable on all highways; (2) the sight distances needed for the passing of overtaken vehicles, applicable only on two-lane highways; (3) the sight distances needed for decisions at complex locations; and (4) the criteria for measuring these sight distances for use in design.The design of alignment and profile to provide sight distances and to satisfy the
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applicable design criteria are described later in this chapter. The special conditions related to sight distances at intersections are discussed in Chapter 9.
Stopping Sight Distance
Sight distance is the length of the roadway ahead that is visible to the driver. The available sight distance on a roadwayshould be sufficiently long to enable a vehicle traveling at or near the design speed to stop before reaching a stationary object in its path. Although greater lengths of visible roadway are desirable, the sight distance at every point along a roadway should be at least that needed for a below-average driver or vehicle to stop.
Stopping sight distance is the sum of two distances: (1) the distancetraversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied; and (2) the distance needed to stop the vehicle from the instant brake application begins. These are referred to as brake reaction distance and braking distance, respectively.
Brake Reaction Time
Brake reaction time is the interval from the instant that the driver...