This length is usually about 75 of that obtained from the headlight sight

# This length is usually about 75 of that obtained from

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This length is usually about 75% of that obtained from the headlight sight distance requirement. This criterion is recommended by GHA Roads Design Guide DRAINAGE REQUIREMENT This criterion for sag curves is essential when the road is curbed. Here, a maximum length requirement is given, unlike in other criteria where the minimum is specified. The requirement usually specified to fulfill this criterion is that a minimum grade of 0.35% be provided within 15m of the level point of the curve AESTHETICS REQUIREMENT This criterion is usually satisfied by the use of a “rule of thumb” expression: L = 100A , where L= minimum length of the sag curve, A- algebraic difference in gradients in (%). Experience however shows that longer curves are often required for high category highways to make them appear better.
Lecture 4 13/04/14 7 Design of Sag Curves Computing Elevations On Sag Curves This is done in similar manner as for crest curves. In this case, however, the offset (y) is added to the appropriate tangent elevation to obtain elevation on the curve since the formation elevation of the curve is higher than the tangent.
Lecture 4 13/04/14 8 Design of Sag Curves Example: A sag curve is required to join a –4 % and +4 % grades. If the design speed is 100kph, determine the minimum length of the curve that will satisfy all minimum criteria. Assume f=0.32 and perception-reaction time =2.5s. Solution: 1. Find MSSD MSSD = => 2. Determine whether S<L or S>L For S>L; For S<L; 3. Determine minimum length for comfort criterion: L = AV 2 /13a =0.08*100 2 /13*0.3 =205.13 4. Determine minimum length for AESTHETICS criterion: L = 100*A =100*8 = 800m Therefore minimum length of sag curve that will satisfy all criteria is = ???? 2 1 2 10 36 254( /100) V d d Vt f g 2 10 100 100 2.5 36 254(0.32 0.04) MSSD 200( tan ) 2 H S B L S A 200(0.6 tan1 ) 2 8 o MSSD MSSD   2 2 0 8 200( tan ) 200(0.6 tan1 ) AS MSSD L H S B MSSD
HORIZONTAL ALIGNMENT Lecture 4 13/04/14 9 The horizontal alignment consists of straight sections of the road, known as TANGENTS, connected by HORIZONTAL CURVES. The curves are usually segments of circles , which have radii that will provide for a smooth flow of traffic along the curve. The design of horizontal alignment therefore entails: Determination of the minimum radii of curves Determination of the length of the curve Computation of horizontal offsets from tangents to the curve (SETTING OUT OF HORIZONTAL CURVES!) In some cases, to avoid a sudden change from a tangent with infinite radius to a curve of finite radius, a curve with radii varying from infinite value to the radius of the circular curve is placed between the circular curve and the tangent. Such a curve is known as a spiral or transition curve
Horizontal Alignment Lecture 4 13/04/14 10 There are two basic types of horizontal curves: 1. Circular Curves Simple Compound Reverse or serpentine curves 2. Spiral or Transition Curves Design of horizontal curvature is one of the most important features influencing the efficiency and safety of a highway. Substandard design results in lower speeds and a lowering of highway capacity.

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• Fall '08
• Devine,D
• Slope, Osculating circle, Sag Curves

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