A Curve in the Road

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Sag vertical curves are those that have a tangent slope at the end of the curve that is higher than that of the beginning of the curve. When driving on a road, a sag curve would appear as a valley, with the vehicle first going downhill before reaching the bottom of the curve and continuing uphill or level.

The most important design criterion for these curves is headlight sight distance. [2] When a driver is driving on a sag curve at night, the sight distance is limited by the higher grade in front of the vehicle. This distance must be long enough that the driver can see any obstruction on the road and stop the vehicle within the headlight sight distance. The headlight sight distance (S) is determined by the angle of the headlight and angle of the tangent slope at the end of the curve. By first finding the headlight sight distance (S) and then solving for the curve length (L) in each of the equations below, the correct curve length can be determined. If the S < L curve length is greater than the headlight sight distance, then this number can be used. If it is smaller, this value cannot be used. Similarly, if the S>L curve length is smaller than the headlight sight distance, then this number can be used. If it is larger, this value cannot be used. [4] Units These equations assume that the headlights are 600 millimetres (2.0ft) above the ground, and the headlight beam diverges 1 degree above the longitudinal axis of the vehicle. [5] Crest curves [ edit ] The most important design criterion for these curves is stopping sight distance. [2] This is the distance a driver can see over the crest of the curve. If the driver cannot see an obstruction in the roadway, such as a stalled vehicle or an animal, the driver may not be able to stop the vehicle in time to avoid a crash. The desired stopping sight distance (S) is determined by the speed of traffic on a road. By first finding the stopping sight distance (S) and then solving for the curve length (L) in each of the equations below, the correct curve length can be determined. The proper equation depends on whether the vertical curve is shorter or longer than the available sight distance. Normally, both equations are solved, then the results are compared to the curve length. [4] [5]

These curves gives a gradual change in grade or slope that provides a sufficient comfort to the passengers. 1.Summit Curve: A curve with a series of two more simple curves of different radii in the same direction is called a compound curve. The profile of a road consists of road slopes, called grades, connected by parabolic vertical curves. Vertical curves are used to provide a gradual change from one road slope to another, so that vehicles may smoothly navigate grade changes as they travel. Road curves are irregular bends in roads to bring a graduation change of direction. Similar curves are on railways and canals.

In the case of horizontal as well as vertical, road curves should ensure safety, comfort, and convenience for traffic, while they are further classified as follows: 1.Simple Curve:Sag vertical curves are curves which, when viewed from the side, are concave upwards. This includes vertical curves at valley bottoms, but it also includes locations where an uphill grade becomes steeper, or a downhill grade becomes less steep.