Current practice regards Design Speed as a design criterion that is aimed at

# Current practice regards design speed as a design

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Most important consideration in geometric design. Current practice regards Design Speed as a “design criterion, that is aimed at providing a consistent and coordinated alignment” . Generally design speeds are chosen administratively, hence variation from country to country or between jurisdictions for the same facility . SIGHT DISTANCE Sight distance is the length of highway visible ahead to the driver of a vehicle. When drivers have a sufficient distance of clear vision ahead that enables them to avoid hitting unexpected obstacles and to pass slower vehicles the highway is safer and operates more efficiently. Ideally, geometric design must always seek to fulfill this condition. There are three types of sight distances which are of interest in geometric design, namely: Stopping sight distance Meeting sight distance Passing sight distance
IDL Lecture 2 on 8-9/03/14 12 DESIGN CONTROLS AND CRITERIA STOPPING SIGHT DISTANCE (MSSD) The stopping distance is the distance required to stop a vehicle traveling near the design speed before it reaches a stationary object in the vehicle’s path. The object may be another vehicle or some other object within the roadway. Ideally, the sight distance at every point should be as long as possible but never less than a given minimum. The minimum stopping sight distance comprises two elements: The distance traveled from the time the object is sighted to the instant the brakes are applied (the “perception-reaction distance”; dependent on initial speed of vehicle and the perception time and brake-reaction time of driver) The distance traveled by the vehicle while braking (the “braking distance”; depends on the initial speed of the vehicle, the condition of the brakes, tires, and roadway surface, and alignment and grade of the highway) Perception-reaction distance (d 1 ): A certain amount of time is required for a driver to perceive and react for the brakes to be actuated. This depends on the physical and mental characteristics of the driver, atmospheric visibility, types and condition of the road and distance, size and color of the hazard. When drivers are keenly attentive, as in urban conditions with high traffic intensity reaction time is 0.5-1.0 seconds but around 2-4 seconds in normal rural traffic conditions. For purposes of design, a combined perception-reaction and brake-reaction time of 2.5 seconds is recommended. The corresponding distance traveled (d 1 ) is 2.5 times the speed in meters/second or Where, d 1 = total reaction distance (m), V= initial speed in kph, t= reaction time in seconds 1 10 36 d V t
IDL Lecture 2 on 8-9/03/14 13 DESIGN CONTROLS AND CRITERIA Braking Distance (d 2 ): The braking distance is dependent on vehicle condition and characteristics, the coefficient of friction between tire and road surface, the gradient of road and the initial speed.

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