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07 - Chapter-7 - Geometric Design Manual-2002 Chapter 7...

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Chapter 7 Geometric Design Manual-2002 Sight Distances Ethiopian Roads Authority Page 7-1 7 S IGHT D ISTANCES 7.1 General Considerations Simply put, sight distance is the distance visible to the driver of a passenger car. For highway safety, the designer must provide sight distances of sufficient length that drivers can control the operation of their vehicles. They must be able to avoid striking an unexpected object on the traveled way. Two-lane highways should also have sufficient sight distance to enable drivers to occupy the opposing traffic lane for passing maneuvers, without risk of accident. Two-lane rural highways should generally provide such passing sight distance at frequent intervals and for substantial portions of their length (see Table 7-2). The length and interval of passing sight distance should be compatible with the criteria established in the chapter 3 for the specific highway design classifications. 7.2 Stopping Sight Distance The stopping sight distance on a roadway must be sufficiently long to enable a vehicle traveling at the design speed to stop before reaching a stationary object in its path. The minimum stopping sight distance is determined from the following formula, which takes into account both the driver reaction time and the distance required to stop the vehicle. The formula is: where d = distance (meter) t = driver reaction time, generally taken to be 2.5 seconds V = initial speed (km/h) F = coefficient of friction between tires and roadway (see Table 7-1) Table 7-1: Sight Distances Design Speed (km/h) Coefficient of Friction (f) Stopping Sight Distance (m) Passing Sight Distance (m) from formulae Reduced Passing Sight Distance for design (m) 20 30 40 50 60 70 85 100 120 .42 .40 .38 .35 .33 .31 .30 .29 .28 20 30 45 55 85 110 155 205 285 160 217 285 345 407 482 573 670 792 50 75 125 175 225 275 340 375 425 f 254 V ) V )( t )( 278 . 0 ( d 2 + =
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Chapter 7 Sight Distances Geometric Design Manual-2002 Page 7-2 Ethiopian Roads Authority The coefficient of friction values shown in Table 7-1 have been determined from test results
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