Road geometry affects the sight distance available to the driver. Sight distance in the context of road design is defined as “the length of roadway ahead visible to the driver.” Sight distance is how far a road user can see before the line of sight is blocked by a hillcrest or an obstacle on the inside of a horizontal curve or intersection. Insufficient sight distance can adversely affect the safety or operations of a roadway or intersection. Road geometry affects the sight distance available to the driver. Sight distance in the context of road design is defined as “the length of roadway ahead visible to the driver.” Sight distance is how far a road user can see before the line of sight is blocked by a hillcrest or an obstacle on the inside of a horizontal curve or intersection. Insufficient sight distance can adversely affect the safety or operations of a roadway or intersection.
Importance:
The sight distance needed for a given situation is the distance traveled during the two phases of a driving maneuver: perception reaction time(PRT), and maneuver time. Perception-reaction time is the time it takes for a road user to realize that a reaction is needed to a road condition, decided what maneuver is appropriate, and starts the maneuver. Maneuver time is the time it takes to complete the maneuver. The distance driven during perception-reaction time and maneuver time is the sight distance needed.
During highway design and traffic safety investigations, highway engineers compare the available sight distance to how much sight distance is needed for the situation. Depending on the situation, one of three types of sight distances will be used.
1) Absolute minimum sight distance:
Safe stopping sight distance, SSD for the design speed is the absolute minimum sight distance. This particular sight distance traveled during perception-reaction time, and maneuver time. Actual stopping distances are also affected by road conditions, the mass of the car, the incline of the road, and numerous other factors. For design, a conservative distance is needed to allow a vehicle traveling at design speed to stop before reaching a stationary object in its path. Typically the design sight distance allows a below-average driver to stop in time to avoid a collision.
2) Overtaking sight distance:
The overtaking sight distance is the minimum distance open to the vision of the driver of a vehicle intending to overtake the slow vehicle ahead safely against the traffic in the opposite direction. The overtaking sight distance or passing sight distance is measured along the centerline of the road over which a driver with his eye level 1.2 m above the road surface can see the top of an object 1.2 m above the road surface. The overtaking sight distance is the minimum distance open to the vision of the driver of a vehicle intending to overtake the slow vehicle ahead safely against the traffic in the opposite direction. The overtaking sight distance or passing sight distance is measured along the centerline of the road over which a driver with his eye level 1.2 m above the road surface can see the top of an object 1.2 m above the road surface.
3) Intermediate sight distance:
At road-stretches where required OSD cannot be provided, as far as possible ‘Intermediate Sight Distance’, ISD equal to twice SSD may be introduced. The measurement of the ISD may be made assuming both the height of the eye level of the driver and the object to be 1.2 meters above the surface of the road.
Therefore, ISD =2SSD.
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