Vertical Geometry(General)

O Sight Distance Parameters

 - Object height

 - Driver eye height

 - Reaction time

 

 

O Longitudinal Deceleration : Longitudinal deceleration measures the friction between tires and the road, influenced by factors like speed, surface type, and condition, with a standard coefficient of d = 0.36 for most roads and d = 0.46 reserved for low-volume, constrained roads.

O  Headlight Sight Distance

On unlit roads, sight distance is limited by headlight beams, providing adequate stopping distances for speeds up to 80–90 km/h, while larger or light-colored objects are visible beyond this range, though small objects remain difficult to perceive at higher speeds.

 

O SSD : The concept of Stopping Sight Distance (SSD) is based on the distance required for a driver to stop a vehicle safely upon spotting a stationary object on the road. The standard measurement is from the driver's eye level (1.1 m) to a 0.2 m high object, which represents an obstacle that cannot be driven over. In certain special cases, such as floodways, a lower object height (e.g., pavement level) may be used.

In constrained conditions, sight distance over barriers may not always accommodate a 0.2 m high object. For these situations, alternate object heights listed in Table 5.5 should be used. Table 5.7 specifies the minimum shoulder width and maneuver time needed when higher object heights are adopted to ensure hazards lower than the design object height can still be avoided.

SSD must be available on all traffic lanes and should be sufficient for drivers to respond safely under all conditions. Values for SSD are provided in Table 5.5, adjusted using deceleration coefficients (Table 5.3) and grade correction factors for uphill or downhill slopes.

Key considerations:

1. d=0.26 is applicable in flat terrain for greenfield major highways or freeways, with prior approval from the road authority.
2. d=0.36 is suitable for most urban and rural road upgrades, especially where terrain restricts other solutions.
3. SSD standards for specific projects must be agreed upon with the road authority.