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The computation of sight distance depends on:
- Reaction time of the driver
Reaction time of a driver is the time taken from the instant the object is
visible to the driver to the instant when the brakes are applied.
The total reaction time may be split up into four components based on PIEV
theory.
In practice, all these times are usually combined into a total
perception-reaction time suitable for design purposes as well as for easy
measurement.
Many of the studies shows that drivers require about 1.5 to 2 secs under normal
conditions.
However, taking into consideration the variability of driver characteristics, a
higher value is normally used in design.
For example, IRC suggests a reaction time of 2.5 secs.
- Speed of the vehicle
The speed of the vehicle very much affects the sight distance.
Higher the speed, more time will be required to stop the vehicle.
Hence it is evident that, as the speed increases, sight distance also
increases.
- Efficiency of brakes
The efficiency of the brakes depends upon the age of the vehicle, vehicle
characteristics etc.
If the brake efficiency is 100%, the vehicle will stop the moment the brakes
are applied.
But practically, it is not possible to achieve 100% brake efficiency.
Therefore the sight distance required will be more when
the efficiency of brakes are less.
Also for safe geometric design, we assume that the vehicles have only 50%
brake efficiency.
- Frictional resistance between the tyre and the road
The frictional resistance between the tyre and road plays an important role to
bring the vehicle to stop.
When the frictional resistance is more, the vehicles stop immediately.
Thus sight required will be less.
No separate provision for brake efficiency is provided while computing the
sight distance.
This is taken into account along with the factor of longitudinal friction.
IRC has specified the value of longitudinal friction in between 0.35 to 0.4.
- Gradient of the road.
Gradient of the road also affects the sight distance.
While climbing up a gradient, the vehicle can stop immediately.
Therefore sight distance required is less.
While descending a gradient, gravity also comes into action and more time will
be required to stop the vehicle.
Sight distance required will be more in this case.
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