affecting Sight distance
The most important consideration in all these is
that at all times the driver traveling at the design speed of the highway must
have sufficient carriageway distance within his line of vision to allow him to
stop his vehicle before colliding with a slowly moving or stationary object
appearing suddenly in his own traffic lane.
computation of sight distance depends on:
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 show 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.
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
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.
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.
of the road.
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