Sight Distance

Sight distance is the actual distance available from a point along the road surface which a driver from a specified height above the road surface has visibility of a stationary or moving object. In other words, sight distance is the length of road visible to the driver at any instant.

Three sight distance situations are considered in the design:

1. Stopping Sight Distance (SSD)
2. Safe Overtaking Distance
3. Safe Sight Distance for entering into uncontrolled intersections

Sight Distances considered by IRC in Highway design:

1. Intermediate Sight Distance which is equal to twice the stopping sight distance.
2. Head Light Sight Distance is the distance visible to a driver during night driving under the illumination of the vehicle head lights. It is critical at up-gradients and at the ascending stretch of the valley curves.

Stopping Sight Distance

The minimum sight distance available on a highway at any spot should be of sufficient length to stop a vehicle travelling at design speed, safely without collision with any other obstruction.
IRC has suggested the height of eye level of a driver as 1.2 m and height of the object as 0.15 m above the road surface.

Stopping Sight Distance depends on the following factors:

• Total reaction time of Driver
• Speed of vehicle
• Efficiency of brakes
• Frictional resistance between tyres and the road
• Gradient, if any.

$\text{SSD = Lag Distance + Braking Distance}$

Lag Distance is the distance covered by the vehicle during reaction time and IRC recommended 2.5 Seconds as total reaction time.

$\text{Lag distance} = v\times t$

Braking Distance is the distance covered by the vehicle after the application of the brakes.

$\text{Braking Distance}=\frac{v^2}{2gf}$

$SSD=v\times t+\frac{{{v}^{2}}}{2gf}$

        $v=$ Speed of vehicle in $m/sec$
        $f=$ Coefficient of friction (0.4 to 0.35 depending on speed, from 30 to 80 KM/H)
        $g=$ Acceleration due to gravity $=9.81 m/sec^2$

Stopping Sight Distance at slopes,

$SSD=v\times t+\frac{{{v}^{2}}}{2g\left( f\pm \frac{n}{100} \right)}$

        $n=$ slope of the pavement
        $+$ is for Ascending Gradient and $-$ is for Descending Gradient.

Over Taking Sight Distance (OSD)

Different vehicles travel at different speeds and this condition is called mixed traffic condition. Some of the vehicles move at a speed which is below design speed while some vehicles move at a speed higher than design speed, in such circumstances, it is necessary for the fast moving vehicles to overtake the slow-moving vehicles.

The minimum distance visible to the driver of a vehicle intending to overtake a slow vehicle ahead with safety against the traffic of opposite direction is known as the minimum overtaking sight distance or the safe passing sight distance available.

Overtaking Sight Distance depends on following factors:

1. Speed of
   
   • Overtaking Vehicle
   • Overtaken Vehicle
   • Vehicle coming from opposite direction, if any
2. Distance between the overtaking and overtaken vehicles
3. Skill and reaction time of the driver
4. the rate of acceleration of overtaking the vehicle
5. Gradient of the road, if any

$OSD={d}_{1}+{d}_{2}+{d}_{3}$

     ${d}_{1}=$ Distance travelled by the overtaking distance during the reaction time $t$ from position $1$ to $2$
             $t$ can be taken as $2sec$
    ${d}_{2}=$ Distance travelled by the overtaking vehicle from position $2$ to $5$during the actual overtaking operation in              time $T$.
    ${d}_{3}=$ Distance travelled by oncoming vehicle from position $6$ to $5$ during overtaking operation in time $T$.

${d}_{1}={v}_{s}\times{t}={v}_{s}\times{2}$

${d}_{2}=b+2s$

$b={v}_{s}\times{T}$

$T=\sqrt{\frac{4s}{a}}$,       $s=0.7\times {{v}_{s}}+6$

${d}_{3}=v\times{T}$

     ${v}_{s}=$ Speed of slow (overtaken)vehicle in $m/sec$
     $v=$ Speed of overtaking vehicle in $m/sec$
     $a=$ acceleration in $m/{sec^2}$

$OSD=({v}_{s}\times{t})+({v}_{s}\times{T}+2s)+(v\times{T})$

The minimum length of overtaking zone should be 3 times the safe OSD and desirable length of the zone is 5 times the safe OSD.

Intermediate Sight Distance (ISD)

Sufficient overtaking sight distance should be available on most of the road stretches however on horizontal curves the OSD requirement may not be always fulfilled. In such cases overtaking should be prohibited by regulatory signs.
At vertical summits where OSD cannot be provided, Intermediate sight distance (ISD) equal to 2 times the SSD may be provided.

Sight Distance at Intersections

It is important that on all approaches of intersecting roads there is a clear view across the corners from a sufficient distance so as to avoid collision of vehicles. It is more important for uncontrolled intersections.

The design of sight distance at intersection may be based on the 3 possible conditions:

1. Enabling the approaching vehicle to change speed: The sight distance should be sufficient to enable either one or both the approaching vehicles to change speed to avoid the collision. The vehicle approaching from the minor road should slow down.
   The total reaction time required for the driver to decide to change speed may be taken as 2 seconds and at least one more second will be needed for making the change in speed.


2. Enabling approaching vehicle to stop: The sight distance should be sufficient to enable either one or both the approaching vehicles to stop to avoid the collision.
   It is the responsibility of the drivers on the minor road who would cross or enter the main road, to stop or change speed to avoid the collision. The traffic of the minor road is generally controlled by an appropriate traffic sign.


3. Enabling stopped vehicle to cross the main road: This case is applicable when the traffic on the minor road is controlled by a stop sign and so these vehicles have to stop before they can proceed to cross the main road. In this case, sufficient sight distance should be available to start the stopped vehicle, accelerate and cross the main road before another vehicle travelling at its design speed on the main road reaches the intersection.

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