The absolute minimum sight distance (SSD) for the design speed shall be made available along the whole length of the road, regardless of the type of road. If SSD is not available on a stretch of road for whatever reason, such as a visual blockage, prompt action should be made to either clear the impediment from the line of sight or erect signs that state the speed limit and the appropriate warnings.

Overtaking Sight Distance:

On the majority of road lengths, it is ideal to have sufficient overtaking sight distance so that fast moving vehicles can pass slowly moving ones as soon as possible.

• Where overtaking is not restricted, the minimum overtaking distance shall be (d1 + d2 + d3) for road segments with two-way traffic activity.
• On roads with only (d1 + d2) and on divided highways where no traffic is anticipated from the opposing direction.
• It is not necessary to give the standard OSD on split highways with four or more lanes, but the sight distance on any highway should be more than the SSD, which is the bare minimum sight distance.

Overtaking Zones:

It is preferable to build highways so that there is always enough road ahead to allow for safe passing. There may be stretches where the safe overtaking distance cannot be supplied since it is rarely practicable. Before restricted zones begin, sign posts indicating No Passing or Overtaking Prohibited should be put up in areas where overtaking or passing is unsafe or not possible.

However, cars travelling at design speed should be given as many opportunities to overtake as practicable. Overtaking Zones are those areas designated for overtaking.

For safe overtaking, the width of the carriageway and the length of the overtaking zone should be adequate. Sign posts should be placed far enough in front to signal the beginning of the overtaking zones; for one-way roads, this distance may equal (d1 + d2); for two-way roads, it may equal (d1 + d2 + d3).

The overtaking zone must be at least 3 feet long (OSD)

The ideal distance between overtaking zones is 5 (OSD)

Intermediate Sight Distance:

Where required OSD cannot be delivered, alternate Sight Distance ISD equal to double SSD may be provided as far as is practicable. The driver’s eye level and the object’s height can both be assumed to be 1.2 metres above the road surface for calculating the ISD.

ISD = 2 SSD

Sight distance at intersections:

Visibility should be provided for drivers coming from either side of an intersection where two or more roads converge. They ought to be able to recognise a danger and halt their car if necessary. The design speed can be used to calculate the stopping sight distance for each road. The drivers on either side should be able to see one another thanks to the sight distance that should be supplied.

In the image above, this is demonstrated. There are three scenarios in which the sight distance design at crossings may be applied:

• Permitting an oncoming car to alter its speed
• Enabling a halted car to cross a major route
• allowing an approaching vehicle to stop

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Over Taking Requirement and Factors governing the distance:

Theoretically, there shouldn’t need to be any overtaking if every vehicle travels down a road at the intended pace. Each driver is free to travel at lesser speeds, therefore all vehicles actually don’t move at the planned speed. This is especially true when there is mixed traffic. Fast moving traffic must be passed or overtaken in order to proceed. The Minimum Overtaking Sight Distance (OSD) or Safe Passing Sight Distance is the shortest distance that the driver of a vehicle must see in order to safely pass a slow-moving vehicle in front of them.

The OSD is the distance measured along the centre line of the road which a driver with his eye level at 1.2m above the road surface can see the top of an object 1.2m above the road surface.

Factors Affecting OSD:

• The velocities of the vehicles being passed, being passed, and the vehicles coming from the opposite direction
• The distance between vehicles that are being passed and those that are being overtaken; the minimum distance depends on the speeds Road gradient, overtaking vehicle’s rate of acceleration, and the driver’s skill and reaction time.

Analysis of OSD on a Two – Way Road:

Simple overtaking process on a 2 – lane highway with 2 – way traffic movement

Vehicle A wants to pass another slow-moving vehicle B going at a speed of vb m/sec or Vb kmph while travelling at its design speed of v m/sec or V kmph. Before oncoming vehicle C approaches the overtaking stretch, vehicle A must speed, change to the adjacent right-side lane, complete the overtaking manoeuvre, and then return to the left lane.

The overtaking manoeuvre may be split up into 3 operations, thus dividing OSD into 3 parts d1, d2and d3.

• d1 is the distance (m) travelled by the overtaking vehicle A during the reaction time, t (secs) of the driver from position A1 to A2 before starting to overtake the slow vehicle B
• d2 is the distance travelled (m) travelled by the vehicle A during the actual overtaking operation during T (secs) from position A2 to A3
• d3 is the distance (m) travelled by oncoming vehicle C during the actual overtaking operation of A during T (secs) from position C1 to C2. Thus, on a 2-lane road with 2-way traffic the OSD = d1 + d2 + d3 in meters

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