Overtaking sight distance (OSD): While computing OSD on two-lane roads, the height of the visible object above the road surface is assumed as

Introduction / Context:
Overtaking sight distance (OSD) ensures a driver can overtake slower traffic safely while seeing an opposing vehicle in time to abort or complete the maneuver. Standard eye and object heights are assumed to reflect typical driver seating and the height of the opposing driver’s eye or vehicle silhouette.

Given Data / Assumptions:

• Two-lane, two-way roadway.
• Typical design values for eye and object heights are used.
• Undulating terrain may limit visibility; hence conservative assumptions are needed.

Concept / Approach:

In OSD calculations, both the overtaking driver’s eye height and the object height are commonly taken as 1.2 m (120 cm). This represents the opposing driver’s eye or a typical vehicle height that remains visible above the crest of a vertical curve. It differs from stopping sight distance (SSD), where the object height is much lower (for example, 0.15 m) to represent small obstacles.

Step-by-Step Reasoning:

Verification / Alternative check:

Sight-distance envelopes drawn for crest vertical curves show that using 1.2 m for both eye and object provides a symmetric line of sight criterion suitable for two-way roads.

Why Other Options Are Wrong:

• Zero/50/75/100 cm: too low for representing an opposing driver’s eye; these are used in other contexts (for example SSD uses a small object height).

Common Pitfalls:

• Using SSD assumptions for OSD leads to underestimation of required visibility and unsafe designs.

Final Answer:

120 cm.