Highway Design – When Overtaking Sight Distance (OSD) Is Not Feasible On stretches where the required OSD cannot be provided, which minimum sight distance criterion should be adopted to maintain safety under mixed traffic?

Introduction / Context:
Overtaking sight distance (OSD) ensures that a vehicle can overtake safely on two-lane highways. When geometric or environmental constraints prevent providing full OSD, standards prescribe a fallback ensuring a base level of safety for drivers who must at least be able to stop safely if a hazard appears.

Given Data / Assumptions:

• Mixed traffic, two-lane highways.
• OSD not achievable due to terrain, right-of-way, or environmental constraints.
• Need a conservative, implementable alternative.

Concept / Approach:

The recommended fallback is Intermediate Sight Distance (ISD), defined as twice the Stopping Sight Distance (SSD). ISD offers better safety than SSD alone by allowing drivers more distance to react and adjust speed in the absence of full OSD.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals and standard practice list ISD = 2 * SSD as the fallback where OSD is infeasible, balancing safety and feasibility.

Why Other Options Are Wrong:

• Half or one third of OSD: Insufficient safety margin.
• Three times SSD: More conservative than typical prescriptions and often impractical.

Common Pitfalls:

Failing to post appropriate signage; not adjusting for gradients or friction conditions when computing SSD.

Final Answer:

At least twice the stopping sight distance (Intermediate Sight Distance)