The ruling design speed on a National Highway in plain terrain as per IRC recommendations is

Solution (By Examveda Team)

Let's understand this important concept in Highway Engineering!

The design speed is a very crucial factor in road design. It's the maximum safe speed at which vehicles can travel on a road under favorable conditions. All the geometric features of a highway, like the curves, sight distances, and gradients, are designed based on this specific speed.

The Indian Road Congress (IRC) provides guidelines and standards for road design in India. They specify different design speeds. These speeds mainly depend on two factors: first, the Type of Road itself (for example, whether it's a National Highway (NH), a State Highway (SH), or a Major District Road (MDR)); and second, the Type of Terrain (which could be plain, rolling, mountainous, or steep terrain).

The question asks for the ruling design speed on a National Highway in plain terrain. A National Highway (NH) is an important road connecting major cities and states, designed for relatively high traffic volumes and speeds. Plain terrain means the area is mostly flat, which allows for straighter roads and fewer design constraints, thus permitting higher speeds.

The term 'ruling' design speed refers to the desirable or *standard* design speed, which is used for most of the highway stretch. There's also a 'minimum' design speed for more challenging sections.

According to IRC recommendations, for a National Highway in plain terrain, the ruling design speed is 100 kmph. This speed ensures safe and efficient travel on such important routes where the flat terrain does not pose significant challenges.

Let's look at why other options might not be correct for this specific scenario:
80 kmph: This speed is often specified for State Highways in plain terrain, or sometimes for National Highways in rolling (gently undulating) terrain.
60 kmph: This speed is more common for major district roads or for highways in mountainous terrain where speeds must be lower due to sharp curves and steep gradients.
120 kmph: While some modern expressways might be designed for speeds around 120 kmph, 100 kmph is the standard ruling design speed for a typical National Highway in plain terrain as per current IRC guidelines.

Therefore, the correct answer is Option C: 100 kmph.