In highway engineering, the design speed is a critical factor determined by the terrain type through which the highway passes. Let's evaluate the given options based on the context of the question:

1. Lower than plain terrain: Rolling terrains typically have more vertical and horizontal curves compared to plain terrains. These curves demand lower design speeds to ensure safety while navigating through changes in the alignment and elevation. Thus, on rolling terrain, the design speed is reduced relative to a plain terrain to allow vehicles to maneuver safely.
2. Same as mountainous terrain: Mountainous terrains tend to have very low design speeds due to sharp curves, steep grades, and challenging topography. While rolling terrains also require reduced speeds, they usually do not require as drastic a reduction as mountainous areas do. Therefore, this option incorrectly ties the same design speed to both terrains.
3. Higher than plain terrain: Plain terrains have the least physical obstructions and typically allow for the highest design speeds due to their flat and straight alignment. Hence, it is inaccurate to say rolling terrains have higher design speeds than plain terrains.
4. Unrelated to terrain type: This option is not accurate as terrain type directly impacts the geometric design of highways, influencing parameters like design speed, curve radius, and grade.

Therefore, analyzing all options, the correct answer is: Lower than plain terrain.

This is in accordance with standard highway design principles, which adjust design speeds lower when moving from plain to rolling and mountainous terrains, taking into account safety and vehicle control.