Q & A: Highway Curves

Q: Is there a purpose for curves on a highway?

A: Highways are not always built in straight lines because curves serve several important purposes.

The biggest reason is the terrain. Roads are designed to follow the natural landscape rather than simply cutting straight through everything. A highway might curve around:

• 🏔️ Hills and mountains — reducing the amount of excavation and steep grades.
• 🌊 Rivers, lakes and wetlands — avoiding expensive bridges or environmentally sensitive areas.
• 🌲 Forests and other natural features — minimizing the amount of land that has to be cleared.
• 🏘️ Communities and developed areas — avoiding homes, businesses and other infrastructure.
• 🚆 Railways and existing roads — allowing the highway to connect with other transportation routes efficiently.

There is also an important engineering reason: curves can help control a highway's speed and alignment.

A very long, perfectly straight highway can encourage drivers to become less attentive and travel faster. Gentle curves provide visual changes and can help maintain driver attention. Highway designers can deliberately introduce horizontal curves while keeping them gradual enough to be safe.

Curves also allow engineers to deal with elevation changes. Instead of building a road straight up a hill, the highway can wind gradually around it. This produces a lower and safer gradient.

Interestingly, curves themselves have specific engineering requirements. A curve that's too sharp requires a lower speed, while a gentle curve can be driven at highway speed. Designers calculate things such as:

curve radius + vehicle speed + superelevation (banking) + friction

to determine how safely vehicles can negotiate the curve.

That's why you'll sometimes see a highway curve that seems to have no obvious reason for being there. It may be following an elevation contour, meeting a required design radius, avoiding underground geology, preserving a drainage system, or simply fitting into the highway's overall geometric design.

And those curves you see on prairie highways around Saskatchewan can be particularly interesting: on seemingly flat land, a slight bend may actually be there because of land ownership, drainage, intersections, wildlife considerations, or the original survey route, rather than because the terrain forced it.

If you want, I can also explain why some highway curves are banked (tilted) and how that lets a car take a curve at 100 km/h without sliding outward. 🛣️

Source: Some or all of the content was generated using an AI language model