It has puzzled many road users on why there should be curves in highways when it is possible to construct the road possibly as the crow flies. This content is set to clear this puzzle. Three prime considerations that guide engineering design decisions are:
- Safety
- Economy
- Sustainability
Bringing it down to highway construction, by safety, it is expected that the road would be safe for road users. By economy, it is expected that road construction and subsequent maintenance does not gulp such money that it becomes difficult to build other roads. By sustainability, it is expected that the construction processes and materials should not affect the climate and topography of the environment that it becomes difficult for future generations to meet their pavement needs.
Safety is very important in geometric design of pavement. Geometric design involves the creation of the route way. It involves location of the route that the road would follow from origin to insertion. It is during this route survey that decisions are taken whether and where the road would bend or climb a hill or cross a river. Below are the reasons why there curves on highways.
1. Presence of an obstruction: When there is an obstruction such as large rock or hill or very thick forest reserve or important structures along the route of the pavement, economic considerations are made on the cost implication of removing the obstruction or bypassing it through an alternative route. The more economical option is usually chosen.
2. A river crossing: It is recommended that where a road would cross a bridge, it should at right angle to the direction of flow of the water. In this case, the intention to locate the road at right angle may introduce bend on the road.
3. To overcome overspeeding and road hypnosis: Road hypnosis is what happens when a driver loses the sense that he is on high speed because the road is very straight and smooth. This kind of situation can make the driver to overspeed. If something happens such as animal crossing or vehicle coming close to a bump or suddenly meeting a bend or a debris capable of affecting the balance of the vehicle, it would be difficult to control the vehicle and accident would probably occur. Going round a corner or bend brings the drivers consciousness of his speed due to the effect of centrifugal force. This force increases with the square power of speed and tends to throw the vehicle out of the road. Bends along a road serves as speed breakers.
4. Earth forms: The earth is made of ridges, coastlines and canyons which are not easy to cross. Vehicles cannot climb an incline or very steep hills. It is safer that roads follow a path with moderate grade which is easier to climb and descend.
5. Property ownership (right of way): This is particularly applicable to roads in densely populated areas. Instead of destroying people’s properties or crossing a road under high voltage electrical grids, it would be better to take the road through bends.
6. Very steep slopes: It would be very dangerous to climb very steep slopes and descend, so it becomes necessary to find an alternative route.
7. Environmental considerations such as wetland, plants and animal, and cultural/archeological sites: Where wetland, cultural and archeological sites are found, it would be more economical to avoid them by changing the alignment of the road thus introducing bends/corners.
8. Soil conditions: When a very poor soil is met along the original alignment of a road, the alignment would be shifted if it would be too costly to excavate and engineer a well-suited subgrade material.
9. Balance of Earthwork: The balance of earthwork is highly considered in pavement construction to mitigate cost and it can be reason to shift alignment of pavement. By balance of earthwork, it is expected that most of the material removed from cut sections would be used in sections needing fill. This would help ensure that much material is not required to be cart to spoil and also much material would not be borrowed.
10. Location of other connecting roads: It is necessary that roads linking to highways should be at right angle. This is basically for safety and can affect the alignment of highway.
11. Proposed bridge and flyover locations: The location of the bridge and flyover can also affect the road alignment. For bridges crossing river, it is safer and more economical to cross the river at right angle.
Even though it is pleasurable and saves motor spirit to drive vehicles over a straight road, the high risk involved in it, makes it very unsafe. The high cost involved in removing rocks or filling valleys or bringing mountains et cetra also makes it imperative that there should be curves in highways.
