Iowa DOT's
policy is to be
sensitive to the environment
and foster and enhance
environmental stewardship.
IOWA DEPARTMENT OF TRANSPORTATION
Presents
ROCKS AND ROADS
Unit 3: Why isn't the highway straight?
D. Why aren't highways always straight?
You may have noticed that highways do not always go in a straight line. Did you ever wonder why? This section discusses some of the issues involved in highway design and how they affect the finished highway.
Engineers and planners need to consider many things when designing a new highway or improving an existing highway. One of the major factors is cost. The goal is always to spend the least amount of money while satisfying all the needs of the project. It would seem apparent that cost is directly related to the length of the highway and the least expensive highway would be the shortest highway. We also know that the shortest distance between two points is a straight line. Why, then, aren't highways always straight? The following design factors must be considered when designing highways:
1. Topography
Topography generally means the "lay of the land." That means the shape and slope of the land surface. The topography may be flat, sloping, hilly, or a combination of any of these.
Why is topography important in highway design? Highways are generally designed to follow the land surface if possible. However, the land surface often slopes steeper than highways can safely be designed for. When this happens, high points must be cut down and low points must be filled up to allow the highway to maintain a gradual slope. Designers try to balance the amount of material they take from high points with the amount they need to fill low points. If this does not balance, as is usually the case, material must either be "wasted" off the site or "borrowed" from off site. There is more information on this topic in Unit 4.
Sometimes the topography is so hilly or steep in places that it is much easier, and requires less earthwork, to slightly change the alignment to avoid these places. While the highway might not be straight for a short stretch, overall it is still generally straight.
2. Environmentally Sensitive Areas
Highway designers have a duty to protect rare and endangered plants and animals, which means protecting the habitat in which these plants and animals live. Other environmentally sensitive areas might include unique natural land forms, such as the Loess Hills along the western edge of the state.
When a new highway is in the planning and preliminary design stage, environmentalists such as biologists survey the planned route to identify any sensitive areas. Certain types of plants and animals depend on the habitat provided by ponded water and saturated soils. These habitats are called wetlands and they are found all over the state. Some types of wetlands are more rare and unique. Those that are very rare are generally avoided altogether when designing a new highway.
When some wetlands must be drained and covered by a new highway, the designers must go through a process called mitigation. Mitigation means that designers must create new wetlands to replace the wetlands lost for the highway construction. The mitigation area is typically equal to or greater than the area lost.
Painted Ridge Farms Wetland Mitigation Site3. Social Factors
Social factors encompass the need and desire to avoid negative impacts to people. Highway designers make every effort to avoid displacing people from their homes in order to construct a new road. Unfortunately, this is not always possible. Many towns in Iowa are centered on highways and intersections of highways. The highway made it easy to get there, so more people came. When it later becomes necessary to improve or widen the highway, it is impossible to do so on the existing alignment. For one thing, no one wants a high-speed road running through the middle of their town, especially in smaller towns.
Additionally, the land required to widen the highway might eliminate many homes and businesses along the route. That is why improved highways near smaller towns often include a bypass of the town itself. The bypass is a segment of highway that goes around the town, so high-speed traffic does not need to slow down to go through the town.
While this sounds like a win-win situation for everyone, sometimes people living on the edge of town or out in the country near the town are not so fortunate. It is sometimes necessary to displace people from their homes in order to construct a bypass. Though rare, when it does happen the state is responsible for paying people fairly for the land. This is one of the hardest decisions that planners and highway designers have to make.
Another important social factor is avoiding impacts to people's cultural identity. This means that highway designers cannot build new highways over land occupied by cemeteries and American Indian burial grounds, or other sites of important cultural significance like historic buildings. Archaeologists survey proposed routes for evidence of American Indian encampments and burial mounds, and if found these areas are protected from disturbance. One well-known area is at Effigy Mounds National Monument in Allamakee County, where American Indians constructed mounds in the shape of various animals.
4. Other Factors
When a highway must cross a river, a bridge must be constructed to safely carry traffic over the river. Sometimes the width of the river can vary significantly along its length. Since bridges are very expensive, designers try to build the shortest bridge they can. This means the highway might turn slightly near a river in order to cross at the best location and angle. This is also true when highways intersect other highways or railroads.
Example of road turning to cross at the best location.
As stated above, highways are sometimes turned to intersect other highways or railroads at a better angle. This is true whether the two intersect at grade or with an overpass. An overpass is when a bridge is constructed to carry one highway over the other so traffic does not need to stop in either direction.
