Road Building: How They're Designed And Constructed
Roads play a key role in our society. They facilitate both national and international trade, as well as provide better access to health care, jobs, education, and other important services. A faster and less costly way to transport goods, roads are vital for the quality of life and economic development of any country.
The process of building a road is much more intensive than many think, especially in the planning phase. Engineers must consider the sharpness of curves, the incline of hills, drainage, location, traffic volume, and so many other factors to effectively create blueprints for these projects. The construction of a roadway, especially a major highway, can take months or even years, and today we'll take an in-depth look at how these critical elements of society are designed and built.
Designing a Road
Before starting the design process, the municipality must determine the scope of the project, the delivery method, and the type of road they need. Taking into consideration location and expected traffic volume, the number of lanes and total length of the roadway must also be decided. Civil engineers and city planners will then begin to undertake the extensive design process before handing the project over to a construction company for completion.
Assessing road usage is one of the most important parts of the planning procedure. A road that supports heavy commercial vehicles, such as semi-trucks, must be straighter to accommodate the limits of the vehicle. These roads must also be tougher than streets that are tailored to low traffic with lighter vehicles, which will affect the building process.
Design guidelines must take into account the intended speed limit, the types of vehicles that will use the road, the slope it's built on, any view obstructions, and stopping distances. Only with the proper application of these guidelines can a team of engineers plan a roadway that is safe and comfortable for drivers.
Lane width is also an important component of road design as it has a direct association with crash rates and severity. When lanes are too wide - generally over 10 and a half feet - drivers feel too comfortable and tend to speed up, leading to more intense collisions.
When lanes are too narrow - less than 9 feet and 2 inches - there is not adequate room for drivers to stay in their lanes, and this also leads to less safe roadways. The sweet spot for lane width is between 9 feet 11 inches and 10 feet two inches, and although drivers may prefer wider lanes, narrow ones cost less to build and maintain.
Sight Distances
Though roads and highways may seem fairly simple to build, there's a considerable amount of math that must be included in the design phase. This math goes into calculating sight distances. There are two major sight distances that need to be a part of the decision-making process: stopping sight distance and decision sight distance.
Stopping sight distance is a conservative estimate of how much time a driver going the speed limit needs to stop before reaching an unmoving object in its path. This is affected by road conditions, the mass of the car, and the incline of the road.
Decision sight distance is more complex than a simple stop for the driver. Always longer than stopping sight distance, this is the distance required for a driver to detect and react to an unexpected hazard, choose the safest path, and complete their maneuver safely. Engineers aim to design roads with decision sight distance as the main consideration, providing 6-10 seconds for perception and 4-5 seconds to avoid danger.
Although these are the two most important distances to consider, corner and intersection sight distances should also be factored into the design process. You can read more about this topic here.
There are a few aspects of road design that can have a significant impact on these calculations. For example, when drivers need to go over a hill, stopping sight is affected by how easily they can see over the top of the curve. Also, if there is a sharp incline, it will be difficult for vehicles to stop for stalled cars, animals, or pedestrians in time to avoid a crash.
Lastly, when a valley appears in the road, an important design consideration is headlight distance. As the road changes from a decline to an upwards slope, a vehicle's headlights will be blocked by the ground in front of them. The distance needs to be far enough for drivers to see any obstructions and have time to stop or avoid the problem.
Other Design Considerations
An important feature of every road is its cross slope. If streets were flat, water would be unable to efficiently drain off of them, and ice would build up in the winter months. Engineers must put slight slopes on each side of the centerline (usually 1-2%), which are usually unnoticeable to drivers. On curved sections of roadways, the outside edge is elevated above the center, meaning all water would drain in the same direction rather than going off both sides.
When designing curves in your road, such as on-ramps to highways, it's important to make the bend as gradual as possible. The tighter the turn, the more difficult it becomes to navigate, and the more crashes occur.
It's also important to be consistent in the design of the road. A bend that is much sharper than previous turns may surprise drivers, which is never a good thing to do. In situations where the structure of the roadway must be inconsistent, include signage to indicate that drivers should decrease speed or anticipate a quick turn.
Building a Road
After finally making it through all the complexities of the design stage, it's time to build yourself a road. Construction technology has made incredible progress since the days of the first paved roads, but all in all, many of the elements of production have remained the same. Follow the basic guidelines of road building and you're sure to succeed, whether you're working on a highway or a driveway.
First and foremost, you need to get the right equipment and materials. Wheel loaders, asphalt mixers, road rollers, excavators, backhoes, skid steers, and compact track loaders are just some of the machines you'll find on a typical road construction project.
When it comes to road building materials, the two main components are asphalt and concrete. Asphalt is a mixture of aggregates and a binder, and is generally made from crushed rock, sand, gravel, or recycled material. The binder, usually bitumen, glues all the other materials together. The quality of the asphalt depends on the quality of the aggregates used in its creation, and higher grades of the material are better for the long-term health of your road.
Next comes the preparation of the site, which consists mostly of clearing and excavation. For the majority of roads, this involves the removal of trees, vegetation, or other obstacles that will hinder construction. Common construction equipment used in this phase is excavators, mini-excavators, compact track loaders, and more that handle the earth-moving and land-clearing jobs.
Depending on where the road is built there may be special preparations like better drainage or lots of bridges. Highways built through or on the side of mountains may require digging tunnels or installing control devices to prevent natural disasters like mudslides or rockfalls. If there is mountain work, articulating dump trucks, wheel loaders, and excavators are going to get needed for the heavy-duty work.
Preparation also requires grading a path for the roadway. Dirt may need to be removed from some areas or added to others, otherwise, the result could be susceptible to potholes. In addition, the surface needs to be properly sloped for drainage as we mentioned in the design stage.
Grading and sloping are especially important for climates that experience both hot and cold temperatures, as it maximizes the durability of the surface. In this phase of road construction, bulldozers, soil compactors, and wheel loaders are very common equipment to use.
The first step after the land clearing and site preparation is laying the sub-base. This goes on top of the soil underneath the road, but stays beneath the asphalt, and can be made of recycled concrete, granular fill, crushed concrete, crushed bricks, or other aggregates of materials.
A sub-base is compacted to help with drainage and the materials allow the road to resist swelling and shrinkage when temperatures fluctuate. The heavier the load it is expected to hold, the thicker and more compact the sub-base will be which is made possible by using equipment like Padfoot and sheepsfoot rollers.
Following the sub-base comes the binding layer. Bitumen has been used in some of the oldest known roads and is still the most popular binding material among construction workers today. This material is able to be heated up to 240 degrees Fahrenheit before melting, making it perfect for use in the scorching temperatures of states like Florida, Texas, and California. The binding layer is the support structure of the road and the primary determinant of its reliability and durability.
Finally comes the asphalt installation. A top layer of this common road material will be poured overtop of the binding layer, giving the smooth surface drivers can't go without. Easy to install and smooth out with asphalt rollers like tandem rollers or double-drum rollers, asphalt is naturally black, which gives the roadway an attractive appearance.
The construction cost of a road can be up to $8 million per mile for a four-lane highway. When possible, it's much less expensive to maintain and repair existing roadways. Resurfacing and expanding an existing road by two lanes will cost about $5.25 million per mile, and is almost always preferable to building a new one.
Although the main aspects of the process are finished, there are still a few more things to consider. Connections with other roads, entrances, and exits will require special grading and some extra attention. It's important to ensure that these areas still achieve proper drainage and are safe for cars to drive over. You may also want to apply a surface-strengthening solution, which helps provide an all-weather surface and increases water resistance.
Final Thoughts
A good best practice is to design and build your road for the worst-case scenario. Keep in mind the heaviest vehicles, the highest traffic volumes, and the worst weather conditions are going to happen at some point. Plan for these and your road will be ready to handle anything that drivers throw at it.
The designing and building of roads may seem extensive, but it is easily one of the most significant aspects of our society. From the time they were first built until today, we've relied heavily on roads for transportation, health care, and economic development. Much of our quality of life and ability to get around is due to the existence of roads. We should be recognizing the work and the workers that take months and years to build them.
