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Roads are an integral part of our everyday life. According to a survey from the AAA Foundation for Traffic Safety, American drivers spend an average of more than 48 minutes behind the wheel per day. However, the average American doesn’t have any idea how much time is spent designing the roadways on their commute. Before the invention of software tools such as AutoCAD or Microstation, civil engineers had to design roadways by hand. Thankfully, the design (and re-design) time has been reduced significantly by using a tool in AutoCAD Civil 3D called “corridors.” In this blog, we will discuss the basic components needed to design a roadway using the corridor tools and how it serves as the foundation for your overall design of a subdivision project.

In the case of subdivisions, how the roadway is designed using the corridor tools will determine where you will set your house pad elevations, where you will place the detention pond, and how the overall storm and sanitary sewer systems will be designed. Additionally, any unforeseen changes that need to be made to the corridor later in the design process are quick and easy because each component is dynamically linked. So, spending time on the front end of the design to ensure the corridor is properly built could save hours or days at the end of a design.

There are three basic design components needed to design a roadway using the corridor tools: horizontal alignment, vertical alignment, and cross section. While we won’t get into the nuts and bolts of setting up a corridor in Civil 3D, below is breakdown of how each component functions within your design.

1. Horizontal Alignment. Horizontal alignment is the centerline of the roadway as shown in plan view, and it consists of two basic components: tangents and circular curves. The alignment is typically roughly placed using just tangents and then curves are placed to connect each tangent to the next. When designing the horizontal alignment, a few things should be considered. First, the engineer should double check the jurisdictional requirements and make sure the alignment is compliant. Typically, each roadway should meet the requirements of the American Association of State Highway and Transportation Officials (AASHTO), the state department of transportation (DOT), and the city or county. These design requirements usually place restrictions on minimum tangent lengths between curves, minimum curve radii, and minimum super-elevation rates. They set the requirements based on sight distance as well as the roadway’s intended classification and design speed. Second, the alignment should follow existing terrain as closely as possible and avoid existing slopes that are so steep that they might create difficulty in construction. Lastly, in the case of a subdivision, the alignment should allow for the desired amount and configuration of lots to be met. Once you have a horizontal alignment, a corresponding vertical alignment can be designed.

2. Vertical Alignment. The vertical alignment, called a profile in Civil3D, is the centerline of the roadway viewed from a profile (or side) view and consists of tangents (grades) and curves. The curves in a vertical alignment differ from the horizontal alignment curves in that they are parabolic curves instead of circular curves. The curves are categorized as sag (an increase in grade) or crest (a decrease in grade). Like the horizontal alignment, it is important to try to match the existing terrain as closely as possible with the vertical alignment to minimize the amount of earthwork that will be required. Additionally, there are requirements for maximum grade and curve lengths specified by jurisdictions. These are selected using the roadway’s design speed and are set to ensure a driver’s comfort and to meet sight distance. The last thing to consider when designing a vertical alignment is the minimum slope of the roadway. Typically, slopes greater than 1.0% are not used because slopes too low will not provide adequate drainage and could cause ponding in the roadway. As always, check with the proper jurisdiction to see if they have their own requirement.

Profile View

3. Cross Section. Called an assembly in Civil3D, this consists of various “subassemblies” connected to create the full cross section. Types of subassemblies used can be road lanes, curb and gutter, shoulders, sidewalks, tie in grades, and many others. Usually, the jurisdiction (state or local) will have their own cross-sectional requirements that will need to be followed. These requirements will tell you widths and slopes of lanes, shoulders, sidewalks, and/or roadside ditches, as well as required tie in slopes. Most roadways will not have the exact same cross section throughout, so several different assemblies may need to be created. The corridor tools in Civil3D allow you to specify different assemblies for different sections of the roadway. Once the assemblies are created, the corridor can be built.

Cross Section

The corridor modeling tools in Civil3D are very helpful for anytime you need to design a roadway or a subdivision, particularly with their ability to dynamically update as any of these components are changed. I’ve found that in the case of subdivisions, it’s especially worth the effort to properly set up a corridor at the start of the design process in order to smooth the way for other design elements.

About Foresite Group

Foresite Group is a multidisciplinary engineering, planning, and consulting firm providing services to public and private sector clients nationwide. Our team’s collaborative process results in creative products and services that help our clients achieve their goals. Our team takes pride in enhancing and developing the cities and communities where we live, work, and raise our families.


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