In order to have a fully functional transportation system, the links that connect the various origins and destinations need to be designed to a level of quality that allows the safe and efficient movement of all vehicles that use them. This level of quality is reflective to the accurate installment of a geometric design. Such a design needs to take various elements into consideration, including number of lanes, lane width, median type, length of acceleration and deceleration lanes, curve radii, and many more. The detailed work of design has made lifelong careers for many engineers in the past. Today, despite advances in computer software, the basic fundamental understanding of building a highway still needs to be understood to guarantee that intuitive roads continue to be built in the future.
Highway design itself is made up of a spectrum of considerations. Some of the main points are discussed below. Details can be found in their respective sections.
Sight Distance is the distance a driver can see from his or her vehicle. This becomes important when determining design speed, as it would be unsafe to allow a driver to drive faster and not be able to stop in time for a potential, unforeseen hazard. Sight distance is applied to two main categories:
- Stopping Sight Distance (SSD)
- Passing Sight Distance (PSD)
Grade is the slope, either upward or downward, of a road. This is especially critical because the terrain on which a road is built is seldom flat, thus requiring inherent "hills" to be present on the road to keep costs down from digging canyons or tunnels. However, too steep of a grade would make it difficult for vehicles to travel along that route. Since a road is built to provide a service to travelers, it is undesirable to have an impassable road.
An appropriate grade is dependent on the engine power in the vehicles that are expected to use the road. This breaks down into a force balance equation, where engine power is countered by various resistances. These include:
- Aerodynamic Resistance
- Rolling Resistance
- Grade Resistance
Horizontal Curves are semicircular curves designed on the horizontal plane to allow roads to weave around obstacles, such as towns, mountains, or lakes. They allow a smooth transition to occur between two nonparallel roads instead of a sharp, pointed turn. When designing horizontal curves, designers must consider the intended design speed, as centripetal force is required to ensure that vehicles can successfully negotiate the curve. Such elements to consider for design include:
- Curve Radius
Vertical Curves are placed to allow the road to follow the terrain, whether it be hills or valleys. Vertical curves are primarily designed as parabolas, using the general form of the parabolic equation with coefficients corresponding to known elements for the road in question. Designers can design a road by adjusting these elements and minimizing costs. These elements include:
- Inbound and Outbound Grades
- Curve Length
- Rate of curvature
Roadway cross sections are important in design primarily for drainage. If roads were flat and level, water would congregate on the surface, reduce the coefficient of friction, and become a safety concern. Therefore, designers implement a "crown" cross section, which has a high elevation along the road's centerline and then tapers off as it leads to the shoulders. This slope is generally very small and unnoticeable by drivers, but succeeds in allowing the water to run into the ditches or storm sewers.
Earthwork plays into highway design as primarily a budgetary concern. The movement of earth, regardless if it is to a site, away from a site, or around a site, is an expensive venture. Designers often make roadway designs with a minimal amount of necessary earthwork to keep project costs down. However, knowing how to minimize dirt movement requires an understanding of the process.
Pavement design generally is not considered part of geometric design, but is still an important part of the design process. In order to facilitate efficient traffic flow, the road beneath the vehicles' tires needs to sturdy, stable, and smooth. Pavement engineers are responsible for determining appropriate pavement depths necessary to allow traffic to pass on a certain road. Inadequate engineering would result in cracking, formation of potholes, and total degradation of the roadway surface.
The calculation and design process of roadway geometry design are often cumbersome and time consuming. The University of Minnesota has an online roadway geometry design tool that was created to assist transportation students in this process. This tool, titled Roadway Online Application for Design (ROAD), allows students to design a roadway on a computer screen using a contour map in the background as reference. This tool makes the design process more efficient and effective over the traditional paper and pencil method. It can be found on the link listed below, is free to use, and covers the topics discussed in the Design section. It can be found at the STREET website.