Design controls and criteria are the basic elements of roadway design. The two main elements are design speed and level of service, followed by design vehicle and its performance, type of users, traffic volumes and mix of type of vehicles.
Refer to Chapter II of the AASHTO Green Book In addition to functional classification, there are a number of design controls that affect the geometry of a highway. (Rt. 71, IL)
In order to design the basic elements of a highway-including its alinement and cross section-the designer must have an understanding of the basic design controls and criteria associated with the highway. One of the most important, highway functional classification, was discussed in Chapter 3. Other important design controls include, but are not limited to the following:
- The design speed of the facility
- The acceptable degree of congestion (i.e., the design-year peak-hour level of service) on the facility
- The physical characteristics of the ﾓdesign vehicleﾔ (i.e., the largest vehicle that is likely to use the facility with considerable frequency); in virtually all instances, the highway design vehicle is an over-the-road tractor-trailer
- The performance of the design vehicle (particularly important in terms of
accommodating heavy trucks in mountainous terrain or buses and recreational
vehicles in areas subject to high levels of tourist activity)
- The capabilities of the typical driver along the facility (i.e., local residents using lowspeed neighborhood streets versus interstate travelers on rural freeways)
- The existing and design-year traffic demands to be placed on the facility (e.g., daily and peak-hour traffic volumes, the mix of passenger cars and trucks on the facility)
Two of the most important of these factors are design speed and peak-hour level of service. This chapter will focus on these two important criteria. The peak-hour level of service, however, only serves as a controlling factor for a small number of highways. For most highways, after the functional classification and associated design speed for a particular highway facility have been established, the degree of flexibility available to the designer is
significantly limited. Speed limits are not the same as the design speed. They are regulatory limits enforceable by law.