In 1985, the FHWA designated 13 specific design elements as controlling criteria for roadway design (see Mitigation Strategies for Design Exceptions). The 13 controlling criteria are (1) design speed, (2) lane width, (3) shoulder width, (4) bridge width, (5) structural capacity, (6) horizontal alignment, (7) vertical alignment, (8) grade, (9) stopping sight distance, (10) cross slope, (11) superelevation, (12) vertical clearance, and (13) horizontal clearance. Federally assisted highway construction and reconstruction projects must meet the established design criteria for these elements, or a formal design exception must be prepared and approved. Different procedures apply to rehabilitation projects, but these design elements are still key considerations in design. Since their designation, the 13 controlling criteria and their application have not been reconsidered as new knowledge has been gained about the relationships between geometric design elements and safety and operations.
In NCHRP Project 17-53, MRIGlobal and their subcontractors (Quincy Engineering and HQE, Inc.) investigated what is known about the safety and operational effects of the 13 controlling and other important geometric design criteria. Several small studies were done to augment the information found in the literature. This information was used to assess the sensitivity of safety and operations to design decisions for these criteria for different types of roads. The research also addressed how to reduce confusion related to the definitions of the controlling criteria.
The use of the controlling criteria in design exception processes was also explored, including through interviews with state department of transportation (DOT) personnel. It is expected that the report will be useful to state DOTs in reviewing their design exception policies for non-federal projects.
