The National Cooperative Highway Research Program (NCHRP) Project 17-18(8), Comprehensive Human Factors Guidelines for Road Systems, concerned the initial development of a new resource document for highway designers, traffic engineers, and other practitioners. The purpose of the planned Human Factors Guidelines (HFG) document, as stated in the project Statement of Work, is "to provide the best factual information and insight on road users' characteristics, in a useful CD-ROM format, to facilitate safe roadway design and operational decisions." The impetus behind this project was the recognition that current design references have limitations in providing the practitioner with adequate guidance for incorporating road user needs and capabilities when dealing with design and operational issues. These limitations may be of various sorts. Design guidelines may represent minimum requirements that are not always appropriate over the full range of roadway users or applications. Guidance may not be based on adequate human factors data. Guidance documents may not offer sufficient explanation so that practitioners can make effective use of behavioral factors. Conflicting requirements or unusual conditions may make it difficult to comply with ideal design parameters and require some basis for a compromise. Design practice may be driven by concerns about cost and compliance, without a basis for also incorporating safety benefits through user-centered design. Because of such limitations to current design guides, it would be beneficial to provide human factors guidelines to assist the practitioner in identifying and addressing human-centered safety concerns in roadway design and operations. The HFG meets this need. The HFG is seen as complement to other primary design guides, such as the AASHTO Geometric Design Guide, 2001 and the Manual on Uniform Traffic Control Devices (MUTCD) 2003.