The level of service (LOS) concept given in the Highway Capacity Manual (HCM) has been used as a qualitative measure representing freeway operational conditions for over 40 years. One key element that is not adequately addressed, however, is how travelers perceive the quality of service. In this book, the author examines traveler perceptions of freeway traffic quality in terms of traveler characteristics and prevailing traffic conditions. Questionnaire surveys are designed and conducted to quantify traveler perceptions and the results are statistically analyzed and compared with the HCM specification. Findings suggest that the use of traffic density as a single performance measure for LOS as currently prescribed in the HCM does not reasonably reflect the perspectives of travelers. Several significant attributes in terms of traveler perceptions are identified through a modeling approach. This book will be of interest not only to traffic engineers and practitioners but also to those who wonder how traffic conditions are evaluated and interacted with perceptions from travelers.