The economy of any country highly depends upon efficient and reliable transportation system. Travel time reliability is a measure of efficiency of a road network system and this is a new evaluation of policy assessment measure of any road network service. The aspects of reliability are useful to the system users and system planners. This book, therefore first time reviewed various modeling approaches to measure the travel time reliability of road transportation system. Basic definitions, concepts, importance and use of travel time reliability measurements have been discussed. Path travel time estimation methods for supersonic traffic detectors data were explained and reliability analysis was carried out for various radial routes of urban expressway. Modeled travel time distribution under various uncertainties by considering probabilistic models. Variation of travel time was modeled under uncertainties from supply side, demand side and external factors of transportation system. Travel time reliability analysis should help in transportation system planning, and should be especially useful to the researchers, students and professional in the field of transportation engineering.