One informal public transport service in Bangkok is Silor (SR), given the meaning in Thai as four-wheeler.� SR facilitates urban mobility both in terms of major travel mode and feeder bus and mass transit lines in the city.� This research aims to investigate service characteristics and challenges of SR service, identify factors affecting the use and non-use of SR, explore travel behavior and attitudes to determine service delivery gaps in order to propose policy recommendations for the better functioning of SR service.� Interviews are conducted through questionnaire survey to collect data from supply side, including drivers and regulators, and demand sides, which involves both users and non-users.� Five SR routes are selected for study namely, Bangbon-Taladplu, Siriraj-Taladplu, Charansanitwong-Klongsan, Vibhavadi-Rachadapisek, and Sukhumvit Soi 39.� Findings from supply side reveal challenges in terms of work hour, working condition, registration of�vehicles and drivers, experiences when called by polices as well as competitions among transport modes.� For demand side, reasons for using SR are convenience, accessibility, and cheap fare whereas difficulties for non-users seem to associate with connection, transfer and crowdedness.� Importance-performance analysis are performed with users' attitudes on service quality and found that reliability, connection and transfer, seat availability and fare are the aspects of high importance and highly satisfied while safety and security seem to be of high importance but low satisfaction.� In addition, exploratory factor analysis and logistic regression model illustrate the four service factors that significantly affect the overall satisfaction of users, including reliability, in-vehicle environment, comfort and convenience, and environmental impact.� Further, the study applies user segmentation through cluster analysis based on the obtained factors which results in four user subgroups having diverse profiles and expectations.� Moreover, travel behavior and perceptions of Thai and Japanese SR users are comparatively analyzed and considerable variations among them are found.� For sustainability aspects, travelling by SR is more desirable than alternative modes in terms of less commute time, more affordable, less energy consumption and emissions; however, there still need more attention on provision of affordable, reliable, comfort and convenient services to users of all socioeconomic groups.� Most feasible options for future policy suggest transport authorities and relevant sectors in formalizing and integrating SR into urban transportation network in order to provide society with efficient alternative mode for a sustainable means of travelling.