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Traffic engineers rely heavily upon the fundamental relationships between the variables speed, flow and capacity which characterize the traffic stream. Accurate knowledge of the operational behaviour of roadway traffic inevitably enhances the reliability of planning and design. Because of the importance of basic relationships to traffic engineering, this study was undertaken. The study involves investigating the important relations between speed, flow and capacity currently existent for uninterrupted flow on multilane highways in Ontario. The results are compared with information established by both the Highway Capacity Manual (HCM) and Polytechnic Institute of New York (PINY). Comparisons are made of 5-minute and 15-minute hourly flow rates, and calculated truck equivalents with those currently recommended. In addition, the impact of adverse weather conditions on fundamental speed-flow relationships and how occupancy (density) relates to speed and flow were investigated. Results of the study indicate that there is a significant difference between relations presently in use and those existing on roadway facilities. In particular, capacity was found to be in excess of 2000 passenger cars per hour and speeds were noted as being higher than established values. Master of Engineering (ME)