Rutting continues to be a major distress in flexible pavements that demands attention in design, maintenance, and preservation. Identifying the probable causes of such distresses is crucial for designing new pavements and for the maintenance and rehabilitation of existing ones. This study aimed to identify the causes of premature rutting through field investigations and laboratory testing. Specifically, both non-destructive and destructive field tests were conducted on a 4.5-mile-long section of flexible pavement along Oklahoma State Highway 7. Field tests, including Ground Penetrating Radar (GPR), Falling Weight Deflectometer (FWD), Dynamic Cone Penetration (DCP), and Face Dipstick, were performed to evaluate the structural health of the pavement. Additionally, soil samples and asphalt cores were collected for laboratory testing. In laboratory testing, asphalt cores were subjected to Hamburg Wheel Tracking (HWT) and roadway density tests. Soil samples were used to determine Atterberg limits and moisture content. The combined results from field and laboratory tests were analyzed to identify the probable causes of premature rutting.