Cantilevered signal, sign, and light support structures are used nationwide on major Interstate highways, national highways, local highways, and at local intersections for traffic control. Recently, a number of failures of these structures have been characterized as wind-induced fatigue failures. It is widely accepted that there is considerable lack of accuracy in the calculation of wind-induced loads on high mast light poles (HMLPs) in both the AASHTO and the "Canadian Highway Bridge Design Code" provisions. A coupled model for predicting buffeting- and vortex shedding-induced response for slender support structures was developed. To accomplish this, monitoring of long-term response behavior of an HMLP subjected to wind-induced vibration and wind tunnel experiments was used to study global behavior and to extract important parameters. From the long-term field monitoring and wind tunnel experiments, the two critical types of wind vibration (natural wind gusts or buffeting and vortex shedding) were individually identified for in-depth analysis. Finally, a coupled dynamic model in time domain was developed for predicting the wind-excited response and was validated by comparing the simulation results with the field-collected data. The fatigue life of a specific HMLP was also estimated with the stress amplitudes predicted by the time-domain model and was validated with statistical extrapolation of the field data.