Slender cylindrical structural members have often been observed to vibrate at large amplitudes under wind excitation. This paper describes a series of experiments conducted to understand the wind-induced vibration of the tapered arms of traffic signal support structures. Vibration and wind measurement transducers were used to monitor the oscillation of a number of such structures with circular and multi-sided mast-arm cross-sections and the corresponding wind conditions. The characteristics of the vibration were correlated with the wind characteristics to reveal the mechanisms of the vibration. To complement the data obtained in the full-scale investigation, wind tunnel tests were conducted to study the aerodynamics of sectional models of tapered circular and multi-sided cylinders with and without a model of traffic signal light cluster attached. The outcome of these experimental studies suggests that the problematic vibrations are due to vortex-shedding at low to moderate wind speed and that at high wind speed, the structures are susceptible to buffeting.