Large trucks, particularly those with over-cab air deflectors, produce dynamic wind pressures on overhead structures. Oscillations of overhead variable-message sign structures have been observed as a result of these truck-induced wind pressures. The purpose of this research was to determine the magnitude, direction, and frequency of these pressure distributions. The choice of instrumentation used, the design of the apparatus, and the data obtained are discussed. A multiprobe device consisting of Pitot tubes and variable-reluctance pressure transducers was combined with a data acquisition system. Field readings were taken by mounting the apparatus on a bridge overpass and collecting data as trucks passed beneath the bridge. It was found that the wind pressures produced by the trucks were not as large as anticipated, but the frequency of the wind was close to the natural frequency of a typical variable-message sign structure. This creates a resonance condition believed to be a major cause of the oscillations. One possible solution is to increase the stiffness of the structure.