Fiber-Reinforced Polymers (FRP) are used as construction material for footbridges because of their high strength-to-mass ratio, resulting in a low mass compared to steel and concrete structures. The lightweight and slender nature, however, makes these footbridges susceptible to human-induced vibrations. Consequently, vibration serviceability is often the most determining design criterion, requiring an accurate prediction of acceleration levels in the design stage. Current design guidelines only consider the forces as induced by crowds on a rigid surface, neglecting the fact that the human body is a mechanical system interacting with the footbridge, known as Human-Structure Interactions (HSI). In this work, the impact of human-structure interactions is quantified for an all-FRP footbridge in Puurs, Belgium, occupied by 31 pedestrians (0.44 pers./m2). The results show that, for this specific experiment, neglecting HSI overestimates the response by more than a factor of two.