We report on the comprehensive experimental characterization of optical crosstalk between waveguides caused by scattering. Our results reveal that a strong power exchange between close-placed waveguides due to sidewall roughness exists also for high-quality, low-loss waveguides. We derive a power-law dependence of the coupling on the distance between the waveguides, confirmed by an ad hoc developed electromagnetic model. Further, we demonstrate higher order mode excitation caused by scattered light and the appearance of decorrelation between the guided modes propagating in waveguides coupled via radiative mechanism, providing a full description of this phenomenon.