The research is targeting at multi-level overpasses which offer one of the solutions for laying the highway in conditions of dense residential development and close proximity to bedroom communities. In terms of acoustics, multi-level overpasses are semi-enclosed structures made of metal and reinforced concrete, where a complex sound field with numerous re-reflections is formed. There is no method of calculating noise from such structures in the literature of our country, therefore forecasting sound levels in the adjacent residential areas while designing noise mitigation measures is extremely challenging. The article proposes the method of calculating the efficiency of the noise barriers installed on the overpass edge based on the statistical theory of acoustics. The main assumption of the proposed model is the allocation of semi-closed volume in an acoustically untreated overpass and generation of a diffuse sound field in it, which is fundamentally different from the currently used methods for calculating the efficiency of the barriers installed in galleries as the barriers located in the open space, similar to motorways on the earth roadbed. When considering the diffuse field, the sound is not diffracted over the upper free edge of the barrier, but radiated outwards through the opening. According to the calculation results using the derived formulas, the efficiency of the barrier installed on the overpass does not exceed 2–6 dBA at its different heights, which corresponds to experimental results.