The paper proposes a calculation method for determining the optimal size of the designed noise barriers, which allows quantifying the level of motor transport noise in the area protected by the barrier. The proposed calculation method allows estimating the sound levels diffracting on the upper and lateral edges of the barrier, as well as reflected from the road surface, from the opposing barrier, if any, and from both barriers. Thus, in case the noise level exceeds the standard values, the designer can determine the source of the greatest noise: from the top of the barrier or from the sides of the barrier. Therefore, it is possible to vary the size of the barrier, selecting through calculations their optimal value in terms of the required noise level in the protected area. It allows to accelerate the design process and to reduce economic expenses for the construction of motor transport noise barriers.