Problem. The article refers to the problem of the concentration of traffic flows in the central business part of a large city. The transport problem in the city center can be solved by the creation of transport ring roads. To do this, it is necessary to know the capacity of the sections of the ring road, and for this purpose it is necessary to calculate traffic flows along the ring. To do this, we use a gravitational model for calculating the origin-destination trip matrix. This matrix is needed for future traffic assignment on the constructed transport ring. There are several types of transport gravity functions proposed by the different authors, and we need to choose the right one or create a new type of the function to solve the current task. Goal. The goal is to suggest a criterion for choosing the optimal form of transport gravity function in a given problem. Methodology. We use the gravity model of transport trip distribution along the road network and the Shatsky-Sheleyhovsky cal-culation procedure. The proposed criteria for choosing the proper gravity function is the sum of absolute values of the differences between the sum of origin-destination trip values for certain destination mega-region and the given arriving capacity of this mega-region. Results. A criterion for choosing the optimal form of transport gravity function is developed. Several forms of gravity function were considered and estimated by the proposed criteria. The best type of the gravity function for the given task is the quadratic function. With increasing travel distance, the motivation for using individual transport also increases. Originality. It is shown that multiplying the function of transport gravitation by a constant (except zero) does not affect the calculation of the matrix. Practical value. The obtained results are used in the developed method of calculating the required capacity of the sections of the transport ring, which can be constructed in a large city to reduce the traffic load on the central part of the city. This is explained by the fact that for those who drive their own cars and make trips in the morning and in the evening, the travel factor is not the most defining factor in choosing the job, as opposed to those who use the public transport.