The paper presents an introduction to the method enabling the estimation of the range of investments necessary for the realisation of the mobility policy understood as the correction of the modal split into the sustainable proportion between car and non-car journeys. The models allow the calculation of the number of travellers required to shift into the public transport mode and the scale of selected investments including the development of the transport network, interchanges, rolling stock, and technical infrastructure. The basis of such calculations is the results of traffic surveys. The worldwide context of the study and similar actions are also presented. The paper consists of five sections. The first section contains a review of current problems connected with the sustainable mobility policy and the role of modal split. The second section focuses on the case study with the presentation of the local mobility policy and selected results of complex traffic surveys. The models used to estimate the investment challenges with exemplary calculations and presentation of similar effects of the intervention are described in the next section (3). Section four contains a discussion on the described methodology. The conclusions in section five end the main part of the paper.