This paper evaluates the impact of changes in selected urban conditions on individual behaviour in terms of mode choice, through the implementation of an urban mobility policy. A System Dynamics model simulates the dynamics of use of the private car and public transport, taking into consideration individual behaviour in terms of mode choice affected by cost and accessibility policy instruments. A simulation using real data from a Brazilian city tested the theoretical applicability of the model. The results highlight the challenge of attracting users to public transport in a private car-based society. Despite improvements in urban conditions and a consequent reduction of the need to own a car provided by the partial implementation of the two urban mobility policy instruments analysed are not sufficient to reverse the current modal share. The users' desire to own and use cars is a key point which needs to be addressed by more instruments to transform policy efforts into changes in user behaviour.