In road traffic, speed-flow curves describe the relationship between vehicle flow rates and average vehicle speeds. These functions are one of the basic elements of transport modelling and they are widely used in project appraisal. Review and re-validation of speed-flow relationship has become timely in recent years as vehicle fleet changed, average traffic volumes increased and more comprehensive, automatic data collection became available. This study used the database of the road operator in Budapest to simultaneously analyse travel times and traffic volumes. The paper aims to examine the validity of the well-known fundamental diagram for urban roads, to provide an estimation for the speed-flow curve based on the Budapest case and to assess the on-set process of congestion.

This paper intends to show that despite limited data availability it is still possible to elaborate semi-sophisticated LUTI models which can be a stepping stone for countries that are less developed in terms of transport modelling practice but eager to improve. It provides an outline of the model and of the calibrating process which was based on data from the city of Budapest. Based on the results it is undeniable that excluding land-use effects of transport in modelling could cause a serious distortion even in a shorter time period. It seems that such land-use effects and feedbacks can no longer be disregarded as it is not in accordance with the desire of improving transport modelling practice. From this aspect, the proposed approach is practical and can overcome general obstacles of time, cost and data availability issues. The next step should be to carry out tests for the estimation of real transport investments and compare the results with other models.