This paper presents an empirical analysis on lane changing behaviour using a trajectory data set collected at a weaving section in Grenoble (France). A detailed literature review shows that a strong empirical understanding of the weaving mechanisms is still lacking. This paper is devoted to fill this gap by investigating the lane changing behaviour at a microscopic level. We distinguished lane changes from the main road and those towards the main road. We conducted a descriptive empirical analysis of the weaving lane changes examining the positions of the lane changes and the accepted gaps. Our results show that under heavy congested traffic conditions the weaving vehicles tend to change lane as soon as possible after the start of the weaving section. When the traffic conditions are fluid, the lane changing positions are more spread along the first part of the weaving section. The weaving vehicles coming from the main road tend to change lane earlier than the weaving vehicles coming from the auxiliary lane. As the weaving vehicles change lane at the beginning of the studied weaving section, our findings ask whether the length of the weaving section is a key variable to estimate its capacity. Our findings raise also some questions about the more relevant micro-simulation models to reproduce the operation of weaving sections.