Maritime traffic safety and capacity raise enormous attention nowadays, especially in ports and inland waterways with high traffic density and restricted waterway geometry. However, a limited number of factors is considered in existing maritime models. Especially influences of external factors (wind, visibility and current), vessel encounters and human factors on vessel behavior (speed, course and lateral position) have not been investigated. In order to provide insight into vessel behavior for the development of a new maritime traffic model, possible impacts from current and vessel encounters are investigated using Automatic Identification System (AIS) data. In this paper, equidistant cross sections approximately perpendicular to the navigation direction are used to extract vessel speed, course and position from AIS data. These vessel behavior data together with corresponding data on external conditions, including wind, visibility and current from the Port of Rotterdam, form the basic data set of this analysis. Vessel behavior under weak external influences is defined as unhindered (or reference) behavior. Vessel behavior under strong current is compared to unhindered behavior to investigate the influence of current. In addition, encounters including overtaking and head-on, are investigated to study the influence of other vessels. Statistical analysis is carried out to investigate changing of vessel speed and lateral position during encounters. Analysis results show that current only has impact on vessel speed. Both vessels involved in overtaking change their speeds and deviate in lateral direction to avoid collision. However, vessels only laterally deviate without speed changing in head-on cases.