A Powered Two-Wheeler (PTW) is defined as a vehicle running on two wheels powered with an engine. As a private transportation mode, PTWs have many advantages over private cars, but the safety issues are a main concern, and there are limited studies on the driving behaviour of PTWs in the literature. The aim of this study is to investigate the driving behaviours of mopeds and motorcycles in urban arterial streets, and their interactions with other vehicles under mixed traffic. Videos of traffic movements were collected and digitalized into vehicle trajectories with a trajectory extracting software. The lane changing behaviour of PTWs is modelled as a discrete choice model. A clustering approach is then proposed to categorize the PTW riders, based on their aggressiveness and level of risk perception under different scenarios of driving environment. The results should be useful for the development of behaviour models for different types of riders.