This paper discusses a naturalistic driving study on the usage of a mobile phone and navigation system while driving. Manual interactions with these devices while driving can cause distraction from the driving task and reduce traffic safety. In this study 21 subjects were observed for five weeks. Their behaviour was logged by four cameras, a GPS sensor and a number of additional sensors. Results are presented on the durations and frequencies of manual interactions with the mobile phone and navigation system. Different manual subtasks with different levels of visual interaction are discussed. The results show that participants spent on average 1% of their driving time interacting with the navigation system and 4% of their driving time interacting with the mobile phone. For the mobile phone 48% of the interactions took longer than 15 seconds, for the navigation system this was 40%. The results are discussed in relation to traffic safety and recommendations for future research.