This report presents the findings derived from a program of research investigating the nature of errors made by drivers, involving a literature review and an on-road study. The review indicates that, despite significant investigation, the role of different error types in road traffic crashes remains unclear, as does the role of the wider road transport system failures in driver error causation. Further, the data systems required to generate this knowledge currently do not exist. A novel framework of methods was used during the on-road study to examine the errors made by 25 participants whilst driving a pre-determined urban route. A total of 298 errors were made, with speeding violations, both intentional and unintentional, being the most common, and over half of the errors made occurring at intersections. More detailed analysis of a sub-set of specific error types indicates that driver errors have various causes, including contributions from the wider road 'system' such as unclear road rules and regulations and poor infrastructure and roadway design. While previous research has suggested that failures across the wider road transport system may influence driver behaviour, the novel approach used here has shown how these factors contribute to driver error. It is therefore concluded that the entire road 'system' has a key role to play in the prevention of driver error. Having demonstrated that this approach does produce valuable in-depth information from real-world driving conditions, the next steps are to apply it to study more specific elements, such as the influence of infrastructure design on safety.