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This paper is based on ongoing research, within CiViTAS ELAN project (co-funded by the EU), about the implementation of public transport (PT) priority system in Zagreb. Prioritising the PT vehicles at signalized intersections has proven to be an effective way to increase the quality of service (QoS) for the PT users in terms of reducing travel times between PT stops. In this paper we elaborate the evaluation methodology applied for the purpose of detecting the impact of PT priority system on the PT QoS. We define a set of evaluation indicators which are used for the decomposition of PT vehicle operation time into different segments (running time, intersection delay, dwell time and driving time). The operation time decomposition enables the identification of different background data impacts (i.e. possibility of modal shift in favour of public transport) on the PT system performances. In addition, different data collection methods for the abovementioned evaluation indicators are described and evaluated: manual time recording, GPS (Global Positioning System) vehicle tracking and manual video data processing. Each method is set against several criteria and comparative analysis is carried out.