Crowdsourcing continues to attract attention from researchers, organizations, and policy makers alike. In particular, crowdsourcing projects that engage intrinsically motivated volunteers and are aimed at endeavors such as policy making, research, and social activism, need to understand how to create sustained engagement in their initiatives. A fair amount of research has been done on identifying users' motivation in crowdsourcing. However, crowdsourcing motivation literature still lacks sufficient theory-driven approaches. In this paper, we derive from theory of work motivation, the technology acceptance model, and the gamification concept to propose a model that can explain participants' motivation in crowdsourcing. To measure our model constructs, we conducted a survey on the users of a European crowdsourcing project, researching truck traffic tracking. The results of the exploratory factor analysis confirm that our constructs can be measured properly using our questionnaire. At the end of this paper, we explain our findings and the contribution of our study.