Dashcams support continuous recording of external views that provide evidence in case of unexpected traffic-related accidents and incidents. Recently, sharing of dashcam videos has gained significant traction for accident investigation and entertainment purposes. Furthermore, there is a growing awareness that dashcam video sharing will greatly extend urban surveillance. Our work aims to identify the major motives and concerns behind the sharing of dashcam videos for urban surveillance. We conducted two survey studies (n=108, n=373) in Korea. Our results show that reciprocal altruism/social justice and monetary reward were the key motives and that participants were strongly motivated by reciprocal altruism and social justice. Our studies have also identified major privacy concerns and found that groups with greater privacy concerns had lower reciprocal altruism and justice motive, but had higher monetary motive. Our main findings have significant implications on the design of dashcam video-sharing services.