Our streets will be soon populated by multitudes of self- driving vehicles, calling for appropriate solutions to coordinate their collective movements in order to ensure safety and efficiency. In this paper, after introducing the general issues associated to coordination of self- driving vehicles, we show that a key engineering issue is identifying the most suitable degree of autonomy in decision making that should be left to vehicles during the coordination process. This issue also includes the possibility, depending on factors such as traffic conditions or the need to enact specific mobility policies, to dynamically adjust such degree of autonomy and thus the adopted coordination scheme. As the paper analyzes with reference to the specific example of intersection crossing, this introduces many theoretical and practical challenges in modeling self- driving vehicles coordination schemes and in their rigorous engineering.