Unmanned Aerial Vehicles (UAVs) are becoming increasingly popular and the amount of UAV traffic in urban environments will largely increase in the future, due to profitable tasks which are particularly suited to UAVs, e.g., parcel delivery and surveillance, in particular in the context of smart cities. Trying to ensure the traffic safety and efficiency by acting on the UAV controller alone might be challenging, since the set of involved players (regulators, manufacturers, business users) is large and diversified. In this work, we address this problem by proposing a language for defining rules suitable for UAV traffic which can be enforced in a decentralized way by the UAVs themselves, without any need for communication and regardless of the UAV navigation algorithm. The language allows to express realistic rules, such as ``when cruising, keep a minimum altitude'', concisely and such that they can be processed online by each single UAV basing on its perception of the nearby environment. We experimentally validate the ability of our proposal to impact on the UAV traffic efficiency and safety by performing a large number of simulations with and without a set of realistic rules.
