They can be used as access point for transfering data between the self-driving car, they can get information from traffic accident area, they can monitor the traffic jum.
Article UAV-Enabled Intelligent Transportation Systems for the Smart...
To me, they can be used in quite large unexpected event. It could be an accident or a delay for a train or an air-fligt. they could be used for the monitoring of the availability of taxis, for a large company. In case of bus dereglation, they can be used by a bus company. They can be used for the real time management of the urban road pricing, accoding to the objectives of the public authority.
Drones recently and now using all of them life. In ITS drones which are check the traffic and roads. ıf there are trafik jam or traffic accident drones can help to show alternative roads...