A definition of interoperability when applied to ITS systems is (World Road Association 2003):
The ability of ITS systems to provide services to and accept services from other systems and to use the ITS services so exchanged to enable them to operate effectively together. ITS systems are interoperable when the ITS services are seamlessly provided in time and space.
ITS interoperability is particularly relevant to the road user and the road network operator. In Australia the arterial road network is usually operated by state or territory road agencies, although some tolled elements may be operated by private sector operators. Other levels of the road hierarchy are usually operated by local government bodies. In New Zealand the state highway network is operated by the national road agency, while the remainder of the network is operated by the road controlling authority, usually the local authority. In both cases operational responsibility is divided across different road classes and interoperability is therefore a potential issue.
For a service to be interoperable three levels of interoperability must be addressed:
- Technical interoperability is the capability of the technical subsystems to communicate with each other by using standardised interfaces and communication protocols. Typical issues are the physical layers and data layers for radio transmission.
- Procedural interoperability is achieved when common procedures are used by all involved road network operators and by the users. Typical issues are harmonised data dictionaries or common human machine interfaces (HMI).
- Contractual interoperability requires agreements between network operators about issues such as operational procedures, service levels, financial transactions, data security, and enforcement.
Where is interoperability needed?
Interoperability is a particular issue if a system is composed of both fixed and mobile subsystems. For example, on-board units in cars that travel across borders or between jurisdictions must be able to communicate with roadside equipment at different geographic locations. Interoperability is also an issue where the impacts of traffic operations cross the borders of the areas of responsibility of adjacent operators and they need to exchange data, information or control. The priority areas for interoperability are:
- Traffic management and control: Cross-jurisdictional traffic management requires the exchange of traffic information among network operators and harmonised procedures for network management (e.g. where do adjacent network operators want to concentrate traffic flow, how to manage major incidents that impact on adjacent network operators).
- Traffic and traveller information (TTI): Data originating from many different sources (roadside traffic sensors, traffic police data, user calls, traffic management centres) that are disseminated to the users by means of different systems (roadside VMS, radio, internet, on‑board navigation equipment) must be harmonised in order to avoid providing conflicting information to drivers. For example, there should be convenient means available to the drivers to acquire digital road maps in order to ensure seamless provision of TTI services across borders.
- Electronic toll collection (ETC): Common ETC payment for cross-concession travel requires on-board units that are able to communicate with roadside beacons at toll stations and requires agreements between the toll operators about clearing procedures.
- Incident and emergency handling: In emergencies, travellers should be able to call services with their own equipment (mobile phone, on-board emergency system) no matter where they travel, and the emergency services should be able to find the relevant information about the vehicle, the persons and freight carried regardless of the origin of a vehicle. While mobile phone networks and on-board emergency systems are not the responsibility of a road agency, traffic management centres operated by road agencies must be able to accept information flowing from the systems of those who are responsible.
Costs and benefits of interoperability
The disadvantages associated with interoperability are:
- a certain loss of autonomy of the network operator combined with what can be time‑consuming procedures for negotiating the procedural and contractual issues
- in some cases, more expensive equipment due to additional functional requirements (e.g. multi-standard roadside equipment or multi-standard on-board units)
- cost of migrating from non-interoperable systems to interoperable systems (e.g. renewal of existing systems that do not comply with new standards).
The benefits of interoperability are:
- more effective management of cross-border traffic operations issues, e.g. incident management
- additional comfort for travellers that can use their ‘home’ equipment and means of payment when travelling in other jurisdictions
- savings to car owners because of avoidance of having to carry more than one on-board unit to carry out the same function
- more competitive bids due to a larger common market when network operators are calling for equipment; multi-sourcing instead of mono-sourcing, e.g. traffic signal controller equipment.
A particular issue for all interoperable systems is non-equipped users, i.e. vehicles that should use a service but do not carry the proper interoperable equipment on board. If the network operators are to ensure they do not discriminate against non-equipped users (such as interstate users of a system implemented in only one state or territory), then solutions must be offered to ensure that a manual procedure is offered for the same function.
How to achieve interoperability?
There are three institutional layers involved in interoperability.
- Governmental and intergovernmental layer: Harmonisation of the road traffic regulations and in particular the technical requirements for vehicles and on-board equipment including harmonisation of driver education with respect to the human machine interface (HMI).
- Architecture and standardisation: The key to interoperability is standardisation, preferably within a common architectural framework. Only when interfaces are standardised can the different subsystems inter-operate to carry out a particular function. Standards must include test procedures so that equipment can be certified by the operators for inter-operable use. For network operators the following standardisation body is of particular importance: Technical Committee TC 204, Intelligent Transport Systems, of the International Organization for Standardization (ISO).
- Business to business agreements: The vehicle manufacturers and the electronics industry have been working together for a long time towards the development of inter-operable systems. However, there are business cases where a strong commercial interest exists for excluding competitors from entering an established system. Road agencies have little role other than encouragement in this layer.