The planning of cooperation and the development of understanding with the offsite services is vital to the success of response to emergency. The first step is obviously establishment of agreements to cooperate in emergencies. These agreements are established mainly by the senior managers of the organizations involved. But this needs to be followed up by active planning and drills by plant personnel.
The activities should be seen not as two disjoint activities, but as a single operation in which a growing weight of resources is brought to bear on the incident. The planning should aim to clarify not only what is to be done but also who is to do it.
Cooperation benefits greatly if there is a full-time liaison officer, and this can be justified in an area where there is large potential for emergencies. What is known in advance is the general location and nature of possible emergencies. Other factors will not be so well defined: time, weather and number of people. Excessive detail in the planning should be avoided. The aim should be to plan broad areas of responsibility, chains of command and systems of communication.
Off-site services will require their own communications. On-site, the practice has been for the police, fire and medical services to be linked by radio to their own communications systems. The EOC should be available to them. Alternatively, they may wish to set up their own mobile control centers.
The off-site services will also normally need to tap into the on-site emergency internal communication. This should be allowed through the EOC. An alternative is to provide for their use portable radios as used by the emergency teams.
Emergency drills are effective in familiarizing personnel with their functions. While drills with on-site personnel can be dictated by a routine, real-time simulation that involves all forces, on-and off-site are much more complicated to perform. However, the effectiveness of the plan, as well as the performance of each of the entities in the response stage can be assessed only in full scale drills. It is common that full-scale emergency drill resulted in major changes in the plan, and sometimes it required conducting process hazard analysis in order to better understand the hazards, and the risks associated. In any case, the emergency plan should consist of a procedure for implementation of change of management.

Reference: Occupational Safety and Health Standard. Process Safety Management of Highly Hazardous Chemicals. Alaska Department of Labor and Workforce Development.
Management/Emergency Planing and Response/Emergency Response