But this can quickly lead to chaos, especiallly on busy days or when the dispatchers do not (or are not allowed) to tell units that they are not needed or can remain in service. A related issue is when two units claim to be "closer". It is the dispatcher's responsibility to be judge and jury in these instances- with no chance of appeal.
One important action that dispatchers can take, especially when units have added themselves is to cancel units that are farther away (this is where the CAD map plays an important role, along with situational awareness). Note, if units become available that are closer then by all means they should be sent- but as replacements, not additions.
Many assume units self dispatching or jumping calls to be victimless crimes. Nothing could be farther from the truth. Response times for the next incident in the area can be dramatically impacted if additional units are responding to a call where they are not required. In a job where seconds count, the added response time when a third or fourth due unit has to respond can have serious consequences.