A service operating an SMR system must maintain a certain number of users for each licensed channel. This is called "loading". The service must show adequate loading before licenses for more channels can be granted. In SMR systems, many factors affect the quality of service. This may include the capacity of the radio system versus the number of radio units using the system. A talkative user community may cause busy signals while a disciplined, professional group of SMR users may keep channels idle. Some systems experience seasonal peaks. For example, a trunked system in an agricultural area used by crop harvesting crews may overload and experience busy signals during peaks in harvesting. A system used by a ski resort may overload on a holiday weekend. Disaster planning experts like to say that trunked system users are likely to hear busy signals for the first time when a disaster occurs. A major flood or earthquake is likely to generate a level of call traffic that loads the system to capacity. Siting of the system affects coverage: the system antennas should overlook the desired coverage area. Federal Communications Commission regulations require systems using the same radio frequency to be spaced 70 miles from one another or do an engineering study to confirm there will be no interference from closer spacing. Maintenance practices also can affect quality. In some systems, the operator may remove one channel of a trunked system from service to perform repairs. In a heavily used system, this could cause busy signals. Engineering documentation for these systems suggests another evaluation would be "delivered audio quality". This is a measurement of the sound quality over all of the involved pieces of communications equipment from end to end. The first clue this may be an issue is that radio users with normal hearing have trouble understanding others when speaking over the radio.