GPS systems typically convey their information in one of three ways - via 900 MHz FHSS, via 2.4 GHz FHSS, or by way of cellular modem. Each of these methodologies has a different set of advantages/disadvantages. 900 MHz FHSS appears to be the best option, because it is capable of the greatest range (often over 3 miles for a 1 watt transceiver). 2.4 GHz is able to communicate more data, but is typically considered more "directional" or "line-of-sight". It only has a maximum range of about 3/4 mile. This can often preclude the system from preempting soon enough to ensure a clear intersection upon arrival. Cellular overcomes the "distance" issue, but can be quite costly when cellular fees are taken into consideration. During an area-wide emergency, it is also well known to those in the industry that the cellular network often will go down. This can make preemption difficult (unless there are other backup systems in place) during a time (of crisis) when preemption may be needed most. Cellular also brings with it a certain amount of "latency". It has been documented that it can sometimes take cellular based preemption systems 10 seconds or more to release the preemption of a traffic signal, even though the emergency vehicle has already cleared the intersection.