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. See also Bus priority Bus rapid transit creep Mobile Infrared Transmitter References External links "U.S. Fire Administration, Emergency Vehicle Safety Initiative, FA-336/February 2014" Guide for Traffic Signal Preemption near Railroad Grade Crossing (PDF) (from the Texas Transportation Institute) Section 4D.13 of the U.S. Manual on Uniform Traffic Control Devices University of Minnesota Intelligent Transportation Systems Institute report on Dynamic route clearance "Hackers target traffic lights" at TMCnet Emergency vehicles Intelligent transportation systems Traffic signals