differentiate between a fire engine and a bus sending a signal simultaneously, and then grant priority access first to the fire engine. Drawbacks of line-of-sight systems include obstructions, lighting and environmental conditions, and undesired activations. Obstructions may be buildings on a curving road that block visual contact with a traffic signal until very close, or perhaps a large freight truck. In the case of a police car, such a blockage would serve to prevent the traffic signal from receiving the police car's emitter signal. Modifying the position of the receiver or even locating it separate from the traffic signal equipment can sometimes correct this problem. Direct sunlight into a receiver may prevent it from detecting an emitter, and severe environmental conditions, such as heavy rain or snow, may reduce the distance at which a line-of-sight system will function. Undesired activations may occur if an emitter's signal is picked up by many traffic lights along a stretch of road, all directed to change to red in that direction, prior to the activating vehicle turning off the road, or being parked without its emitter being deactivated. Bus signal allow buses to turn go green lights to get to destinations faster. Line of sight emitters can use IR diodes. They are pulsed with a low-priority signal (10 Hz) or a high-priority signal (14 Hz). Localized radio signal Radio-based traffic-preemption systems using a local, short-range radio signal in the 900MHz band, can usually avoid the weaknesses of line-of-sight systems (2.4 GHz and optical). A radio-based system still uses a directional signal transmitted from an emitter, but being radio-based, its signal is not blocked by visual obstructions, lighting or weather conditions. Until recently, the major drawback of radio-based traffic signal preemption systems was the possibility of interference from other devices that may be using the same frequency at a given time and location. The advent of FHSS (Frequency Hopping Spread Spectrum) broadcasting has allowed radio-based systems to not only overcome this limitation, but also the aforementioned limitations associated with acoustic and line of sight (optical) systems. It was not until recently that cost effective GPS preemption systems were introduced, supplanting FHSS radio-based preemption as the preemption method of choice, particularly for cities that had experienced the myriad of issues associated with other (acoustic and optical) preemption systems. Radio-based systems also began to offer some additional benefits — adjustable range and collision avoidance. The operating range