Edited sentence: A more prudent approach seems to be notification directly back to the driver of the emergency vehicle in the cab of the vehicle itself, as in the instance of the “EliminatorTM” described below.Examples of commonly encountered obstacles include large vehicles such as busses, semi-tractors, curves in the roadway, overhanging foliage, bridges, and even fog.GPS systems must be “taught” where preemption is desired and require satellite triangulation for the emergency vehicle to “know where it is”.However, they are susceptible to a new host of limitations.In recent years, GPS systems have been introduced to overcome some of these line-of-sight constraints.More sophisticated systems such as the "Eliminator" overcome these limitations and give advance warning of collisions with similarly equipped emergency vehicles.Reception can also be disturbed by atmospheric phenomena.Satellite acquision is not always timely or even possible, such as in large metropolitan areas where one or more skyscrapers may hinder immediate satellite acquisition.Their reliability can be compromised by anything which visually obstructs the emitter’s ability to communicate data to the receiver.There are limitations inherent in these standard optical signal preemption systems, which rely on line-of-sight communication between the emergency vehicle and the receiver