You don't actually out run the signal, that is a metaphor for loss of signal strength while moving at highway speeds. The signal characteristics/field strength pattern actually changes while moving and gives the results of reduced signal strength at the receiver in the cab ahead of the transmitter. This in not a serious problem and results in an intermittent frame freeze for digital signals. I could get more technical about this but it would probably make you go to sleep :-)
While backing or moving slowly this is not a problem and the signal is steady and clear.