To be seen from a long distance you have to compromise.
Increasing visibility from a single point or a certain direction is possible by reducing the angle of radiation without adding a huge amount of power. The narrower the cone the radiation is confined by the housing of the LED the more power will be directed to the eye of the beholder. This works well if your sign has to be seen from a linear road, i.e. if beholders follow a certain path approaching the sign.
Another method to increase visibility without investing lots into high power LEDs is to increase contrast by surrounding all LEDs with dark non-reflective matter like cardboard with black paint.
These are methods which do not depend on the progress of LED production in terms of power and efficiency, so I dared to add them to this old question.