The problem I've always had with this poll was the use of the word "income" which to me has a different meaning than does sales, volume or revenue. I answered it on the basis of gross sales which would be the most meaningful figure because what you end up earning from the gross sales will depend on a whole bunch of other factors.
There was something similar at Letterville a couple of years ago. My interpretation of both this poll and that one was that $70K a year in gross sales was the median (not the average) point of the responders. In other words, half the responders do less than $70K a year in
sign related business.
With that as a conclusion any answers more detailed would be skewed. Consider
This Poll which showed that only 28% of the responders are fulltime and work from a commercial location. Yet another ended up with 78% of the responders are owners.