Billboards are aimed at motorists (and their passengers). Motorists have a 3 second viewing time when passing by a billboard. This explains why the general rule in designing a billboard layout is to keep the text within 7 words.
I just have a few notes on this billboard recently posted on C5.
-the Art Director must be oblivious about the seven word rule
-the client obviously had too much to say
-both must have been aware about the heavy traffic on C5 during the morning rush hour which is why both of them gambled on this type of a layout.