Mark Ackerman wrote:
Hmmm, I'll have to try this again on our next flight - but I'm pretty sure we've been able to "see" adsb traffic much farther away than 15 miles 
 
Yes, as I mentioned before, while the data broadcast from FAA towers only delivers traffic targets within your "hockey puck", which is limited to 15 mile radius, direct air-to-air transmissions between aircraft can *possibly* be farther than that.  But reception of air-to-air signals is highly variable, because it depends on many factors as described above.  Thus, "your mileage may vary."