RE: Radio interference issues
There have been some cases where multiple transmitters operating in the same area have caused interference. The type of interference is called "third order intermodulation interference" (3IM). 3IM interference is caused when a number of different frequencies come into a receiver and hit the mixer stage. Those frequencies are then subject to the normal addition and subtraction effects that happen with waves. Bascially, if any combination of the channels added together and then subtracted equals the frequency of the receiver, you get interference. Usually though, it works out like this:
(2 x Channel A) - (Channel B) = (Channel C) Channel C gets hit.
With some real numbers: (2 x 72.030) - 72.050 = 72.010 72.010 (channel 11) gets hit.
You can even use the channel numbers: (2 x 12) - (13 ) = 11
This formula works with any 3 channels that are evenly-spaced apart.
If a receiver is far enough away from its own transmitter, and the other two are much closer to it, then there's a possibility it can get a hit. However, most receivers are well-designed enough to do a very good job of rejecting 3IM interference. That's why it's not really considered much of a problem. If multiple transmitters are in close proximity to each other, then the likelihood of 3IM interference is slightly higher than when the transmitters are spaced out across the field. The RC car guys, operating shoulder-to-shoulder present the worst-case scenario for 3IM interference, and it's not been much of a problem at all with modern systems.