This depends on the FM signal of your location. In general, most vacant/empty channels are found below 90.0MHz and above 107.0MHz. You can use your car radio to find a channel that has no broadcasting going on. Listen for a channel that contains constant static/buzz sound.
NOTE: Empty channels vary by region. If you are on the road and you move out of your current region, you may notice that the FM transmitter begins to experience interference and poor audio quality. This means that the empty FM channels have changed. In this case, safely search for a new empty channel with no voices or music and constant static to use with the Roadtrip.