[quote=Eugene]Rad is not a unit of fallout. That map is meaningless.[/quote]
But Rad is a measurement of ionizing (high energy) radiation, which is related to fallout. It indicates the strength if its related to units of time.

http://www.britannica.com/EBchecked/topic/488258/rad
http://en.wikipedia.org/wiki/Rad_%28unit%29