Let's consider an example of why this question has been discussed and debated ad nauseam with no results: In New York City on 9/11 all bridges and tunnels were closed. If your backup site was across the river in New Jersey, or even in a different borough of New York, you were pretty much prevented from getting to it. And if you had to fly there, you were obviously out of luck.
It can really help if employees live in different directions from your main site. At least someone would likely have a way to get to your recovery location if others can't. The problem is that whether the recovery site is near or far, people still need to be able to get there for it to be useful.
Consider a few things when deciding on an actual distance: make sure that the recovery site is on a different power grid and keep it at a distance were it is accessible under emergency conditions.
But the only absolute regarding distance is that there is none. And in my opinion, there never should be.
ABOUT THE AUTHOR: Robert McFarlane is a pioneer in the field of building cabling design. He has been asked to speak at countless seminars on building infrastructure for electronic communications, evolving technologies and the requirements of trading floor and data center design. Mr. McFarlane served for twelve years as President of Interport Financial, Inc., a firm specializing in designs for financial trading floors and critical data centers.