Entryway systems length - Why 10 Feet not 9?

10 is a proxy for 2 full steps per shoe from an average person. Eg. Left... right... left... right.. off

I always wondered what was magical about 10' as well. I did some looking online to see if there is any research has been done, and found some conducted by a mat manufacturer, but nothing suggesting 10' is optimal. Another site said about 52% of debris on shoes is eliminated in 10' (depending on composition of mat) and 100% in 30'. I'm assuming LEED adopted 10' as a reasonable distance and, as Dylan points out, is typically the distance of 2 full strides. I think the most effective mat is the one that comes with a sign that say "Wipe Your Feet!"....just like mom raised us to do. )

A little history: Neither LEED versions 2.0 nor 2.1 specified a minimum walk-off distance, but a 2003 LEED Interpretation dictated, “the length should ensure that all foot traffic will encounter the entryway system.” In 2005, LEEDv2.2 set the minimum length at 6-feet (1.8 m). That was only about enough for “left-right” and little more than what many commercial properties provided for conventional, non-green buildings. Ultimately, EBOM-2008 & LEED-2009 increased the minimum to 10-feet (3 m), presumably for the reason that Dylan cites. I have seen references that cite various percentages of dirt removal at various distances for various mat types. These numbers may have come from some kind of empirical research, but I have never been able to find a primary source for any of this data. However, it is intuitive that more mat is better than less.

Dylan's wipe each foot twice explanation is the same I have heard from Steve Ashkin.