The distance between the back of the guardrail

1
N 3
The distance between the back of the
guardrail post and the shoulder break is
important if the rail is to perform as tested.
3' from the face of the rail is the correct
distance. Consider this when the initial slope
staking takes place to assure the typical section
and the guardrail will fit _at_ the top of the
embankment.
2002 Roadside Design Guide Pg. 5-25
And don't forget to begin a 3'-1 1/2" post
spacing 25' prior to a hazard when the distance
between the face of the rail and an obstruction
is between 3.5' and 5.5'