30 CFR § 77.213
Draw-off tunnel escapeways.
When it is necessary for a tunnel to be closed at one end, an escapeway not
less than 30 inches in diameter (or of the equivalent, if the escapeway does
not have a circular cross section) shall be installed which extends from the
closed end of the tunnel to a safe location on the surface; and, if the
escapeway is inclined more than 30 degrees from the horizontal it shall be
equipped with a ladder which runs the full length of the inclined portion of