Cal. Code Regs. tit. 8, § 7080 – Escapeways and Refuge Stations.

(a) (11-50) Every mine shall have two separate escapeways to the surface which are so positioned that damage to one shall not lessen the effectiveness of the other, or a method of refuge shall be provided when only one opening to the surface is possible.
(b) The escape exit shall be maintained within reasonable distance of the toping areas of the mine.
When the distance between the escape exit and any stope is computed by the following formula and the answer is one or more, the escape exit shall be extended in the direction of such stope until it is reasonable close.
T
+
S
= D
1000
300
T = distance in feet of horizontal travel.
S = distance in feet of vertical and inclined travel.
D = symbol for total distance of travel.
For the purpose of this formula, an incline of less than 20 degrees from the horizontal will be considered as a horizontal passageway.
(c) (11-51a) Escape exits shall be maintained in good condition at all times so as to provide a ready means of escape for employees in case of emergency.
(d) (11-51b) All underground passageways and shaft stations shall be signed and marked by arrows pointing the way to and through the escape exit in a manner to expedite escape.
(e) The surface outlets from escape exits shall be not less than 100 feet from the exit most frequently used by the men.
The surface outlet of no two exits shall be covered by one building or by connected buildings.
(f) Where the use of a series or raises is contemplated for an emergency exit, the mine management is strongly advised to select the position of the raises so as to have them in line with one another, so that hoisting equipment can be installed if advisable at some future time.