92 Ill. Adm. Code 1535.341
Mounting Lamp Units
Section 1535
Section 1535.341 Mounting
Lamp Units
a) Lamp units shall be mounted horizontally on 2 foot 6 inch
centers. The mountings for the lamp unit shall be provided with means for
adjusting the units in both the horizontal and the vertical planes and for
positive locking of the adjustments.
b) The lamp units preferably should be mounted with a minimum
clearance of 7 feet above the crown surface of the highway opposite the signal
mast. Special conditions, such as a sharp approach grade, may require a
variation in the height of lamps but in general the endeavor should be to place
these lamps in the best position with respect to the normal line of vision of
drivers of approaching vehicles and still maintain reasonable protection
against accidental breakage.