§ 234.217 Flashing light units.
(a) Each flashing light unit shall be properly positioned and aligned and shall be visible to a highway user approaching the crossing.
(b) Each flashing light unit shall be maintained to prevent dust and moisture from entering the interior of the unit. Roundels and reflectors shall be clean and in good condition.
(c) All light units shall flash alternately. The number of flashes per minute for each light unit shall be 35 minimum and 65 maximum.