Any motor vehicle may be operated under the conditions specified in R.S. 32:301 when equipped with two lighted lamps upon the front thereof capable of revealing persons and objects 75 feet ahead in lieu of lamps required in R.S 32:321 or 32:323, provided, however, that at no time shall it be operated at a speed in excess of 20 miles per hour but only on such highways, or parts thereof, that such slow speeds are permitted.
La. R.S. § 32:325