The commissioners may also originally locate, lay out and establish in whole or in part, relocate, straighten, widen, extend, alter and open, construct and improve roads not parts of said system nor deemed main thoroughfares, upon parties interested therein paying or securing to be paid such proportion of the cost of the original construction and improvement as the commissioners may deem just, which shall not be less than one-fourth of the cost in any case.
16 P.S. § 2723