Every resolution ordering any improvement mentioned in this chapter, payment for which shall be in whole or in part by special assessments shall establish a road improvement district which shall embrace as near as may be all the property specially benefited by such improvement and the board shall apply thereto such method of assessment as shall be deemed most practical and equitable under the conditions prevailing: PROVIDED, That no assessment as determined by the board of commissioners shall be levied which shall be greater than the special benefits derived from the improvements.