Whenever by reason of thawing of frost, or rains, or due to new construction the roads are in a soft condition, the maximum weights on all vehicles mentioned in section 304.180, including trucks, tractors, trailers and semitrailers and other vehicles therein mentioned may be limited by the state highways and transportation commission to such an amount and in such manner as will preserve the road under such conditions; and said commission shall give due notice thereof by posting notices at convenient and public places along said road or roads or parts thereof which are subject to said regulations and reduction of weights.