Question-Why is span length ratio of end span, approach span to its neighbouring inner spans typically about 0.75?

Answer-From aesthetic point of view, odd number of spans with reduce in length in direction of abutment is desirable. In addition, spans of equal length are found to be tedious. However, an arrangement of irregular span lengths is not recommended since it gives a feeling of disquiet.

From structural point of view, for a multi-span bridge with alike span length, sagging moment at mid-span of the end span, approach span is largest. In order to cut this moment, span length of an end span, approach span is designing to be 0.75 of inner spans.
However, ratio should not be less than 0.40 because of effect of uplifting at end span, come close to span support.