In General Specification for Civil Engineering Works (1992 Edition), the design of roadbase material is based on recipe approach. Why?

The design of roadbase material is based on recipe approach (David Croney and Paul Croney (1992)) because Hong Kong government follows the traditional British practice by adopting recipe design in which the
aggregate grading envelope, the quantity and grade of bitumen are specified in the bituminous mix. This recipe of bituminous mix is derived based on past experience and good workmanship during construction. In
fact, many countries nowadays adopt special design mix of roadbase which proves to produce satisfactory bituminous mixes to suit different site and design conditions.

In fact, recipe specification of bituminous materials does suffer from several drawbacks. Firstly, the conditions of traffic and climate of newly constructed bituminous road may differ from the conditions on which the recipe design is based. In case adjustment has to be made to the recipe design, it is very difficult to determine and assess the modifications required. Secondly, it poses problem to site engineers to assess the effects of minor non-compliance if recipe specification is adopted. Finally, the recipe mix may not be the most economical design which is dependent on site conditions.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.