Historically, the two major aspects of road design have been (i) The design principles – like AASHTO 1993 Empirical Design or lately, Mechanistic Empirical Pavement Design Method (MEPDM) (ii) The materials & technologies of pavement construction The fundamental design principles have not undergone major changes, however, the advancement in materials and technologies have improved tremendously over last few decades and this makes it necessary to revisit some of the conventional concepts in road design. The new technology now challenges the conventional wisdom and has brought us to the threshold of an era of all new sustainable green roads of tomorrow. AASHTO 1993 Empirical Pavement Design is the basis for pavement design today; in most parts of the world. In some parts of the world, there is a movement towards Mechanistic Empirical Pavement Design Guideline (MEPDG), but the movement is slow and gradual as this approach is expensive and heavily dependent on software programs due to its inherent computational complexities. The concept of Structural Number and Layer Coefficients of pavement layers under AASHTO 1993 Empirical Pavement Design was derived from AASHO road test carried out in Ottawa, Illinois between 1958 & 1960. The conventional Layer Coefficient value of 0.44 which is used today was recommended considering the strength of the construction materials available at that time. But, in view of the new technology available now, this needs to be revisited. Secondly, AASHTO 1993 Empirical Pavement Design provides for assuming certain values for Drainage Coefficients, ranging between 0.4 to 1.4, based on certain criteria. It is quite common to assume a value of 1 for drainage coefficient, in most parts of the world. Now, with the advent of new nanotechnology for waterproofing of road layers, it is possible to consider higher values for drainage coefficients. The new nanotechnology for soil stabilization can make subgrade soils well bonded, strong and flexible, allowing the designer to assume higher values of Resilient Moduli in the AASHTO 1993 design equation, which may bring the required structural number down and allow a lighter design of cross-section of structural layers on top of the subgrade. Indicative calculations for a typical 100 MSA, two lane (10 m wide) highway show that the new technology, while remaining within the AASHTO 1993 design guidelines, enables designing a pavement that is 3 times better (it will now be a 300 MSA pavement instead of 100 MSA) with a cost reduction of about USD 114000 per km. This paper takes an overview of latest trends in USA regarding pavement design approaches. It also puts forth, the opportunities presented to a pavement designer, by the new nanotechnology and proposes a new design approach, for optimized pavement design - green, sustainable & economical; while remaining within the AASHTO 1993 guidelines or MEPDG.

• Vivek Kane(Executive Vice President, Zydex, India)