The Tamil Nadu government has asked the National Highways Authority of India (NHAI) to prepare an integrated design for a double-decker viaduct comprising a six-lane elevated highway on the lower deck and a metro rail corridor on the upper deck for the proposed 18.4 kilometre (km) corridor from Kilambakkam bus terminus (KCBT) to Chettipunniyam. The state granted in-principle approval in December for extension of the metro corridor from KCBT to Chettipunniyam. Officials said the request requires a revision of the Detailed Project Report (DPR) that was earlier prepared for a single-deck elevated corridor. They said the piers will need to be redesigned to bear combined loads for both highway and metro infrastructure and that the cost-sharing arrangement between the NHAI and the state government will have to be reworked before the DPR can be finalised. Design finalisation is expected to take nearly five months, after which the state government’s share of the project cost can be determined. NHAI officials noted that constructing both decks simultaneously, as done on the Mugaliwakkam-Ramapuram stretch, would be more feasible than adding the metro later. Officials added that NHAI is prepared to proceed either by building the elevated highway first and developing the metro corridor later or by incorporating the civil infrastructure for the metro during highway construction, subject to approvals from the Union government. If the state opts to include pier caps and the launching of U-girders and I-girders as part of the elevated corridor work, its financial contribution would rise but future disruption to traffic could be minimised. Subsequent work such as track laying and installation of overhead electrical systems would then be able to proceed with reduced inconvenience. The proposal to extend the Blue Line metro from the airport to KCBT remains pending Union approval and the further extension from Kilambakkam to Chengalpattu is expected to be delayed as a result. A rail activist said that the DPR has been with the Union government for more than a year and urged that approval be granted at the earliest to commence work.