The Tamil Nadu government has asked the National Highways Authority of India (NHAI) to prepare an integrated design for an 18.4-km double-decker viaduct from Kilambakkam bus terminus (KCBT) to Chettipunniyam near Mahindra City on GST Road. It would carry a six-lane elevated highway below and a Metro rail corridor above.

The revised detailed project report (DPR) must account for stronger piers and a new cost-sharing arrangement. Design finalisation is expected to take nearly five months; the state’s project contribution will be determined afterward. The redesign is likely to delay the corridor by another year.

Tamil Nadu granted in-principle approval in December 2025 for the Metro extension from KCBT to Chettipunniyam. NHAI has recommended building the highway and Metro civil infrastructure simultaneously; alternatively, the state could add Metro pier caps and launch U- and I-girders during highway construction. This would avoid disrupting lower-deck traffic during later track and overhead-electrical installation, but would increase the state’s contribution. NHAI can build the highway first, though the Metro requires Union government approval. The Blue Line extension from Airport to KCBT awaits that approval, and the further extension to Chengalpattu is also expected to be delayed. The earlier DPR was submitted to the Centre more than a year ago.

Tamil Nadu granted in-principle clearance for an 18.4-km elevated corridor from Kalaignar Centenary Bus Terminus (KCBT) at Kilambakkam to Chettipunniyam, near Mahindra City on GST Road. The National Highways Authority of India (NHAI) will lead the project, with a six-lane highway on the lower deck and pillars designed to support a future metro line above. India’s longest existing double-decker metro viaduct is 5.6 km on Nagpur’s Kamptee Road.

Chief Secretary N Muruganandam chaired a meeting with state highways, Chennai Metro Rail Limited and NHAI officials. The state will fund additional structural costs for the future metro corridor. NHAI is updating the detailed project report to reflect a revised alignment; tenders will follow necessary approvals. A Bus Rapid Transit System remains likely in the revised plan.

First proposed in 2022, the project stalled for nearly two years over metro design conflicts. It was initially estimated at ₹3,400 crore, with three proposed entry and exit ramps.

Separately, the state approved extending Chennai Metro’s Blue Line from the airport to KCBT via Pallavaram, Chromepet, Tambaram and Vandalur, and allocated funds for related land and utility works.

• NHAI plans an 18.4-km, six-lane elevated corridor from Kilambakkam bus terminus (KCBT) to Chettipunniyam near Mahindra City junction, using pillars in GST Road’s central median. Three locations will have entry and exit ramps. The estimated cost is ₹3,200 crore.
• A 30-km Tambaram–Chengalpattu elevated road proposed by the Union Road Transport and Highways Ministry in 2018 was scaled down because of feasibility, traffic and alignment issues. NHAI finalised the detailed project report months ago but has not invited tenders; the Airport–KCBT metro line’s ramps must be integrated. Tamil Nadu approved the metro extension last week.
• In 2024, NHAI widened 18.5 km of GST Road from Vandalur to Chettipunniyam from four to eight lanes. The road is 35–45 metres wide and carries over 75,000 passenger cars daily. One lane on each side will be used during corridor construction, leaving six lanes for traffic.
• NHAI is also widening 2.4 km of GST Road between Tambaram road overbridge and Perungalathur from four to eight lanes. The overbridge widening is complete; work on the adjoining stretch toward Chennai Bypass/Tambaram was expected to finish within a month. At Irumbuliyur junction, the road narrows from 35 metres to 11 metres.
• Festival congestion can stretch 3–4 km in both directions. On the weekend before Onam, Tambaram to Chengalpattu took nearly 70 minutes.