In the late 1990s, the city's transformation commenced as numerous flyovers started dominating the landscape. Due to the escalating vehicular density and the constraints of horizontal expansion within the city, the flyovers are now extended to their maximum capacity. As bridges and elevated metro corridors intersect various parts of the city, planners believe that opting for an underground approach, despite its costliness, is a viable solution not only for metro projects but also for road corridors. Presently, there are at least 11 tunnel-based projects in progress, encompassing east-west corridors and rail systems such as the metro and bullet train, throughout the Mumbai Metropolitan Region (MMR). The Metro 3 underground corridor (Colaba-Bandra-Seepz), one of the initial underground mass transit projects in the city, is nearly complete. The finishing touches are being applied to the country's first undersea twin tunnels, which are part of the Mumbai Coastal Road project. Contractors for three east-west tunnel-based road corridors have been finalised by authorities. Tenders have been invited for four of the nine underpasses on Western Express Highway (WEH) and Eastern Express Highway (EEH) for grade separation projects. Simultaneously, progress is underway on the bullet train tunnel and Metro Line 7A (Andheri to domestic airport). Planners have stated that despite entering the scene relatively late, the city will discover a suitable solution in underground mass transit expansion, considering its island nature that prohibits horizontal expansion. Facing a vehicle count of 45 lakh, including 13 lakh private cars, the city perpetually grapples with traffic chaos. The resulting congestion often leads to lengthy traffic jams, pollution, and inefficient fuel consumption. The city boasts the highest density of private cars in the country, reaching 650 per kilometre of road.