The Hindhead Tunnel, opened in 2011, is part of the 4-mile (6.4 km) dual-carriageway Hindhead bypass that replaced one of the last remaining stretches of single-carriageway on the 68-mile (109 km) A3, the London to Portsmouth road. The bypass was constructed to improve road safety, reduce congestion and improve air quality. At 1.15 miles (1.85 kilometres) in length, the tunnel is the longest non-estuarial road tunnel in the United Kingdom, and takes the road beneath the Devil's Punch Bowl, a Site of Special Scientific Interest.