 At 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.A naval dockyard has existed in Portsmouth since at least Tudor times, giving significant importance to the road linking that city with London.The original  route skirted the north-western limits of The Weald climbing to the summit of Gibbet Hill close to Hindhead.