to extract foul air from the tunnel however the low height of these shaft buildings prevents efficient dilution and dispersal of pollutants. Shaft 4 is located on the corner of The Highway and Heckford Street. The original redbrick circular building was demolished in 1967 to allow widening of The Highway to four lanes which now extends over half the shaft. A plain surface building was constructed to house the fresh air supply fans. Pedestrian access to the tunnel is now limited to the portals; however there are granite staircases topped with ornate lighting columns close to each portal as an alternative to the footpaths on the approach ramps. The tunnel is entered via a sloping brick-lined cutting at each end leading to the entrance portal. Both portals are faced with pink granite and were given Grade II heritage listing in 1983. On the Rotherhithe side (south side) the approach road spans over the platforms of Rotherhithe London Overground station. It was originally planned the tunnel would commence before the railway was reached however during construction this was changed to be north of this instead thus shortening the tunnel by . It is followed by a short cut-and-cover section until the first of the four shafts is reached. The tunnelled section is between shafts 1 and 4, measures long and is lined with cast iron segments. The tunnel bore was sized to the maximum that the technology of the time allowed. Even then it was recognised this would result in less than ideal lane widths with each lane only in width (modern equivalent road width is ) and footways typically wide on each side of the roadway. The tunnel is shallow, with a maximum gradient of 1 in 36 (2.8%), to cater for non-mechanised traffic. It includes sharp bends at the location of the shafts. This allowed the sections between the shafts to be straight, thus reducing the complexity of construction and avoiding the foundations of the docks. An urban myth is that the bends were installed to prevent horses from seeing daylight at the end of the tunnel too early, which might make them bolt for the exit. The sharp bends and narrow lanes do however make the tunnel unsuitable for large vehicles. Due to this, and issues with vehicle emissions, Transport for London enforces a ban on vehicles more than 2 metres high or wide. Also banned are vehicles