Before the Holland tunnel was constructed, residents of New York and New Jersey seeking to cross the Hudson River used ferries which carried vehicles onboard. At the turn of the century as automobile ownership became more common, up to 30 million vehicles were transported across the Hudson using ferries. In 1920 the New Jersey Interstate Bridge and Tunnel Commission and the New York State Bridge and Tunnel Commission allocated funds for the creation of Holland Tunnel, then referred to as the Hudson River Vehicular Tunnel project. Initially, it was thought that building a bridge would be the most practical and affordable solution to connect New Jersey and Manhattan. However, commercial ships required a minimum clearance of 200 feet, and since the Manhattan side of the Hudson River did not have sufficient clearance to meet this requirement, it was determined that a long approach to the bridge would also be required. Since a long approach to the bridge on the Manhattan side would not be feasible, in 1913 it was decided that a tunnel was the best method of achieving the desired result. There were two proposals put forth for the creation of the Holland Tunnel. The first plan was by a firm called Jacobs and Davis who proposed a bi-level tunnel, measuring 31 feet in diameter. The second proposal was put forth by George Goethals, and was larger in scale than the plan proposed by Jacobs and Davis. Despite these earlier proposals, the well regarded engineer that was finally chosen to construct the Holland tunnel was Clifford Holland. In the initial testing, Clifton Holland and his team of engineers discovered that the carbon monoxide from the vehicles could reach lethal levels if ventilation systems were not installed. Clifton Holland and his team were responsible for inventing the two-duct automatic ventilation system that would later be adopted by tunnel engineers across the globe. The automatic ventilation system required two ventilation buildings at each end of the Hudson. These buildings house a total of 84 massive fans: half of the fans pump clean air into the tunnel, and the other half serve as exhaust fans. Although only 56 of the 84 fans are operational at all times, (the other 28 are reserved for emergencies) this system has the ability to completely replace the air in the tunnel in just 90 seconds. When construction of the Holland tunnel began in 1920, a sudden and tragic death of the chief engineer, Clifton Holland stalled the project. His successor, Milton Freedman also passed away before the project was complete. The third and final chief engineer of the project became Ole Singstad. The construction of the Holland Tunnel was finally complete in 1927. In 1931 the Port Authority took over the jurisdiction of the Holland Tunnel. On several occasions the Holland Tunnel had been threatened by outbreaks of fire: the first incident took place in 1949 and was caused by fire erupting from a truck carrying chemicals. In 2002 the tunnel was threatened once again when a fire erupted in an abandoned warehouse facility located near the western entrance of the Holland Tunnel; the tunnel was temporarily closed off to deal with the fire.