carrying the tube's roadway was placed in between two plenums. A lower plenum below the roadway floor could supply fresh air, and an upper plenum above the ceiling could exhaust fumes at regular intervals. Two thousand tests were performed with the ventilation system prototype. The system was determined to be of sufficiently low cost, relative to the safety benefits, that it was ultimately integrated into the tunnel's design. By the time the tunnel was in service, the average carbon monoxide content in both tunnels was 0.69 parts per 10,000 parts of air. The highest recorded carbon monoxide level in the Holland Tunnel was 1.60 parts per 10,000, well below the permissible maximum of 4 parts per 10,000. The public and the press proclaimed air conditions were better in the tubes than in some streets of New York City; after the tunnel opened, Singstad stated that the carbon monoxide content in the tubes were half of those recorded on the streets. Prefabricated tunnel sections During construction of Baltimore Harbor Tunnel from 1955 to 1957, Singstad adopted a cost-saving method for the construction of the tunnel in the river mud. Previously, hydraulic shields or pressurized caissons had been used — with the constant danger of divers suffering the bends, and the necessity for constant diligence. A sunk-tube method had been earlier proposed and used by Olaf Hoff on the Detroit River tunnel and Harlem River Tunnel. Singstad advanced Hoff's ideas and proposed first digging a large ditch in the river bottom and lowering cable-suspended pre-fabricated tunnel sections in length (weighing 23,000 tons each) into the ditch from overhead barges. Interior chambers were filled with water to lower the sections, the sections then aligned, bolted together by divers, the water pumped out, and the tunnels finally covered with earth. This technique was followed in numerous later tunnel projects by other engineers, on the Chesapeake Bay Bridge–Tunnel, for example. Personal life Ole Singstad married Else Johansen (June 28, 1890 – July 8, 1964). Together they had two children, Rita (May 2, 1918 – May 3, 1975) and Paul (April 25, 1925 – March 27, 1976). Singstad returned to Norway five times in his life: first for his mother's 80th birthday in 1923, to Lensvik in 1930 and again in 1933 while working on the tunnel at the river Schelde. He missed his mother's 100th birthday in 1943 because of World War II, but he