When those portions of the cast-iron lining rings forming the lower half of the tunnel had been cleared out, they were filled with concrete, thus providing a smooth surface for what would eventually become the main ventilation duct. This surface was also utilised by the road transport vehicles, which were now needed to deliver materials for the completion of the tunnel.

At the same time, concrete haunches were formed to support the road deck units. These units, each eight feet long by twenty-six feet wide and weighing twenty-two tons, are of reinforced concrete. They were pre-cast in a casting yard at Coble Dene adjoining the tunnel. Steam curing ensured a twenty-four hour casting cycle on each of the eight moulds. By this means an output of seven units a day was achieved. As each unit was completed it was swung by crane onto a special low-loading vehicle equipped with a turntable, which could be raised or lowered. In the tunnel, this turntable enabled the units to be swung through 90 degrees and dropped into their approximate positions on the steps of the haunches. Final levelling and placing at intervals of 2 ft. was done by a special traveller equipped with jacks, rolling upon a lower step of the haunch. The space of 2 ft. between the deck units allowed the projecting reinforcing bars to intermesh and it was afterwards filled with in situ concrete. Kerbs were laid and the pre-cast walkway units concreted in place on either side of the roadway. The road deck was then surfaced with hand laid mastic asphalt. The width from kerb to kerb allows on 12 ft. wide traffic lane in each direction with minimum headroom of 16 ft. at the kerb line.