This gas readily replaces oxygen in the human circulatory system. With 55 miles per hour traffic, a driver would be exposed to the tunnel atmosphere for about one minute. Only an extremely high concentration of carbon monoxide would have any effect at this short an exposure. If the tunnel is not ventilated, a fairly high concentration can develop. Then a mechanical breakdown, a car out of gas, or any other minor mishap would serve to increase the exposure time.
Ventilation becomes a necessity.
The remaining power costs operate fans to keep carbon monoxide below two hundred parts per million. This force would consist of three shifts for a total of 20 men, with slightly larger shift crews during the morning and evening peak periods. Within this item is an allowance for overbreak, which means that a contractor is not expected to remove the rock exactly to the neat lines of the tunnel. The overbreak allowance chosen was based on the speculation that the shales, sandstones and granites would break reasonably well, but that the expected highly shattered shale next to the sandstone w9uld be much less controllable. At either portal, for a distance into the mountain until the cover over the tunnel is equal to one tunnel diameter, a cut and cover technique was planned.