I believe the typical car provides some ventilation from the outside, even when the heater/AC is off. Thus any carbon-monoxide fumes in the tunnel would be drawn into the cabin and likely affect the driver. It's not a matter of oxygen..................................
I think that the internal volume of a car cabin carries enough oxygen for at least 10 minutes of occupant safety. As long as traffic doesn't gridlock, zero emmision vehicles travelling through the tunnel at a mile a minute will carry all the ventilation needed for a ten mile tunnel span!