"The air inside the volume of a coffin can last you a few hours. Unless you are literally shoulder to shoulder with no other air, your oxygen will last as long as you need."

... unless there is a fire:

"Most drivers rolled up their windows and waited for rescue. The ventilation system in the tunnel drove toxic smoke back down the tunnel faster than anyone could run to safety. These fumes quickly filled the tunnel and caused vehicle engines to stall because of lack of oxygen. This included fire engines which, once affected, had to be abandoned by the firefighters. Many drivers near the blaze who attempted to leave their cars and seek refuge points were quickly overcome."[1]

[1] https://en.wikipedia.org/wiki/Mont_Blanc_Tunnel#1999_fire