Locate the nearest one and get out of the tunnel. If you are close enough to see the tunnel entrance, exit that way. Do not look for an emergency exit. Do not hesitate to leave your car behind. Cars with engines running produce carbon monoxide, which is deadly.
Fires produce even more.
A severe fire may cause ventilation systems to fail. While extremely rare, tunnels can collapse due to structural deterioration caused by the heat from a severe fire. Bricks tunnels are very safe, but concrete can break down badly if it has no fire protection. For this reason, most concrete is fireproofed. Death in a tunnel fire is usually due to fire or smoke, not structural failure.