Any tunnel of appreciable size will have designed ventilation and other auxiliary systems(life support/escape, maintenance, power, etc). It isn't just a tube in the ground.
But the ventilation doesn't go to the void, it will just be blown into the same neighborhood. So unless it's filtered in the tunnel ventilation system, you still get the same fumes there.
You can have ventilation stacks that at least distribute the exhaust higher into the atmosphere, or at key points where fewer people live. I don't know specifically if they do that here though.
I know. They're designed with fire in mind. What I don't know is what they're rated at and if they run at higher flow rates depending on traffic and/or pollution levels.
The alternative was to run the highway around the city, which is probably also cheaper. Though it's nice that they recovered some space and planted grass and trees over it.