My understanding is that most "last mile" gas lines are pretty low pressure and low flow. On purpose, so that if theirs a leak they aren't releasing massive amounts of gas into the atmosphere in a hurry, so the concentration doesn't raise to dangerous levels.
Then when they are working on the lines and in those pits to make connections, they have detectors to tell if there's a dangerous build up, and ventilation to ensure it doesn't happen anyway.
The system generally being low pressure is why sometimes during really cold periods when demand is high, the gas company has to adjust field regulators to increase flow so that demand can be met. They normally run at low enough flow that if everyone's furnace and stove and water heater all run at once, the lines would effectively run out of gas.