The core, real question re: single stair buildings to rank and file firefighters should be this: Which of these 2 buildings would you prefer in an active unit fire with risk of spread? Which requires more resources and is more likely to extend them to a point where you need to call in equipment from elsewhere, further delaying full response and increasing risk for anyone still inside? The one with 12 units per floor, or the one with just 2? Reality is the one on the left has a much greater fuel load, far more area to spread, higher risk of large scale collapse, and 6x more spaces and people to clear. And this double loaded corridor example is smaller than the norm. In practice the difference between the two is even greater. Common single stair limits are a total floor area of just 4,000 sq ft. A double loaded corridor can be 46,000 sq ft - 11x the size and risk in a fire.