Think about it! How long will it take you to search a school gym or a large restaurant or a small convention center or even a grocery store in IDLH (zero visibility) conditions? With a rope or not?
The answer is TOO LONG!
Think aggressive ventilation. I was tought to go into an IDLH atmosphere and locate (by feeling) and remove the victim. Wouldn't it be faster to remove the IDLH from the victom and then go in and "look" for victims as opposed to blindly feeling for them. Imagers are not the answer until everyone has an imager. With one imager per crew, as soon as a victim is located, the entire crew must leave with the victom and imager.
Positive Pressure ventilation will improve conditions to where visibility acceptable in 2 to 5 minutes in a large area.
Just think about it!