They were banned at Indy after one collapsed and killed several people in 1960. The way I understand it, overnight rains made the ground wet and soft. When the field went by on the initial parade lap, everyone on top of the scaffolding went to one side to get a better look and the whole thing went down from all the weight shifted to one side. There's pictures out there if you want to look for them. Jim Rathmann won. My dad was there. He didn't know about the tragedy until well after the race ended. I still have the Indy Star newspaper he bought after the race.
