Worst I ever saw was I-80, going up to my friend's house in Vallejo (CA) for Thanksgiving. Some trucker tipped his truck over, and spilled a load of mystery powder (I'm sure the CHP knew what it was, but I didn't, thus the mystery!) on 3 of 4 lanes going eastbound.
I got lucky, and was close to the accident when it happened. Only took me a half hour to go 1/4 mile to get past the wreckage/cleanup. That works out to ONE HALF OF ONE MILE PER HOUR, for half an hour.
Thank GOD that I was driving an automatic at the time (hadn't learned how to drive stick at that time......yeah, I know.....what a HEATHEN!!!)