Meaning of "went through the floor"?
Solution 1:
Yes, it means to collapse, or go way down, or whatever. Contrast with "through the roof".
I can't find a reference but I'm pretty sure the expression originated in the financial markets, wherein a seller or regulator would set a price floor on a commodity or security, and if the value of said commodity fell below the floor, it was very undervalued and no one would buy it.
Solution 2:
To go through the floor means "to fall to very low levels". So, your quote basically says that the traffic decreased dramatically.