The traffic Index is defined as:
Q: What is a "traffic index?"

A: The "traffic index" is a score from 0 to
100 where 0 is "slow" and 100 is "fast". It is
determined by comparing the current
response of a ping echo to all previous
responses from the same router over the
past 7 days. A score of 0 to 100 is then
assigned to the current response depending
on if this response is better or worse than all
previous responses from that router.

Therefore 0 (spike down) would indicate very slow at that time (around midnight MST) traffic was very slow.

Canie