Agree this is a bug in the description IMO.
My original choice of treating everything as uppercase lead me to a much more complex than necessary implementation that still failed on the randomized tests...

Personally, I had the basic idea right away... stacking partially applied functions and using something like id as the initial value, but I couldn't get the form right and resorted to looking it up in the wiki. I guess I cheated, but I plan to come back to this one in the future and see if I can get it without peeking.

now I feel silly...

This sentance really was confusing: "If x length is less than or equal to 1 return 0: the car didn't move."
I ended up passing all the tests including random tests with rule:
"If the average speed for a given interval is less than 1, count the average speed is as if it where 0 for that interval."