[Tagging] Mapping gentle slopes?

[Nathan Edgars II]

I'm interested in mapping which direction of a street slopes down
(mainly for cycling purposes). The contour lines used by OpenCycleMap
don't have nearly enough resolution for this, yet many streets have a
noticeable slope (which can also be seen in how the roadside drains
are designed). Is there a way to tag that a street is downhill in the
forward or backward direction?