A little bit of tread will do practically nothing with gravel, sand or other loose surfaces.
Notice how those Tour de France riders still have slicks on, even when it's raining on a downhill descent with switchbacks. That's because they have the best traction.
The only thing that will improve your traction in the rain is to increase the width of the tire. It'll slow you down a bit on the straight but will increase your rolling surface area on cornering.
Nothing short of caterpiller tracks will save you from a skid on a well polished manhole cover. Best thing to is contact your local authority and ask them nicely to surface it.