Wet vs. Dry Traction

[rac156]

I was driving around in the rain tonight and I started thinking. How much less traction does a wet road provide vs. the same road when its dry? I know there are many factors such as surface condition, tires, temperature, etc. But as an overall average how much less traction is there? 25% less? 50% less?

 

[Fred(DrivingSchool)Kinder]

For rain traction must include tread depth, water depth on road surface. I would expect a tire manufacture to have the best info. Yesterday I was on a Skid Pad with water and some dry areas. I cant tell by the seat of the pants feel that the wet surface grip varied by at least 100% of a percent.

Road slope, resulting drainage etc. will make a big difference in grip.