In my neck of the woods the safe speed is always much lower than the posted speed in foul weather. If the sign says Speed Limit 65, that means under ideal conditions. If it is raining, the SAFE speed is nearer 45.
I was tooling along at about 50 in a rain storm, checked my left mirror, and here comes a red pickup in the passing lane going way too fast. He hit a puddle with a sound like someone kicking a trash can, spun around twice and stopped in the median. Guess he never heard of "hydroplaning".

R.B. consider if you will, just how much rubber is in contact with the road at any given time. THAT is what stops you when you hit the brakes, and if you compare the amount of rubber on the road on a car, then on a big truck, then consider the weight of each you will see that there is damn little difference.
Is it Newton's second law that says things in motion tend to stay in motion? Have you ever driven a 9 ton vehicle?