Wet surfaces OR wintry/icy surfaces == trouble. Even for those of us who consider ourselves expert drivers assessing road and speed conditions. I've hydro'ed @ 20 mph, losing all braking and steering ability on freshly poured, wet asphalt. Lesson learned!

Hydroplaning can happen at even 20 mph or less when braking (locking them up) believe it or not. It is many times dependent on so many things, it's impossible to speculate on any specific case without seeking the reasons at that specific time, place, and conditions - speed, water depth, road surface, oils, tire condition, braking, accelerating, the whole ball of wax. When I hydro'ed at 20 mph a lot of years ago in my Lincoln Town Car on fresh, wet blacktop in a junior college parking lot, I knew this was a totally unpredictable deal. I had hydro-ed when applying the braskes, and instead of stopping, the car seemed like the front tires were on glare ice going dead-straight ahead, and most terrifyingly, running a stop sign. Best safe than sorry to exercise a lower speed on public thoroughfares. Ask me how I know ! Try and let the crazies go by/ahead of you to save your own vehicles. Just as driving on wintry surfaces. Totally unpredictable.