Hydroplaning: Why It Happens and What to Do

What hydroplaning is

On a dry road, the tread presses rubber straight onto the pavement. On a wet road, the tread has a second job first: it has to push water out of the way so the rubber can reach the road at all.

Hydroplaning is what happens when it can't keep up. Bridgestone describes it as the tires losing their grip on the road surface and traveling on a film of water sitting on top of it. Water builds up in front of the tire faster than the grooves can clear it, pressure rises underneath, and the tread lifts off the pavement.

From the driver's seat, it feels like the steering has gone light or disconnected. That's because it has. Bridgestone notes that hydroplaning dramatically reduces your ability to steer and brake until the tires touch down again.

The four things that set it up

Bridgestone lists the causes as a combination of road conditions, vehicle speed, tread depth and vehicle weight. Three of them are worth understanding in detail.

Speed

Tires need time to move water out from under the tread. The faster you go, the less time each part of the tread has to do it. Bridgestone says hydroplaning can happen at speeds as low as 35 mph, depending on the tires' tread depth and design and how much water is on the road.

That's not a line where it starts for every car, and you should be skeptical of anyone who gives you one. It's a reminder that you don't need to be on the highway for it to happen.

Water depth

More water means more for the tread to move. Bridgestone says the risk worsens as standing water gets deeper than about 1/10 of an inch. That's not much: a puddle in a low spot, the outside lane where water collects, or ruts worn into the pavement can all hold that much in a heavy shower.

Tread depth

Tread grooves are the channels water escapes through. As tread wears, those channels get shallower and hold less.

Two lines matter here:

• 2/32 of an inch is where NHTSA says to replace tires, the point where the built-in wear bars sit flush with the tread. Bridgestone says even the best tires offer little resistance to hydroplaning at 2/32 or less.
• 4/32 of an inch is where AAA recommends replacing. In AAA's tests on wet pavement at 60 mph, tires worn to 4/32 took an average of 87 feet longer to stop than new tires.

Our tread depth chart shows both lines in 32nds and millimeters, and the gauge below shows what each reading means.

A note on the UTQG traction grade printed on the sidewall: it comes from straight-line wet braking tests and doesn't measure hydroplaning. Our UTQG guide explains what it does and doesn't tell you.

What to do if the car starts to float

The instinct is to brake hard or yank the wheel. Both work against you while the tires are on water. AAA's guidance is:

1. Ease off the accelerator. Let the car slow on its own.
2. Avoid hard braking. Braking hard while the tires have no grip can make things worse.
3. Steer gently in the direction you want to go. Small inputs, not a jerk of the wheel.
4. Wait for the tires to grip again before making normal steering or braking inputs.

Once you're through it, slow down further for the rest of the drive. The conditions that caused it are still there.

How to lower the odds

Most of the work happens before the rain starts.

Keep tread above 4/32. Check three grooves on every tire and go by the shallowest reading. The quarter test is a quick check: put a quarter upside down in a groove, and if you can see all of Washington's head, you're at about 4/32. If you're close, run your readings through Do I Need Tires Yet?.

Set pressure to the door placard, cold. The number on the door-jamb sticker is the vehicle maker's recommendation. The number on the sidewall is a maximum, not a target.

In the rain, the habits that help come from Bridgestone's and AAA's wet-weather advice:

• Slow down. Bridgestone suggests 5 to 10 mph below your usual speed on wet roads, and more where you see standing water.
• Turn off cruise control. You want to be able to ease off the gas yourself.
• Steer around puddles and standing water when you can do it without a sudden lane change.
• Leave more following distance so you're not forced into hard braking.
• Never drive into a flooded road. You can't see how deep it is.

The South Florida factor

Hydroplaning isn't a once-a-year problem here. The National Weather Service office in Miami puts South Florida's rainy season at May 15 to October 15, with showers and thunderstorms developing almost every day. For five months a year, wet pavement is part of the normal commute.

The national numbers back up the respect. Per the Federal Highway Administration's 10-year averages (2007 to 2016), 75 percent of weather-related crashes happened on wet pavement and 47 percent during rainfall.

Florida law adds one requirement: Statute 316.217 says headlights must be on during any rain, smoke or fog. It also helps the driver behind you see you through the spray.

Our post on hydroplaning in Broward's afternoon storms turns all of this into a storm-day checklist. Heat matters to tires here too; our guide to Florida heat and tires covers that side.

Quick reference

When it's time for new tires

If your tread is at or near 4/32, replace before the next rainy season rather than during it. The Tire Size Finder reads your size off the door-jamb sticker, and you can shop tires by size from there.

Extreme Tires installs at the Sunrise shop or by mobile van in Miami-Dade, Broward and Palm Beach counties. Anywhere else in the 48 contiguous states and DC, your order ships to your address for a local installer. See how installation works, or call (954) 773-1896 with questions.