Rain changes how a car behaves more than most drivers realize. The road looks almost the same, the car feels almost the same, and then you brake for a red light and the car takes much longer to stop than you expected. Wet roads are involved in a large share of weather-related crashes, and most of those happen in ordinary rain, not storms.
This article explains what water does to your tires' grip, what hydroplaning is, how to handle a skid, and the simple habits that keep you in control when the road is wet.
Why wet roads are slippery
Your car's grip on the road comes from four small patches of rubber, each roughly the size of your hand, where the tires touch the pavement. Everything the car does, whether that's braking, steering or accelerating, goes through those four patches.
Water gets between the tire and the road and acts like a lubricant. That cuts the amount of grip available, often by a third or more compared with dry pavement. With less grip, everything takes longer: stopping, turning and speeding up. Stopping distances on wet pavement can be much longer than on dry. The article on following distance and stopping distance has a calculator that shows how much.
The first rain is the worst
Roads collect oil, rubber and dirt during dry weather. When rain first starts, especially after a long dry spell, that buildup mixes with water into a slick film. The first half hour or so of rain can be the most slippery, until enough rain falls to wash the road clean.
Hydroplaning
The grooves in your tires are there to channel water away so the rubber can reach the road. If there's more water than the grooves can move, a wedge of water builds up in front of the tire and lifts it off the pavement. The tire is now riding on water, and you've lost most of your ability to steer or brake. That's hydroplaning, also called aquaplaning.
Hydroplaning becomes more likely when:
- You're going faster. Speed gives water less time to escape from under the tire. The risk rises steeply as you go faster.
- Water is deeper. Puddles, standing water in ruts, and water flowing across the road are the danger spots.
- Your tires are worn. Shallow tread can't move as much water.
- Your tires are underinflated. A soft tire's tread can't push water away as well.
Signs you might be hydroplaning include the steering suddenly feeling light, the engine revving up without the car speeding up, or the back of the car feeling loose.
What to do if you hydroplane
- Ease off the accelerator. Don't brake hard. Let the car slow down gradually so the tires can find the road again.
- Keep the steering steady. Hold the wheel pointed where you want to go. Avoid sudden turns, which can make the car spin when the tires regain grip.
- Brake gently if you must. If your car has anti-lock brakes (ABS), you can press the brake firmly and steadily, and the system will pulse the brakes for you. If not, use light, gentle pressure.
- Wait for grip to come back. You'll feel the steering get heavier again as the tires reconnect with the road.
Skids
A skid happens when you ask the tires for more grip than the road can give. Braking too hard, turning too sharply, or accelerating too quickly on a slippery surface can all cause one.
The most important rule in any skid is to look and steer where you want the car to go. Your hands follow your eyes. If you stare at the tree or the ditch you're afraid of hitting, you'll tend to steer toward it.
- Front-wheel skid (understeer): You turn the wheel but the car keeps going straight. Ease off the accelerator and unwind the steering a little. Turning the wheel even more won't help, since the front tires already can't grip. Once they bite again, steer gently.
- Rear-wheel skid (oversteer): The back of the car slides out to one side. Ease off the accelerator and steer smoothly in the direction you want the front of the car to go. Be ready to straighten the wheel as the car comes back in line so you don't overcorrect into a slide the other way.
In both cases, smoothness is everything. Sudden, jerky inputs are what start most skids, and they're what turn small slides into spins.
Habits for driving in the rain
- Slow down. Driving below the speed limit in heavy rain is good judgment, not timidity. Speed limits are set for good conditions.
- Double your following distance. If three seconds is your normal gap, aim for at least four to six in the rain.
- Do your braking in a straight line. Slow down before a curve, then go through it with steady speed and a steady wheel.
- Be smooth. Gentle steering, gentle braking, gentle acceleration.
- Turn on your headlights. It helps other drivers see you. Many states require headlights whenever your wipers are on.
- Don't use cruise control. On a wet road, you want full control of the throttle, and cruise control can make a hydroplane worse by keeping the wheels spinning.
- Avoid standing water. Drive around big puddles when it's safe, or drive in the tire tracks of the car ahead, where water has been pushed aside.
- Never drive into flooded roads. It doesn't take much moving water to float a car, and you can't see how deep it is or whether the road underneath is still there.
Check your tires
Tires matter more in the rain than at any other time. In most U.S. states the legal minimum tread depth is 2/32 of an inch, but tires lose a lot of their wet grip well before that, and many tire experts suggest replacing them sooner if you drive in the rain often. A quick way to check: put a penny into the tread with Lincoln's head pointing down. If you can see the top of his head, the tread is very shallow. Also check your tire pressure monthly against the number on the sticker inside the driver's door.
Practicing in The Driver Tester
The Driver Tester's Slippery When Wet scenario uses the same road, traffic and scoring as Driving in Traffic, but adds three wet patches: one going into the curve, one at the bottom of the curve, and one at the intersection. While any part of your vehicle is on a wet patch, your tires have only half their normal grip for both steering and braking.
That makes the habits above very easy to see in action. Brake hard or turn sharply on a wet patch and the car keeps going where it was pointed, often off the road. Slow down before each patch, cross it with the wheel steady and your foot off the brake, and leave a bigger gap than usual to the car in front. Each clean lap earns 40 points, plus up to 30 for lane position and up to 30 for following distance.