From tire tread to floodwater: 9 tips for driving in heavy rain

Rain takes away two things a driver depends on at the same time: traction and visibility. A film of water between rubber and asphalt reduces grip. Spray, glare and fogged glass shrink the view ahead. Floodwater adds a third problem. Its depth and current are hard to judge from behind the wheel, and the road beneath it may be washed out.

That combination makes driving in heavy rain a different task from ordinary driving, not just a slower version of it. Stopping takes longer. Abrupt steering or braking that a car would absorb on dry pavement can start a skid. A dip that looks like a puddle can hold enough water to stall an engine or move the car. Choices that seem minor in good weather carry more weight. Two examples are how closely to follow a truck and whether to take a familiar underpass.

The tips below are a selection of practical steps, not a complete rulebook. They follow the order in which a driver tends to face them. The first covers the decision made before leaving home. That means reading weather alerts and knowing what the National Weather Service means by a flood watch versus a flood warning. The next tip deals with the car itself: tire tread depth, tire pressure and wiper blades. Those three items set how much margin a vehicle has on wet pavement.

From there, the list moves to technique on the road. That includes speed and following distance, headlight use, recovering from hydroplaning, clearing a fogged windshield and handling spray from trucks and buses. Each of these deals with grip or visibility.

The final tips address floodwater directly. They explain why official guidance tells drivers to turn around rather than cross a flooded road. They also cover what to do if a car stalls in rising water. The last one covers what to check once a vehicle is past standing water, from the brakes to the roadside hazards a storm leaves behind.

Each tip stands on its own. A reader can go straight to the situation at hand, whether that is a storm in the forecast, a skid on the highway or water across the road ahead.

Illustration: Quartz

Check the National Weather Service forecast and active alerts before leaving, then decide whether the trip needs to happen at all. Knowing a flood watch from a flood warning tells a driver how serious the threat is.

The wording of those alerts matters. The National Weather Service issues a flood watch when conditions could produce flooding. A watch means the ingredients are in place, but water has not yet started covering roads. It is a signal to prepare and to keep checking for updates.

A flood warning is a step up. The National Weather Service issues one when flooding is happening or about to happen in the area named. For a driver, that changes the question from whether to be ready to whether to be on the road at all. Postponing the trip is the simplest response.

A flash flood warning carries the most urgency of the three. It means a flash flood is occurring or imminent. Flash flooding is a rapid rise in water that can develop within minutes or hours of intense rain. Small streams, dry creek beds and city streets with overwhelmed storm drains can fill quickly. In the U.S., flash flood warnings can be pushed to mobile phones through the Wireless Emergency Alerts system. A driver who receives one mid-drive should pull over somewhere safe before reading it. NOAA Weather Radio also broadcasts National Weather Service alerts around the clock.

If the trip goes ahead, the route deserves as much thought as the departure time. Water collects first in the lowest places. Underpasses, dips in the road and streets that run beside creeks or rivers are the spots most likely to flood. Choosing roads on higher ground cuts the chance of meeting standing water, even if the drive takes longer. Knowing a second route helps in case the first one closes.

Two simple preparations round out the plan. Leave with a fully charged phone. It keeps maps, alerts and emergency calls available if conditions change. Leave with enough fuel, too. Detours around closed roads add miles, and traffic can back up for long stretches during a storm.

Illustration: Quartz

Tire tread and wiper blades decide how well a car holds wet pavement and how clearly its driver sees in a downpour. Check both before the wet season begins.

Tread has a specific job on wet pavement. The grooves cut into a tire channel water out from under it. That keeps the rubber in contact with the road. As the tread wears, those grooves get shallower and move less water. Worn tires lose grip on wet pavement much sooner than tires with deeper tread.

The legal minimum tread depth in most of the U.S. is 2/32 of an inch. In the U.K., the limit is 1.6 millimeters. The U.K. rule applies across the central three-quarters of the tread and around its full circumference. That minimum marks the point at which a tire must be replaced, not the point at which wet grip begins to fall off.

Two quick checks show where a tire stands. Most tires have tread wear indicators, which are small raised bars molded into the bottom of the grooves. When the surrounding tread wears down level with those bars, the tire has reached the minimum. In the U.S., the penny test offers a rough reading. Insert a penny into a groove with Lincoln's head pointing down. If the top of his head is fully visible, the tread is at or below 2/32 of an inch.

Correct tire pressure matters as well. The right figure is listed in the owner's manual and on the placard inside the driver's door jamb. The number molded into the sidewall is the tire's maximum pressure, not the recommended setting. Pressure reads most accurately when the tires are cold, before the car has been driven far.

Wiper blades are the other half of the check. Replace blades that streak, skip or leave unwiped bands across the glass before the rainy months arrive. The rubber edge hardens and cracks with age and with exposure to sun and heat. A worn blade smears water across the windshield rather than clearing it.

Top up the washer fluid reservoir at the same time. The fluid helps clear the road grime that wet roads leave on the glass. On hatchbacks and SUVs, the rear wiper blade wears too and needs the same inspection.

Illustration: Quartz

Reduce speed and at least double the space to the car ahead when roads are wet. The U.K. Highway Code states that stopping distances on wet roads are at least double those on dry roads.

Both changes work together. A lower speed shortens the distance a car travels before it stops. A longer gap gives the driver more time to react before that distance runs out.

The Highway Code also turns the idea into a simple timing method. It advises at least a two-second gap behind the vehicle in front on roads carrying faster traffic. On wet roads, it says that gap should be at least doubled. That works out to a minimum of four seconds.

To measure the gap, pick a fixed roadside marker, such as a sign or a bridge. Start counting seconds when the car ahead passes it. If your car reaches the marker before four seconds have passed, the gap is too short for wet conditions.

A posted speed limit is a maximum, not a target. The Highway Code makes this point directly, noting that a limit does not mean the speed is safe in every condition. Heavy rain is a condition where the safe speed can sit well below the number on the sign.

Smooth control is the other half of the technique. Each tire has only so much traction to share between braking, steering and accelerating. On wet pavement that supply shrinks, so a sudden demand on any one of them can use it all up.

Brake earlier and more gently than usual. Steer in steady, gradual movements. Accelerate gently from intersections and traffic lights, where painted road markings can be slick in the wet.

Avoid cruise control in heavy rain. The system holds a set speed on its own, and the driver's foot is often away from the pedals. That can slow the driver's response when conditions change and the car needs to lose speed.

Illustration: Quartz

Switch on low-beam headlights whenever heavy rain cuts visibility. In a downpour, headlights mainly help other drivers see the car. Low beams do that without throwing glare back into the driver's own eyes.

High beams work against a driver in heavy rain. Their light hits the falling drops and reflects back toward the windshield. The result is a bright wall of glare that can make the road ahead harder to see, not easier. The same effect happens in fog and snow.

Low beams point down toward the road surface. That angle lights up lane markings and the pavement edge while sending less light into the rain directly ahead. It also avoids dazzling oncoming drivers, who are already dealing with spray and a wet windshield.

Daytime rain is where many drivers skip this step. A gray afternoon can seem bright enough to drive without lights. Yet an unlit car blends into wet pavement and spray. That is especially true for a driver glancing at a mirror before changing lanes.

Daytime running lights are not a substitute. On many vehicles, they light only the front of the car and leave the tail lights dark. A driver approaching from behind in heavy rain may not spot that car until late. Turning on the low beams usually switches on the tail lights as well.

Automatic headlight systems can also fall short. Many respond to ambient light levels rather than to rain. They may stay off during a daytime storm. Checking the dashboard indicator, or moving the switch to the on position, removes the doubt.

The law points the same way across much of the U.S. Many U.S. states require headlights whenever windshield wipers are in use. The details differ from state to state. Drivers who cross state lines should check the rules where they are driving.

Hazard lights are a different matter. Using them while the car is moving is restricted in many places. Other drivers may also read flashing hazards as a sign of a stopped or disabled vehicle.

Clean, working lights matter as much as the switch position. A burned-out bulb or a lens clouded with road grime puts out less light. A quick walk around the car with the lights on, before a storm arrives, shows whether every lamp is working.

Illustration: Quartz

If a car starts to hydroplane, ease off the accelerator, keep the steering wheel pointed straight and avoid hard braking. Hold that steady course until the tires grip the pavement again.

Hydroplaning happens when a layer of water builds up between the tires and the road and lifts the tires off the surface. Once that happens, the tires are riding on water rather than asphalt. Steering and braking lose most of their effect. In the U.K., the same thing is usually called aquaplaning.

The warning signs tend to arrive quickly. The steering can suddenly feel light or loose, as if the wheel has come disconnected from the road. If the driven wheels lose contact, the engine revs may jump while the car fails to gain speed. That rise in engine noise can happen even though the driver's foot has not moved.

Lifting off the accelerator is the first step. It lets the car slow down gradually. A lower speed gives the tires a chance to push through the water film and reconnect with the road. Hard braking on a surface with almost no grip can unsettle the car and make a skid more likely.

Keeping the wheel straight matters for a similar reason. Any steering angle applied while the tires are floating does little at the time. When the tires touch down again, though, the car will head where the front wheels are pointing. A sharp turn of the wheel during hydroplaning can become a sudden swerve the moment grip returns.

If the rear of the car starts to slide, small and gentle steering corrections work better than large ones. Steer toward the direction the front of the car should go. Avoid jerking the wheel back and forth.

Anti-lock brakes do not prevent hydroplaning. ABS stops the wheels from locking under braking. It cannot create grip when the tires are not touching the road. Once the car has slowed and traction has returned, any braking should use steady, moderate pressure. A sudden stab at the pedal can break the grip again.

Some conditions make hydroplaning more likely than others. Higher speeds, deeper standing water and tires with worn tread or low pressure all raise the risk. Puddles along the road's edge are common places for it to start. So are ruts worn into the lanes, where water collects. Driving in the tracks of the vehicle ahead can reduce how much water the tires meet. Its tires have already pushed some of the water aside.

Illustration: Quartz

To clear a fogged windshield in the rain, run the defroster together with the air conditioning. The air conditioning pulls moisture out of the cabin air, and the defroster blows that drier air across the glass.

Fog forms on the inside of the windshield when warm, humid cabin air meets glass that rain has cooled. Wet clothes, dripping umbrellas and passengers' breath all add moisture inside the car. Running the air conditioning with the heat turned up is not a contradiction. The system removes water from the air even while the heater warms it.

Most cars also have a rear defroster, a grid of thin heating lines built into the back window. Switching it on clears the view behind the car. That view matters for lane changes in poor conditions. A film of grime on the inside of the windshield gives condensation more surface to cling to. Cleaning the inside of the glass before the wet season helps it clear faster.

Trucks and buses create a separate visibility problem. Their large tires throw up heavy spray that can coat a following car's windshield in seconds. That spray hides the road ahead, the lane markings and the brake lights of vehicles farther up. Keeping a larger gap behind a truck or bus than behind a passenger car reduces how much spray reaches the glass.

Overtaking a large vehicle in heavy rain calls for a clear decision. Either pass it decisively, without lingering alongside in the spray, or stay back. Sitting beside a truck's rear wheels for a long stretch keeps the car in the worst of the water. It also keeps the car where the truck driver has a limited view of it.

Rain can become heavy enough that visibility drops to near zero. In that case, the general practice is to pull off the road, well away from traffic, and wait for the downpour to ease. Turn on the parking lights while stopped.

Leaving the tail lights on at the roadside carries its own risk. Drivers behind who are struggling to see may follow those lights and end up leaving the road behind the stopped car. Choosing a spot well clear of the travel lanes lowers that danger.

Illustration: Quartz

Do not drive into a flooded road, however shallow the water looks. The National Weather Service built its "Turn Around, Don't Drown" campaign around that single instruction. The slogan asks drivers to make the decision before the front wheels reach the water.

The NWS uses specific depths to show how quickly the danger grows. A foot of water can float many vehicles, according to the agency. Two feet of water can carry away most vehicles, including SUVs and trucks. A higher ride height does not make a vehicle safe in moving water.

Leaving the car to wade across carries its own danger. Six inches of fast-moving water can knock an adult off their feet, according to the NWS. That depth sits well below the knee of most adults.

The CDC reports that roughly half of flood-related drownings involve people driving into hazardous floodwater. The figure ties a large share of flood deaths to a single decision made behind the wheel.

From the driver's seat, floodwater gives away little about how deep or fast it is. Muddy water hides what lies beneath it. A flooded stretch can be deeper in the middle than it appears at the edge. Moving water is also harder to spot at night or in heavy rain, when headlights and wipers limit what a driver can see.

The road itself may no longer be there. Floodwater can wash away pavement or erode the ground beneath it. That can leave a gap or a collapsed section that is invisible from the surface. A driver who assumes the road continues under the water has no way to confirm it.

Barricades and road-closed signs mark roads that authorities have shut, and drivers should never go around them. Water levels can keep rising after a car starts across. A crossing that looked passable can become impassable before the car reaches the other side.

The alternative is to stop, turn around and find another route, or wait for the water to recede. A detour costs time. Driving into floodwater can cost a vehicle or a life.

Illustration: Quartz

If a car stalls in floodwater, do not try to restart it. If the water around the car is rising, get out and move to higher ground as soon as it is safe, then call emergency services.

Trying to start it again can turn a stall into a badly damaged engine. An engine runs on air drawn in through an intake. When that intake sits below the waterline, the engine pulls in water instead.

Water cannot be compressed the way air can. A piston rising against a cylinder full of water stops hard, a failure mechanics call hydrolock. The force can bend connecting rods or crack engine parts. Cranking the engine again makes the starter push the pistons against that water once more. That can add to the damage.

A stalled car also loses its main advantage in a flood: the ability to drive out. Once the engine is off, the vehicle sits wherever it stopped, and any current pushes against it. Water can also reach electrical components and cause them to short or fail.

Switching on the hazard lights, if they still work, makes the car easier for other drivers and rescuers to spot. The bigger question is whether to stay or go. Official guidance on that varies with the situation, including how fast the water is rising, how strong the current is and how close dry ground is.

Where water is climbing around the car, the conservative choice is to leave the vehicle rather than wait inside it. Before stepping out, look for a route that avoids moving water and leads to solid, higher ground.

Once out of immediate danger, call for help. The emergency number is 911 in the U.S., 999 in the U.K. and 112 across the European Union. Give the dispatcher the road name, the direction of travel and any nearby landmark or exit number so responders can find the car quickly.

Illustration: Quartz

Test the brakes with a few light presses as soon as the car is clear of standing water. Wet brake components can reduce stopping power, and a quick test shows whether the car will stop normally.

Pick a stretch of road with no one close behind. Press the pedal gently a few times at low speed. Light pressure creates friction and heat, which helps dry the pads and rotors. If the car pulls to one side, the pedal feels soft or stopping takes longer than usual, leave extra room ahead. Then get the brakes checked.

A car that sat in deep water is a different case. Do not start a vehicle that was submerged, even if the water has drained away and the car looks normal. Water can reach the engine, the fuel system, the transmission and the electrical wiring. Turning the key can add to the damage. Have the car towed and inspected by a mechanic before anyone drives it.

Some signs of water intrusion are visible to an owner. A line of silt or debris on the door panels or seats marks how high the water rose. Damp carpets, a musty smell and moisture inside headlight or taillight housings point the same way. Engine oil that looks milky on the dipstick suggests water has mixed with it. Mention any of these signs to the mechanic.

Roads leaving a flooded area carry their own hazards. Floodwater can undermine pavement and leave road edges crumbled or washed away. It can also leave branches, rocks and other debris across lanes. Choose routes that avoid broken shoulders and scattered debris, even if the detour is longer.

Downed power lines are a separate hazard. Treat any fallen line as live, and do not drive over it or through water touching it. Report it to the local utility or emergency services.

If the car was flooded, report it to the insurer promptly. Take photos of the water line, the interior and the engine bay before any cleanup begins. Coverage depends on the specific policy. The insurer can explain what a claim includes.