Recognizing Rain Dangers on the Road

November 16, 2001 at 2:43 AM CST - Updated July 12 at 3:16 AM

Rainy driving tips - In stormy conditions, it is more difficult to see other vehicles, road signs and the road itself. It is critical to make sure you can see and be seen.

  • First and foremost: slow down! It takes longer to stop or adjust in wet weather.
  • Stay toward the middle lanes - water tends to pool in the outside lanes.
  • Maintain proper following distance 3 Second Rule. This needs to be increased in wet weather.
  • Drive in the tracks of a car ahead of you.
  • Don't follow large trucks or busses too closely. The spray created by their large tires reduces your vision. Take care when passing them as well; if you must pass, do so quickly and safely.
  • Be more alert when driving in wet or slippery conditions. Watch out for brake lights in front of you.
  • Avoid using your brakes; if possible, take your foot off the accelerator to slow down.
  • Turn your headlights on even in a light rain, or in gloomy, foggy or overcast conditions. Not only do they help you see the road, but they'll help other drivers see you. If your car has daytime running lights you still should put them on, so vehicles behind you can see you better.
  • Before it starts to rain, replace old or brittle wipers.
  • Avoid off-road driving: it's hard to judge the actual depth of puddles and you can easily become stuck, even in an SUV.
  • Never drive beyond the limits of visibility. At night rainy roads become especially treacherous. The glare of oncoming lights, amplified by the rain on your windscreen, can cause temporary loss of visibility while substantially increasing driver fatigue. In rainy conditions pedestrians, livestock, and wildlife are extremely hard to spot and even harder to avoid.
  • Never drive through moving water if you can't see the ground through it; your car could be swept off the road.
  • When driving through a puddle of uncertain depth, go slow. If it's deeper than the bottom of your doors, turn around and find another route. Deep water can cause serious damage to a modern car's electrical system.
  • Avoid splashing pedestrians.
  • If possible, stay off the road during heavy thunderstorms. Large flashes of lightning can temporarily blind and disorient drivers, and the accompanying high winds and heavy rain can create deadly driving conditions.
Slow down! This should be obvious but it also very important. People are so used to driving certain speeds on certain roads that sometimes they forget the need to slow down when inclement weather presents itself.

Before you go - Wet-weather driving demands gentle use of all the main controls - steering, clutch, brake and accelerator - and a larger allowance for errors and emergencies. When you begin a journey in rain, your shoes will be wet and liable to slip off the pedals. Scuff the soles on the rubber matting or carpeting of the car before you start the engine. All motorists should regularly check that their headlights, tail lights, brake lights and turn signals are working properly.

How are your tires? - Check your tires on a regular basis. Bald tires significantly reduce your traction on wet roadways, and offer little resistance to hydroplaning. When your tires run over water, the water is displaced and it needs somewhere to go quickly. The best place is between the treads of your tires. If your tires are bald, the water has no place to go and you end up riding on a layer of water, like a boat. (See Hydroplaning, below.)

Turn on your wipers -  Replace your wipers regularly, at least once a year. Wiper blades in bad condition don't clear water from the windshield very well and distort your view. Older vehicles may need to have the whole wiper arm replaced.  The arms bend over time and sometimes can't keep enough downward pressure to clear the windscreen, even with new blades installed. Wipers will often clear light rain from the windscreen with a few sweeps, then run on an almost-dry screen and leave smears of drying dirt. Don't be afraid to use the windscreen washers liberally: the fluid is cheap (99 cents a gallon) and the safety benefit is high. Carry extra during the winter.

Don't follow large trucks or busses closely. Splash and spray from these vehicles can obscure your vision, creating a potentially disastrous driving situation. Keep your distance, and your windshield wipers on, when other traffic is in front of you.

Turn on your lights - Whenever visibility is poor or it rains, headlights are a good way to let other drivers know where you are. It's both helpful to other travelers and makes you more safe. Remember, you are not the only one affected by poor visibility. You may be able to see cars without their headlights on but others may not have vision or windshield wipers as good as yours. Many states require headlights to be turned on when it is raining or when visibility is reduced to less than 500 feet.

Heavy rain - Heavy rain can overload the wiper blades, allowing an almost continuous sheet of water to flow over the screen. When visibility is so limited that the edges of the road or other vehicles cannot be seen at a safe distance, it is time to pull over and wait for the rain to ease up. It is best to stop at rest areas or other protected areas. If the roadside is your only option, pull off as far as possible, preferably past the end of a guard rail, and wait until the storm passes, seldom more than a few minutes. Keep your headlights on and turn on emergency flashers to alert other drivers.

Foggy windows - Rain or high humidity can quickly cause windows to mist up inside the car. In a car equipped with air conditioning, turn up the heat and direct the airflow to your defrosters with the AC switch engaged. (Many cars automatically engage the AC when switched to the defrost mode.) In a car without AC the procedure is the same, but you may need to open your side windows to get the air moving. Most modern cars have a built-in rear window defroster that easily clears a misted rear windscreen by heating up electrodes embedded in the glass.  If you don't have one, put your defroster on high and its hot air will eventually follow the inside of the roof down to the rear window. If the car has swiveling dashboard vents, adjust them so that the air flow strikes the upper edge of the side windows. The airflow will clear the side windows first, finally traveling to the rear of the car. If all else fails, a rag or article of clothing will work as well; you'll just need to clear the window more often. Drivers should regularly clean their windshield and windows, both on the inside and outside, to help them see in good and bad weather. Smokers need to take extra care to make sure their interior windows are clear of a buildup of smoke residue.

Handling a skid - Losing control of your car on wet pavement is a frightening experience. You can prevent skids by driving slowly and carefully, especially on curves. Brake before entering the curves. Steer and brake with a light touch. If you find yourself in a skid, remain calm, ease your foot off the gas, and carefully steer in the direction you want the front of the car to go. This procedure, known as "steering into the skid," will bring the back end of your car in line with the front. For cars without anti-lock brakes, avoid using your brakes. If your car has ABS, brake firmly as you "steer into the skid."

Expressway driving - Leave lots of space between you and the car in front because it takes longer to stop. You're supposed to leave a few seconds between cars in dry weather. Make sure you add space in wet weather because if you have to hit the brakes hard, your tires will lock up, you will hydroplane and you will most likely hit the car in front of you. If available, drive in the fast lane, where there are fewer cars and less oil deposited on the road. Also, because of the built-in slope of the road, water drains towards the slower lanes. Avoid lane changes, as water tends to build up between the tire ruts in the lanes.

Oily deposits - Watch for intersections because of the oil spots in the road. Rain is most dangerous when it falls after a long, dry spell on to roads that have become polished and smooth: the rain blends with oil and rubber-dust deposits on the road surface to form a highly dangerous skid mixture. That mixture builds up at intersections, where cars stop and start frequently. Be extra careful immediately after it starts raining because it takes a while for the worst of the dirt and oil to get washed off the road.

Driving Through Water - Where water has flooded onto the road, drive very slowly and cautiously. Never drive through moving water if you can't see the ground through it: you and your car could be swept off the road, possibly finishing you both. Stop the car before entering the flooded area and check the water level ahead. Generally, if the water is deeper than the bottom of your doors or the bottom third of your wheels, it is inadvisable to attempt driving through it. Seek a detour rather than braving the flood and risking damage to your electronic control systems. Attempting to go through deep water can ruin any of these systems, creating a repair bill in the thousands of dollars.

At night it's much harder to see water hazards.You'll need good road observational skills to notice the difference between a wet road surface and flood water. Watch the contours not only of the road but also of fences, trees, hedges and buildings at the side of the road ahead. If they appear to be unnaturally low, slow down at once as the road is probably flooded. If you don't slow down and hit flood water at speed, the effect is almost like hitting a wall: you will first lose control, then come to a violent stop, possibly injuring your passengers as well. Watch out for places where floodwater collects, particularly low-lying roads adjacent to streams, and dips under rail or highway bridges.

If you determine it's safe to go on, proceed slowly and avoid making large waves in the water. If you have a manual transmission, engage first gear and keep the engine running fast by releasing the clutch just enough to partially engage gear and giving more acceleration than usual. This keeps the exhaust gases moving, helping to prevent water from entering your tailpipe. Vehicles with automatic transmissions should place the car in first gear and feather your brake, slowing the vehicle while at the same time keeping your revs up. Doing this for longer than a few seconds can seriously damage your vehicle and is not recommended. If you're submerged too deeply, your engine will stall and water might enter your engine through your air intake, causing a condition known engine hydro-lock, forcing you to replace it. 

What to do if you stall in deep water - If possible, have someone pull your vehicle out using a tow rope or cable winch. It may be possible to drive a manual transmission car out using the starter motor. You can make the job easier by removing your spark-plugs, thereby lowering your compression and making your starter turn more easily. Take great care not to let water enter the cylinders, as it will hydro-lock your engine, ruining it. This is a last resort for rescuing a flooded vehicle and is not recommended. 

Check your brakes - If you successfully pass through a deep water hazard, test your brakes. They may be saturated, and only driving very slowly and braking lightly at the same time will generate enough heat to dry them out. Be sure they are pulling evenly on all wheels before building up speed again.

Hydroplaning

Hydroplaning (called aquaplaning in Europe and Asia) occurs when water on the roadway accumulates in front of your vehicle's tires faster that the weight of your vehicle can push it out of the way. The water pressure can cause your car to rise up and slide on top of a thin layer of water between your tires and the road. While hydroplaning your vehicle rides on top of the water, like a water skier on a lake. In less than a second, your car can completely lose contact with the road, putting you in immediate danger of sliding out of your lane. This usually happens at higher speeds, over 40 miles per hour. Try to imagine your vehicle traveling at a high rate of speed on a sheet of ice: that image approximates what will happen if you try to brake or steer while hydroplaning. 

The 3 main factors that contribute to hydroplaning:
  • Vehicle speed. As speed increases, wet traction is considerably reduced. Since hydroplaning can result in a complete loss of traction and vehicle control, you should always reduce speed, paying attention to the traffic around you.
  • Tire tread depth. As your tires become worn, their ability to resist hydroplaning is reduced. 
  • Water depth. The deeper the water, the sooner you will lose traction, although even thin water layers can cause a loss of traction, including at low speeds.

Let's examine what happens to a tire in the midst of a hydroplane. When entering a puddle, the surface of the tire must moves the water out of the way in order for the tire to stay in contact with the pavement. The tire compresses some of the water to the sides, and forces the remaining water through the tire treads.  With good tires, a moderate rate of precipitation, and a well-drained roadway surface, hydroplaning rarely occurs below 55 mph. However, if any of those conditions are not met, it can happen at speeds as low as 35 mph.

On a smooth polished road in moderate rain at 60 mph, each tire has to displace about a gallon of water every second from beneath a contact patch no bigger than a size nine shoe. Each gripping element of the tread is on the ground for only 1/150th of a second; during this time it must displace the bulk of the water, press through the remaining thin film, and then begin to grip the road surface. Although bald tires give better grip on dry roads than treaded tires, they are unsafe in rain because water is a lubricant on rubber. (Also, punctures are more common in the rain.)

Hydroplaning science - Hydroplaning is the result of your tires moving quickly across a wet surface - so fast that they do not have sufficient time to channel that moisture away from the center of the tire. The result is that the tire is lifted by the water away from the road, losing all traction.

Of course the word 'quickly' is a relative term. Tread design, tread depth, weight of your vehicle, tire pressure, depth of water and even the consistency of that water - (whether it is highly aerated or not, for example) - all play a part in determining at what speed the tire will begin to hydroplane. It is a pretty safe bet to assume that any speed in excess of 60 MPH is fast enough to support hydroplaning regardless of the other variables. This is not to say that at 55 MPH you are safe, however. 
The exact point at which your vehicle becomes waterborne is complicated and depends on all of the following variables:
  • tire size - the size and shape of a tire's contact patch has a direct influence on the probability of a hydroplane.  The wider the contact patch is relative to its length, the higher the speed required to support hydroplaning. 
  • tire tread pattern - certain tread patterns channel water more effectively, reducing the risk of hydroplaning.
  • tire tread depth - as your tires become worn, their ability to resist hydroplaning is reduced. 
  • tire pressure - keep your tire pressure within the manufacturers recommended pressures.
  • water depth - the deeper the water, the sooner you will lose traction, although at higher speeds even thin water layers can cause a loss of traction.
  • water composition (oil, temperature, dirt, & salt can change its properties and density)
  • vehicle drive-train: because of their computer-assisted differentials, all-wheel-drive vehicles are more likely to hydroplane than two wheel drive vehicles in certain situations. A sudden uncontrolled transfer of power from the front tires to the rear tires can put a hydroplaning AWD vehicle out of control.
  • vehicle speed - as speed increases, wet traction is considerably reduced. Since hydroplaning can result in a complete loss of traction and vehicle control, you should always reduce speed.
  • vehicle weight - the lighter the vehicle, the more likely it is to hydroplane.
  • road surface type - non-grooved asphalt is considerably more hydroplane-prone than ribbed or grooved concrete surfaces. 
How can you tell that you're hydroplaning? It is often hard to tell when you are hydroplaning. The rear end of your vehicle may feel a little squirrelly (loose, giving you the sensation that it has moved to one side or the other), especially in a high crosswind. The steering may also suddenly feel loose or little too easy.  Watch the road ahead for standing or running water. You can also pay attention to the spray being kicked up by the cars in front. If it suddenly increases it's possible that the driver has hit a patch of water that could cause you to hydroplane.
What to do if you start to hydroplane -  There are two absolutely essential no-no's to remember should you experience the beginning of hydroplaning:
  • Do not apply your brakes 
  • Do not turn your steering wheel 
If you find yourself hydroplaning, do not brake or turn suddenly. This could throw your car into a skid. Think of your steering wheel as the rudder of a boat(your vehicle is a boat when in the middle of a hydroplane). Hold the wheel firmly and don't steer in any other direction but straight ahead. Ease your foot off the gas until the car slows and your steering returns to normal. If you need to brake, do it gently with light pumping actions. If your car has anti-lock brakes, then brake normally; the car's computer will mimic a pumping action, when necessary. If your vehicle’s tires are still in partial contact with the road surface, you should be able to regain control of the vehicle in the same way that you would on snow or ice.