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Wet Weather Driving

Wet weather driving is the operation of a motor vehicle in the rain, snow, sleet and fog. It is considerably more dangerous than motor vehicle operation in dry conditions. While driving through snow, many drivers slow down, limit distractions and drive carefully. However, in the rain and fog, people do not take the same precautions, leading to thousands of accidents a year in the United States alone. Most of these accidents are preventable; caused by drivers who fail to realize that wet weather driving is fundamentally different than dry weather driving.

Road Hazards

Engine oil and grease build up on the roads

Over time, grease and engine oil can build up on road surfaces, especially roads that are frequently used such as interstate and state highways. The longer the roads are dry, the longer oil and grease have to build up. When the roads are dry, this is not a problem, however, when the rain starts to fall, it brings this oil and grease up to the surface and it rises above the water coating the road. This makes the wet conditions even more dangerous. Although this oil and grease is washed away after a few hours of rain, those first few hours are critical.

Crowned Roads

In the United States, most roads are crowned, or built so that the center of the road is slightly higher than the sides. The purpose of crowning is to allow rain to run off the roads. However, in many cases, especially in the spring, buildup such as sand from winter plowings, and dead leaves has built up along the sides of the roads, trapping the water by preventing natural runoff. This creates puddles on the roadsides, which can lead to hydroplaning and spray that obstructs view.

Puddles and Cross Running Water

Also see Hydroplaning (tires)

A hydroplaning tire.

Running through a puddle can be very dangerous for a variety of reasons. They could be hiding a very large pothole that can damage a tire or even damage electrical equipment in your engine compartment. They can splash a large quantity of water up onto the car windshield that can obscure vision for extended periods depending on the puddle size. Another danger of puddles is hydroplaning. This is when the tires lose contact with the road and instead ride on a pocket of water. This is very dangerous because it makes the vehicle very hard to control as it loses traction with the road, and prevents the vehicle from steering, breaking or accelerating. Running water can also be very dangerous when driven through. If the current is powerful enough it can push the car off of the road.

Other Cars

Other cars on the road can provide a significant hazard to any driver during the rain. The driver has no idea what the other drivers that are only feet from them are doing, their level of visibility, if they are paying attention or if their cars are in good repair with good tires. Other cars can also kick spray, obstructing the driver’s view. When driving in the rain, buses and large trucks kick up the most spray and should be followed from a distance three times larger than when the road is dry.

Vehicle Hazards

Window Fog

When it is raining, the windshield and other windows will quickly fog up, obscuring the driver’s vision. This is easy to fix, all the driver needs to do to avoid this fog is to make sure that the window defoggers are on and that the air conditioning is running. Many rear windshield defoggers consist of wires that run across the glass. When the defogger is activated, these wires heat up to clear the fog from the glass. However, in many cars some of these wires are broken, many times due to hasty and overzealous cleaning. Most of the time, these wires can be repaired, or a new windshield can be installed with new wires.

Balding and Improperly Inflated Tires

Over time, tires lose their tread in a process that is generally known as balding. Tires will all wear down over time, although aggressive driving and alignment problems can speed up the process. The tire tread is what allows the tire to grip the road and take away water on a wet road, and a vehicle that lacks adequate tread has a greater risk of [hydroplaning] on wet surfaces. Tires should have a tread of at least 1/16 of an inch, anything less and it is time to replace it. A trick to see if the tire in question has enough tread is to stick a Lincoln penny into the tire tread, with Lincoln’s head pressed into the grove. If the top of Lincoln’s head is visible it means there is not enough tread. Tires that are not properly inflated are also a driving hazard that is magnified in wet road conditions, and are at risk of blowing out should the car go into a skid.

Breaking Power/Stopping Distances

Average Dry Stopping Distance (Assuming two-thirds of a second reaction time):

Speed	Thinking Distance	Braking Distance	Total Stopping Distance
20 mph	20 feet	20 feet	40 feet
30 mph	30 feet	45 feet	75 feet
40 mph	40 feet	80 feet	120 feet
50 mph	50 feet	125 feet	175 feet
60 mph	60 feet	180 feet	240 feet
70 mph	70 feet	245 feet	315 feet
80 mph	80 feet	320 feet	400 feet

Average Wet Stopping Distance (Assuming two-thirds of a second reaction time):

Speed	Thinking Distance	Braking Distance	Total Stopping Distance
20 mph	20 feet	40 feet	60 feet
30 mph	30 feet	90 feet	120 feet
40 mph	40 feet	160 feet	200 feet
50 mph	50 feet	250 feet	300 feet
60 mph	60 feet	360 feet	420 feet
70 mph	70 feet	490 feet	560 feet
80 mph	80 feet	640 feet	720 feet

When the road is wet, the average stopping distance five passenger automobile is twice what it is on a dry road. Coupled with poor visibility, which can lengthen the time the driver takes to react, the stopping distances can be even larger. Therefore, drivers should position themselves twice as far away from the vehicle in front of them.

Water buildup on brakes

After driving through a puddle, brake rotors can become coated with water that can affect their ability to safely slow the car. In order to prevent this from happening, the driver should lightly tap the brakes after each puddle; this removes the excess water and insures the brakes will be there when they are needed.

Windshield wipers

The largest factor that affects driver visibility in the rain is the windshield wipers. Older wipers tend to become cracked and brittle after long periods of use, lasting only six months to a year in areas with frequent rain. However, wipers do not last forever even if they do not get much use. They can become brittle and warped by the sun, rendering them useless in heavy rain. There are several indicators that the wipers need to be replaced including: windshield streaking, windshield smearing, rubber that flakes off when touched, wiper screeching, and frayed wipers.

Driver Techniques

Visibility

The best way to stay safe in the rain is to be able to see. Auto glass should always be cleaned inside and outside to avoid oily buildup that will obstruct vision. Headlights should always be replaced immediately after they go out, not only because it is safe but it is the law in the United States. While driving in the rain, always have low beams on, never use high beams. When the high beams are on, the light reflects off of the rain drops and impairs vision even worse. Lastly, spray on wax from car washes can cause water to bead up on windows. Always clean windows again after a car wash to prevent this from happening.

Being Seen

Even during the day, when driving in wet weather headlights should always be on. This can help other drivers see the car from both the front and the back. If road conditions cause a major slowdown, or if driving in the breakdown lane under the speed limit, it is a good idea to have the hazard lights flashing just to alert other drivers of the car’s presence.

Leave early and slow down

Traffic slows down in wet weather. Plan on leaving earlier and spending more time in transit. It may be that the way that you were planning to go was blocked or flooded, and leaving early allows you to go a different way. There is another key benefit of going slow. Peter Cunningham, a championship-winning race car driver, has this to say about wet weather driving, “Slow down. As your speed increases, the tire footprint - the amount of the tire's tread contacting the road surface - decreases, providing less traction. By slowing down, you also reduce the risk of hydroplaning should you run into [puddles] on the road.” When in doubt, slow down.

Know when you are fit to drive

Driving in wet weather requires more focus, energy and stamina than driving in dry weather. For safety, driver distractions such as the radio, cell phone usage, drinks and passenger conversation should be kept to a minimum so that the driver can concentrate on the road with a minimum amount of distraction. If the road conditions become too dangerous, for example if the driver can no longer see the road in front of them, or if they feel tired and unable to devote enough energy to driving they should find a safe spot to pull off the road to rest or wait for better conditions.

Watch for Pedestrians

This is very important if not on a highway. With your vision impaired by the rain, it may be harder to spot pedestrians, especially at night and if they are wearing dark clothing. Rain can also affect their ability to hear traffic, and if distracted, they will not see the car’s headlights. Always drive slower than normal in heavily populated areas to avoid any kind of accident.

Be prepared

Even after taking all necessary precautions, accidents happen, and it is a good idea to prepare for them before hand. For more info on emergency kits go here.