Driving/Safety/Wet weather driving

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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

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Road debris and other hazards contribute to potential problems.

Engine oil and grease build up on the roads

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Over time, grease and engine oil can build up on road surfaces, especially roads that are frequently used such as interstate and state highways.[1] 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.[1] Although this oil and grease is washed away after a few hours of rain, those first few hours are critical. The stopping distance for a car traveling at 50mph is 65m.[1]

Crowned roads

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In the United States, most roads are crowned, or built so that the center of the road is slightly higher than the sides.[1] The purpose of crowning is to allow rain to run off the roads. However, in many cases, especially in the spring, sand from winter plowings and dead leaves build up along the sides of the roads, trapping the water and preventing natural runoff. This creates puddles on the roadsides, which can lead to hydroplaning and spray that obstructs a driver's view.[1]

Puddles and cross-running water

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A hydroplaning tire.

A puddle may hide a very large pothole that can damage a tire or an electrical component.[1][2] A puddle may also splash a large quantity of water up onto the windshield and obscure vision for an extended period depending on the puddle size. Another danger of puddles is hydroplaning, when the tires lose contact with the road and instead ride on a pocket of water. This makes the vehicle very hard to control as it loses traction with the road and prevents the vehicle from steering, braking, or accelerating.

Running water may also be dangerous when driven through. If the current is powerful enough, it can push the car off of the road.

Vehicle Hazards

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Window Fog

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When it is raining, the windshield and other windows may quickly fog up, obscuring the driver’s vision. Window defoggers can eliminate the problem.[1][2] 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.[2] Most modern cars are fitted with Air Conditioning, running the A/C in wet or winter months also removes moisture from inside the car and is a very quick way to defog a windscreen, whilst turning your A/C off whilst driving can very quickly cause your windscreen to fog dangerously.

Balding and Improperly Inflated Tires

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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.[3] 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.[3]

Braking Power/Stopping Distances

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Average Dry Stopping Distance (Assuming two-thirds of a second reaction time):[4]

Speed Thinking Distance Braking Distance Total Stopping Distance
20 mph 20 feet (6.1 m) 20 feet 40 feet (12 m)
30 mph 30 feet (9.1 m) 45 feet 75 feet (23 m)
40 mph 40 feet (12 m) 80 feet 120 feet (37 m)
50 mph 50 feet (15 m) 125 feet 175 feet (53 m)
60 mph 60 feet (18 m) 180 feet 240 feet (73 m)
70 mph 70 feet (21 m) 245 feet 315 feet (96 m)
80 mph 80 feet (24 m) 320 feet 400 feet (120 m)

Average Wet Stopping Distance (Assuming two-thirds of a second reaction time):[4]

Speed Thinking Distance Braking Distance Total Stopping Distance
20 mph 20 feet (6.1 m) 40 feet 60 feet (18 m)
30 mph 30 feet (9.1 m) 90 feet 120 feet (37 m)
40 mph 40 feet (12 m) 160 feet 200 feet (61 m)
50 mph 50 feet (15 m) 250 feet 300 feet (91 m)
60 mph 60 feet (18 m) 360 feet 420 feet (130 m)
70 mph 70 feet (21 m) 490 feet 560 feet (170 m)
80 mph 80 feet (24 m) 640 feet 720 feet (220 m)

When the road is wet, the average stopping distance for a 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 may be even larger. Therefore, drivers should position themselves twice as far away from the vehicle in front of them.

Water buildup on brakes

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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 ensures that the brakes will be there when they are needed.[1]

Windshield wipers

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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.


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  1. a b c d e f g h Kim, Liz, and Joanne Helperin. "Tips and Techniques for Driving in Rain". Edmunds.com. Retrieved February 13, 2011.{{cite web}}: CS1 maint: multiple names: authors list (link)
  2. a b c Clemens, Kevin. "Safer Wet-Weather Driving". Mobil 1. Retrieved February 13, 2011.
  3. a b "What are Bald Tires?". Wisegeek. Retrieved February 13, 2011.
  4. a b Wren, Eddie (August 2004). "Stopping Distances". Drive and Stay Alive. Retrieved February 13, 2011.