Driving in Rain & Fog


Statistics

Based on the most current data, there were almost four times more fatal crashes during rainy conditions than during snow/sleet conditions – Bureau of Transportation Statistics

Roughly a quarter of all accidents (one in four) occurs during poor weather and are labeled “Weather Related”.  Weather-related crashes are defined as those crashes that occur in adverse weather (i.e., rain, sleet, snow, and/or fog) or on slick pavement (i.e., wet pavement, snowy/slushy pavement, or icy pavement). – National Highway Traffic Safety Administration (NHTSA)  

Visibility distance is reduced by fog and heavy precipitation, as well as wind-blown snow, dust and smoke. Low visibility conditions cause increased speed variance, which increases crash risk. Each year, over 38,700 vehicle crashes occur in fog. Over 600 people are killed and more than 16,300 people are injured in these crashes annually. – Federal Highway Administration

Overview

Driving when the road is wet presents special challenges - reduced traction, longer braking distances, and less control.  This is even more of a challenge when the road is covered with oils, mud, fallen leaves or other accumulated debris that become very slick with the first rainfall after a dry spell.  Add to this mix thick fogs that can dramatically reduce your visibility and you really have to be on guard as you drive.

Pre-Trip Preparation

Most drivers have a plan to be prepared for rain and fog.  Since rain and fog reduce visibility, most pre-trip inspections include checking headlights, tail lights, brake lights, and turn signals so that your vehicle can be seen by other vehicles.  It’s also important to check your vehicle’s tire tread condition and inflation level to provide control on wet roads. Tires that are improperly maintained or have poor treads will generally make handling and stopping more difficult (and potentially more dangerous).

Visibility in rain or fog can be made worse if your vehicle has a dirty windshield covered with smears from bug impacts or road grime and oils.   Taking time to thoroughly clean your windshield (inside and out) can help you have the best visibility possible.  

Normal maintenance should extend to replacing windshield wipers on a periodic basis.  Waiting until you can’t see through the wash of water on your windshield is usually too late.

Multiple Issues with Driving in Rain and Fog

Rain creates several potential problems for drivers:

reduced traction,

longer braking distances,

generally less control of steering, and in some cases, “hydroplaning”

reduced visibility from the rain and fog creating a veil between vehicles  

Generally, the first rainfall after a dry spell creates the most difficulty for drivers.  During dry periods, the sun heats the asphalt and brings up the surface oils.  Additionally, dripping oil and grease from passing vehicles slowly accumulates (especially at intersections where traffic stops for signals).  These oils become very slick with the introduction of rain.  It is this combination of oils and water that can make it tough to stop or control your vehicle.

When heavier rains gradually wash the surface of the road, traction at lower speeds can be regained.  At higher speeds, your tires can actually ride up on top of the water that is on the road surface.  This is called “hydroplaning” and occurs when the tires are no longer capable of pushing away the water that accumulates in front of and under them.  Tires with adequate tread depth help to minimize this occurrence and tires that have very shallow treads will tend to hydroplane more easily.

The falling rain reduces visibility, contrast and light levels.  The rain, itself, makes it harder to spot other cars, pedestrians and changes in road conditions.  During heavy rain, like fog, the use of low beam headlights can help, but high beams may produce glare.  If your vehicle has “Daytime Running Lights”, these are not an adequate substitute for turning on your low beam lights (normal setting).  Some states actually require the use of lights whenever it is raining.

Basic Tips for Rain

There are many things to watch out for when driving in the rain:

Allow more travel time so you can drive slower than normal. This will give you the freedom to allow greater following distances, cope with heavy traffic that is moving more slowly than normal, and enable you to navigate any unexpected detours.

Turn on your headlights.  This helps other vehicles spot you sooner, and it helps you to see what’s ahead.

Watch out for pedestrians. Under difficult weather conditions, pedestrians may be distracted by fiddling with an umbrella or other rain gear. Additionally, falling rain tends to deaden sound, so pedestrians may not hear your vehicle approaching.

Give large trucks and buses extra room.  They have a harder time seeing other vehicles, and their larger tires tend to throw more spray when traveling at highway speeds.

Increase your following distance – allow more room between you and the vehicle in front of you – you need the extra time to react and decide what to do.  

Brake earlier and with less force than you would normally. This can increase the stopping distance between you and the car in front of you, and it also lets the driver behind you know that you're slowing down. Also, be careful about using turn signals early, so that other drivers know your intentions.  In general, it may help to take turns and curves with less speed than you would in dry conditions.

If you start to hydroplane, don't brake suddenly or turn the wheel, or you might spin into a skid. Release the gas pedal slowly and steer straight until the vehicle regains traction.

Never drive through deep puddles or flooded roads, particularly if you can't see the pavement through the water!  According to the National Oceanic & Atmospheric Administration, 50% of flood fatalities are caused by people attempting to drive through water. The depth of water on roadways is not always obvious and it can hide dangerous debris, potholes or the roadbed could be washed out.  Rising water could stall the vehicle's engine, trap the vehicle and sweep it away.

Roads that go through higher elevations can become especially dangerous as the weather may be different on top of a ridge than in the valley below.  When traveling in mountain areas, be sure to get a complete weather forecast for all elevation ranges.

Tips for dealing with Fog

When driving at night, or in heavy fog, your low beam headlights will illuminate a specific distance in front of your vehicle.  The area in front of your vehicle that is clearly illuminated and visible is your “sight zone”.  If an object, person or vehicle enters the “sight zone” in front of your vehicle, you may require a sudden stop or evasive maneuver.  

Most people take a moment to react to the object, recognize the threat (stopped car, pedestrian, startled deer, etc.) and then take additional time to react by shifting their foot to the brake pedal or by steering to avoid a collision.  In general, most people who are in top condition (not drowsy, ill, or distracted) take 1.5 seconds to perceive and recognize the danger and begin to react (call this the “reaction time”).   At 40mph, your vehicle travels 90 feet during the reaction time – this 90 ft. area is covered at full speed before you can begin to brake or steer around the obstacle (the “reaction zone”).   Even more time and space is needed to bring your vehicle to a stop or to steer a path around the problem.

When you are driving so fast that your lights don’t fully cover this “reaction zone”, you are driving too fast to react to a problem.  Basically, by the time you see the problem; you will not have enough time to react to it.  Many experts call this “overdriving” the headlights – setting your reaction zone out beyond the distance you can actually see.  

Another basic tip for dealing with fog includes using your low beam headlights.  High beam headlight settings will usually produce glare that simply lights up the fog and doesn’t increase the distance you can see ahead of your vehicle (the light is mostly reflected back towards your vehicle).  Finally, remember that there may be drivers coming behind you – make sure that they can spot your vehicle by using appropriate flashers or warning lights.

Summary

Rain storms and fog can occur at most any time of the year depending on your territory.  Pay attention to the weather conditions and take time to prepare. Leave extra time to get to your destination, and slow down when weather conditions interfere with your visibility or your vehicle’s control.

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