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9. Auto Accidents: Causes & Prevention

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23. Snow & Ice

Sometimes a road that is normally safe becomes dangerous when slippery. Ice and packed snow on the road can cause your vehicle to skid, especially if you are driving fast or traveling downhill.

You should always use your low beams on frosty mornings when other drivers' windows may be icy or foggy. Do not use your high beams in a snowstorm because they will reflect off of the falling snow and make it difficult to see.

To drive safely in snow or ice you should:

  • carry special tires or chains and know how to install them
  • obtain maximum visibility of the details ahead by use of low beam headlights, windshield wipers, and defrosters
  • reduce speed before turns, curves, and shady areas
  • allow a greater distance in front of your vehicle for stopping
  • watch for signs warning of potential icy conditions
  • watch for areas in which ice is likely to collect such as on bridges
  • avoid stopping or turning suddenly
  • exercise caution in down-shifting
  • should avoid unnecessary stops

  • If you will be driving in an area where it has snowed or may snow, carry chains in case you find yourself in conditions where you can't drive without them. Make sure you carry the correct number of chains and that they will fit your drive wheels (which is your front wheels in a front-wheel drive vehicle, your rear wheels in a rear-wheel drive vehicle, and all wheels in an all-wheel drive vehicle).

    Learn how to put the chains on your vehicle before you need to use them. Check the weather before you leave.

    When driving on roadways where snow chains are required, temporary special speed limits may be posted which you must obey. These speed limits range from 25 to 40 MPH.

    When driving on packed snow, you should drive at least half as slow as you normally would. If there is ice on the road, you should slow to a crawl.

    Some road surfaces are more slippery than others when wet. These roads often have warning signs. Here are some clues to help you spot slippery roads:

  • on cold and/or wet days, shade from trees, buildings, and under over-overpasses can hide spots of ice-- these areas freeze first and dry out last
  • bridges and overpasses can also hide spots of ice -- they tend to freeze before the rest of the road does
  • close to the freezing point, the road is icy and may be more slippery than at colder temperatures

  • In California, we also have a special problem with black ice. Black ice is very difficult to see on the roadway and appears more like a wet spot than a patch of ice.

    Even when you see no apparent ice on the roadway, black ice may be present. Black ice is likely to form on shady parts of the roadway such as the back sides of turns or hills. Slow down and be especially careful when the weather is cold enough to form black ice.

    Black ice typically refers to a thin coating of glazed ice on a surface, often a roadway. While not truly black, it is transparent, allowing the usually-black asphalt to be seen through it. It also is unusually slick compared to other forms of ice on roadways.

    It is usually deposited by extremely cold rain droplets, mist, or fog. The process of freezing is slowed down due to latent heat given off in sublimation, allowing the rain droplets to flow and merge together on the surface forming a film before freezing into clear ice.

    Because it contains relatively few air bubbles, black ice is transparent and thus very difficult to see (as compared to snow, frozen slush, rime ice, or other typical forms of ice on roadways). In addition, it often has a matte appearance rather than the expected gloss; and often is interleaved with wet pavement, which is identical in appearance. For this reason, it is especially hazardous when driving or walking because it is both hard to see and extremely slick.

    Black ice may form even when the ambient temperature is several degrees above the freezing point of water. This may occur when:

    • terrain contours and/or winds cause a steep difference of atmospheric pressure and/or temperature
    • the atmosphere has warmed up after a prolonged cold spell that leaves the temperature of the ground and roadway well below the freezing point

    To help avoid skidding on slippery surfaces, you should:

  • drive more slowly and stay farther behind the vehicle ahead
  • slow down as you approach curves and intersections
  • avoid quick stops and use threshold braking as outlined earlier (unless you have antilock brakes)
  • shift to a low gear before going down steep hills
  • Lesson Summary


    Lesson 9 Quiz

    You will now answer 5 questions to test what you learned during this lesson. You must answer all questions correctly to receive completion credit for this lesson. You may answer the questions as many times as necessary to get them right.

    You should review the lesson material if you don't do well on the quiz.

    1. To avoid tailgating, and help avoid a rear-end collision, you should give yourself a gap of how many seconds behind the car in front of you?


    2. Teenage drivers have a total accident rate that is _____ times that of adults:


    3. Failure to yield is the primary cause of what percentage of fatal and injury collisions?


    4. If you are distracted for one second, by a cell phone, passenger, or other distraction, at 30 mph you will travel how far "blindly"?


    5. If you have a tire blowout:



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