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Module 6

Forces that Can Affect Driving

Earlier, we talked about inner forces—internal forces that relate to the human body, such as anger or sleep deprivation and how they affect our ability to drive safely; now we are going to explore the different external forces that can affect how well your car performs under different conditions.

We will discuss gravity and its impact on the movement of motor vehicles. We’ll review the definition of inertia and its role in crashes. We’ll explore the concept of force impact – what happens when two heavy objects collide. By the end of this module, you will have a clear idea of how different forces – natural, mechanical and human – affect your car’s handling and your safety. prc

Part 1: Understanding Natural Forces

When it comes to controlling the motion of our cars, we have three basic mechanisms that are under our control. They are:
  1. Speed
  2. Braking
  3. Steering
If you think about it for a moment, you can see that these three actually boil down to only two things that we can do to control the movement of a vehicle:
  1. You can control how fast you’re going (changing the speed by using accelerator or brake)
  2. You can change the direction you’re going in (steering).

But these are not the only forces at play, nor are they even the most significant in some cases. When it comes to controlling our cars, we work in tandem with natural laws, which determine how vehicles may be safely operated.  In fact, many crashes – some serious or sadly, some even fatal – are caused by drivers who misjudge the power of natural forces. The forces that have the greatest impact on driving are:

  • Gravity:  Gravity, as you may recall from your study of Sir Isaac Newton, is the force that attracts objects to the center of the earth. It affects your speed when driving uphill and driving downhill.  Gravity also affects the amount of power your car needs to accelerate.  Perhaps most significantly, gravity affects how much time you need to speed up and to brake.
  • Friction: Friction is the resistance that is caused when one object or surface moves over another.  In driving, it is most relevant to a vehicle’s ability to brake. The heavier the vehicle, the more friction is created with the road. This means that a heavier car will be able to stop more easily than a lighter one. 
  • Traction:  Traction is the result of friction between the road and your car. It causes your tires to grip the road, and it is what allows you to control your steering.
  • Centrifugal force: Centrifugal force is what makes an object move outward when rotating around a center.  You may notice this when you turn your car; centrifugal force pulls your car away from the direction you want to turn (and is one of many reasons why you need to slow down before you turn).
  • Inertia: If you’ve ever been in a car when the driver had to brake suddenly, you may have noticed that loose objects in the car continue to move forward even after the driver slows or stops the car. This is called inertia, and it comes into play during a collision.
  • Kinetic energy: Kinetic energy is the energy your car gains because it is in motion.  The faster you and your car go, the more kinetic energy you acquire. This energy must be absorbed by the brakes and engine as the driver slows down and stops. If that doesn’t occur, then the energy will be absorbed by your body, the car and any objects you hit if you are in a collision.  Kinetic energy determines how much time and distance is needed to stop a car.
  • Force impact:  There are a number of things that determine the force of impact in a collision: kinetic energy, speed and weight of the car, safety features of the car, its so-called “crush zones” and the other object involved.  If two vehicles crash, the lighter one will take the greater impact.
If there is a collision, the type of crash has an effect on the how hard the vehicles involved will hit each other:
  • As you can imagine, the force of impact is highest in a head-on crash.  If both vehicles are moving at 50 miles per hour, the force of impact would be the same as hitting a brick wall at 100 miles per hour.
  • Impact force is still quite high in a side-impact crash, though slightly less than in head-on crashes.  Among the types of collisions, this is the second deadliest.
  • Impact force drops down to  “moderate” in a sideswipe collision.  Should you find yourself headed for a head-on or side-impact crash, you should try and maneuver into a sideswipe position if at all possible.
  • Impact force drops all the way down to “small” in a read-end collision. This is because the impact is transferred to the car in front of you, which is also moving forward.

Part 2: Mechanical and Human Factors

Here’s something you may not have thought about:  The kind of car you drive matters – for safety, and not just for aesthetics. Age, weight, size and model all impact how a car operates and how it handles under different conditions. These factors include how much power is needed for the engine to run, how quickly it is able to brake and even how much gas it uses.

Add to all of the above the different kinds of safety features – which affect how a car handles in an accident and how well it protects passengers ­– and you can see why car buyers have a lot to consider when selecting a car that is right for them.

While you never want to get into a crash, it is important to have a basic understanding of the safety features on cars that can help protect the driver and passengers in the event of a crash. Some of those features include:

  • Crumple zones: Car manufacturers now include crumple zones to protect people in front-end, rear and offset crashes (when about 40 percent of the front of the car is struck in a collision). As the name suggests, crumple zones were designed to ensure that the car, not its occupants, absorb the brunt of the impact of a crash.
  • Strengthened passenger compartment: Cars now come with reinforced “cabins” – the compartment where the driver and passengers sit.  Cabins now keep their shape better in head-on collisions. Car manufacturers have also tweaked the steering column, dashboard, roof pillars, pedals and floor panels so they are less likely to invade the cabin area in a crash than they used to be. Car roofs now are designed to better protect car occupants in a rollover crash, and doors are designed to stay closed during a crash.
  • Side impact protection: We’ve all seen that scene in a movie where a person is inside his car, unaware that a larger vehicle is about to come crashing into the driver’s side panel. That happens in real life, but recent safety improvements now protect passengers better with Increased side door strength, internal padding and better seats. Many new cars also have side intrusion beams or other reinforcements within the door structure and additional padding on the inside door panels.
  • Airbags: Airbags are installed in the front and side of cars to protect people from the types of severe injuries that result from serious collisions. Side airbags, also known as “curtain” airbags, provide a lot of protection during side impact and rollover crashes. But airbags have an important safety partner – you must also wear your seatbelt!  Airbags are designed to work together with a seat belt, not instead of it.
  • Head Restraints: These are often referred to as “head rests” because they are installed behind where your head will be when you’re driving.  But it’s a mistake to call them by that name.  Although you may want to lean your head back on a head restraint to rest when you are in the driver’s seat of a stopped vehicle, the important reason for head restraints being built-in is not for resting at all.

You might not have thought of it before, but head restraints are an important part of a car’s safety features. They should be fitted to all seats in the front and back of the car. But it’s not enough to simply have them there; head restraints are adjustable, and must be positioned properly, so that they are directly behind the back of a person’s head, not below it (as sometimes happens in the case of a tall person).

Remember:  the function of the head restraint is to protect the driver and passengers from whiplash caused by rear impact collisions. People get whiplash in these types of crashes because their necks extend backwards in the first stage of the crash before being thrown forward in the second stage.

It’s important to understand the features and limits of every car you drive – even if you are only planning to drive that car once! As a safe and responsible driver, you need to adjust your driving behavior to the car you are operating.  You also need to understand that the following factors can be different for every single car you drive. You should grasp their influence on you and the car under normal, hazardous or crash conditions:

  • Individual reaction time
  • Weather and road conditions
  • Vehicle weight
  • Brake conditions
  • Speed

Driving in Adverse Conditions

When you first practice driving with your parent or another qualified adult, probably that adult will insist that you only drive under ideal conditions: daytime driving, quiet streets, good weather. Eventually, though, you will need to gain experience driving in less than perfect conditions. You need to know how weather impacts your ability to brake, for example, or how fast you can safely drive in fog.

Driving in such hazardous weather conditions – such as snow, heavy rain, or thick fog – requires practice, preparation, caution and, above all, common sense.

For example, even a little snow can be hazardous – it doesn’t have to reach the status of blizzard to cause problems for drivers. Even a brief rain shower can wreak havoc on the road: that little bit of water mixes with the layer of oil and exhaust that accumulates on road surfaces during dry conditions, creating at mixture that can make roads exceptionally slick when it first starts to rain. And fog can suddenly reduce your visibility to near zero without much warning.

As we’ve mentioned above the first rule of driving in hazardous conditions is to use common sense. If your visibility is reduced due to fog or rain, slow down and turn on your lights. Leave more space between you and the car in front of you. If the weather conditions worsen and you no longer feel comfortable driving, get off the road and wait out the worst of it.

When you have to get off the road due to weather conditions or any other road hazard, you should:
  • Turn on your headlights
  • Signal to other drivers that you are pulling over
  • Pull off the road as far as you can (or turn onto a side street if you can)
  • Turn on your hazard lights so other drivers see you
  • If you are on a busy road, get away from the vehicle and the road in case your car is struck by     another motorist

The second rule of driving in hazardous conditions is to be aware ahead of time how to handle different situations. For example, ask your parents or your driving instructor to take you to a safe place – such as a large parking lot or quiet roadway – to practice driving in certain weather conditions. Try to stop quickly to see how much time and distance you need to brake in the rain or snow. Practicing controlled braking in all conditions is how you will strengthen your driving skills, including reacting appropriately to avoid a crash or losing control of your car.

Night driving

Driving at night can be tricky for all drivers, but it is particularly dangerous for new teen drivers. Need proof? For teenagers, the nighttime fatal crash rate is roughly double that of the daytime fatal crash rate. There are several reasons why driving at night is so dangerous: every driver, even an experienced one, is limited by nighttime conditions. Safe and responsible drivers adjust their driving behavior to accommodate those conditions.

  • Reduced road visibility:  Not surprisingly, it is harder to see anything at night. There are streetlights, but they are not as bright as daylight. Streetlights also light the road – not the buildings or street addresses you may be looking for as you drive. It’s also harder to judge distance in the dark.
  • Night vision is much worse than day vision: The effects of reduced light on vision are the same for all drivers – new and experienced. Peripheral vision  (the ability to see out the corner of your eye) is reduced. So is your depth perception. Less light makes it harder to focus your vision, and you could suffer more eyestrain if you drive a lot at night.
  • Glare from headlights: This is a big deal. Glare from the headlights of oncoming cars can really impact your night vision. Try not to look directly into other cars’ headlights. To avoid being blinded by the glare, direct your vision slightly to the right of oncoming cars. If the light from cars behind you is reflecting into your rearview mirror, flip the setting of the mirror to glare reduction. And definitely make sure your windshield is clean – dirty glass amplifies glare.
  • Animals: Nocturnal animals, such as raccoons, are more active at night. It can be harder to see —and tougher to avoid— these animals when they cross the road at night.  Be on the lookout.
  • Dangerous drivers:  Nighttime can be the dangerous driver’s playground – and you want no part of that. Law enforcement can tell you that there are more drunk and impaired drivers on the road at night – as well as on the weekends. Be on alert for strange driving behavior. Be especially careful if you notice someone weaving back and forth or drifting across the center line. If you see such a driver, increase the distance between you and that car.  If possible, turn on to a different road or pull over to wait until the impaired driver is gone.
  • SLOW DOWN! Just because the speed limit is 40 miles per hour, it doesn’t mean you have to drive that fast. It’s important to remember that speed limits are what the authorities have decided is safe for that road UNDER IDEAL CONDITIONS. Nighttime driving is not an ideal condition, so you should drive between 5 and 10 miles per hour below the posted limit.
  • Following distance: Always, always, always leave at least a four- or five-second following distance between you and the car in front of you. That’s the rule for ideal conditions, and as we’ve already said, nighttime does not represent ideal driving conditions. So leave even more space between your car and the car ahead of you at night or in bad weather. You need this time and distance in order to be able to respond safely.
  • Don’t out-drive your headlights! Only drive as fast as your headlights will allow. This means that you shouldn’t drive so fast that you can’t stop within the area lit up by your headlights.

Driving in the rain

Nighttime isn’t the only time you need to drive with extra care. Wet roads can be quite dangerous because they decrease your braking ability. In other words, you need more time and more distance to stop in the rain than you do in dry conditions. And it doesn’t have to be pouring: even a light mist can make roads slippery because the rain mixes with the oil on the road to create a greasy surface.

When driving in the rain, it’s very important to remember to:
  • Turn on your headlights and windshield wipers  –
  • Stay below the posted speed limit – that limit is set for dry conditions, not rain
  • Slow down when you come upon a curve – centrifugal force will kick in
  • Stay on the paved portion of the road and off the shoulder
  • Drive in the tracks of car ahead of you – this will help you avoid excessive water on the road
  • Allow more distance between you and the car in front of you – really! You will need this extra time and space to brake if the car ahead of you brakes
  • Brake carefully  – remember, there’s water on the road.
  • Avoid sudden maneuvers – again, don’t underestimate the impact that water on the road will have on how your car handles.


If you remember to take all the precautions listed above, you should be able to avoid getting into a skid, which is a really harrowing experience that can happen even on a road that you’ve been driving on smoothly for a while.  For example, you can be tooling along at 35 or 40 mph on a wide city street that’s already dried nicely after several days of rain—or so you thought.  But what you didn’t notice was a rather large size puddle, or even a big patch of wet leaves, that’s right ahead of you.  And before you can do anything about it, you’ve arrived at that spot and the car suddenly veers off — more or less “doing its own thing.”

What is actually happening in a skid is that your wheels are “locked up,” meaning they aren’t able to roll as usual, and instead remain fixed in place, and the car is then sliding instead of rolling, and what’s more, they’re sliding in the wrong direction, and basically going where you don’t want it to go.

It’s out of your control, and the way to get control back is counter-intuitive…that is, it’s not at all what you would logically think of.  Consider:  our usual habit when we want the car to stop doing whatever it’s doing is to put our foot on the brake.  And that’s the worst thing you can do in a skid.  It would only make things worse.  So the first thing to remember when you get into a skid is to take your foot off the brake.   This should unlock the wheels.

Keeping your foot off the gas pedal as well, you need to regain control by very gently maneuvering the  steering wheel so that you are steering in the direction of the skid.  As the vehicle slows down, straighten the front wheels gradually until you regain control.


Slow down when you see a lot of water on the roadway. One of the biggest risks when driving in the rain is hydroplaning. Hydroplaning happens when you drive too fast on roads that are covered in water – this makes your tires lose contact with road. Your tires are literally “surfing” on top of the water on the road. A slight change of direction or a gust of wind can throw your vehicle into a skid.

If you find yourself hydroplaning, you can regain control by taking your foot off the accelerator. You should not brake! As with a skid, this can make things  worse.

Flash Floods

Our climate in Nevada makes us susceptible to flash flood conditions. Considering the fact that over 50% of flash flood fatalities are automobile-related, it’s important that Nevada drivers be extremely cautious of weather conditions that can result in flash flooding. In Nevada, flash flooding is particularly common during the summer months but it is also common during the spring thaw and prolonged periods of rain.

Standing water may not seem like much, but it can be very dangerous.  Under no circumstance should you drive into deep water or fast moving water! Your car can be flooded or swept away quickly, with no warning and no chance of escape.  Once your tires lose contact with the road, you are at the mercy of the water.

Even if you are driving through shallow standing water, you should be extra careful, as you have no idea whether or not the road has washed away beneath you. Keep in mind that water is the most erosive force on earth. It takes only minutes to completely destroy a roadway.

When attempting to drive through shallow, standing water, you should do so very slowly to avoid splashing water into your engine compartment.  If you get water in your engine it can cause your car to stall, and you will be stranded. It’s also important to know the area you are driving through.  Don’t drive through low-lying or flood-prone areas during rain or flood warnings.

Always be especially cautious at night, as flood dangers are more difficult to see.

Here are some fast facts to keep in mind about flash flooding:
  • A driver can lose control of their vehicle in less than one inch of water
  • Most cars can float in 2 feet of water
  • Cars traveling at a high rate of speed can be completely pushed off the road in 6 inches of water

Driving in snow and ice

Even experienced drivers often dread winter driving, Snow, sleet, and ice are extremely dangerous and should be treated with a great deal of caution and respect. You should practice driving in snowy conditions with your parents or your instructor every chance you get.

Just as you do with rain, the first thing to do when driving in snow is to slow down and increase the distance between you and the car ahead of you.  Here are some important things to keep in mind when driving in winter conditions:

  • Bridges freeze before roads. The road leading up to the bridge may be fine, but that doesn’t mean there isn’t ice on that bridge. This is because a bridge comes in contact with freezing air above, below and on either side of it— unlike a road, which is only in contact with the air above it.
  • There could be ice underneath the snow. Your tires can grip snow, allowing you to drive. But it’s very difficult for tires to grip ice, and often there is ice underneath the snow on a road.
  • Drive in the tire tracks other cars have made in the snow. You do this for the same reason as you do with rain: there will be more traction in these areas and you are less likely to skid.
  • Be wary of changing lanes. This is really important. Very often the space in between lanes has a built-up layer of compacted ice that you should avoid. You do not want to go into a skid while you are trying to change lanes, so it’s better not to change lanes in snowy conditions if you can avoid it.
  • Test your brakes in a safe place on the road before you need to use them, in order to get a sense of how they react to the conditions.
  • Beware of black ice.  Unlike the name suggests, this type of ice isn’t black – it’s called black ice because when it forms on roads the black color of the road shows through. That’s one reason why this type of ice is so treacherous – it’s hard to spot on roads because the lack of air bubbles makes it transparent. In other words, it’s very difficult to see when a road is covered in black ice. Your first indication may be when you hit a patch and all the other “road noise” stops because you are coasting on ice. If this happens, take your foot off both pedals and hold the wheel straight until you cross that patch and hit asphalt again.

The best idea is to stay off the road when any winter travel advisories are in place.  If you must drive, go slow, stay in the right lane and keep your headlights on! Remember the following safety tips when driving in winter conditions:
  • Always use your headlights and windshield wipers (when it is snowing or sleeting).
  • Keep your speed slow and steady.
  • Slow down when you approach corners, curves and shaded areas. This is where black ice often lives.
  • Keep more distance between you and the car in front of you.
  • Keep an eye out for patches of ice. Remember, sometimes it’s underneath the snow
  • Try to avoid any sudden moves, such as turning, swerving or hard braking.

Driving in the fog

If it’s so foggy that visibility is significantly compromised, you should not drive. Wait it out. If you must drive in fog, however, make sure that you turn on your headlights. You should use your low beams, not your high beams (your high beams will reflect off the fog, bouncing the bright light back to you, making it more difficult for you and even other drivers to see). Turn on your fog lights if your car has them.

As with other poor weather conditions, you should slow down when driving in fog and increase the space between you and the car ahead of you. Other drivers are in the same position as you are; they can’t see the road ahead of them very well, either, and it’s important for you to remember that.

If you absolutely must drive in light fog, remember the following safety tips:
  • Stay in the far right lane
  • Use your low beam headlights (or fog lights, if your car has them)
  • Do not use your high beams
  • Slow down, but keep moving when you enter a thick fog bank
  • Turn off your radio  – you don’t need any distractions!
  • Keep the windshield clear by using your wipers and the defroster
  • Be on the lookout for slow moving vehicles ahead of you
  • Avoid changing lanes or passing long lines of cars  – you don’t know what’s up ahead
  • Keep an eye on the rearview mirror for vehicles approaching you from behind
  • Tap your brakes lightly before braking to give the driver behind you a heads-up
Remember, if the fog gets heavy, you should stop driving until it passes.  If you develop car trouble in the fog or you must pull over to the side of the road, remember to:
  • Move off the roadway as quickly as possible
  • Get out of your car and move away– you don’t want to be near it if another car strikes it
  • Be aware of when you can and can’t use flashers or flares

Driving in construction zones

Construction zones can be pretty tricky to navigate. They require patience and caution as you make your way — more slowly than usual—through narrow lanes and rough, graded surfaces.

The visual cues of a construction zone are easy to spot: Orange signs, cones and barrels mark a work zone. When you are nearing a work zone, look for signs warning you to slow down. Always follow signs and directions from construction workers – it’s the safe and responsible thing to do.

Besides, if you fail to heed the warnings, barriers, flaggers, and reduced speed signs, you could face fines, community service or even jail time! Oftentimes these penalties are double what they would normally be for a traffic violation in a non-construction zone, and they apply even if there are no workers present (although the area must be marked as a double penalty zone). Always be careful when driving in construction zones!

When driving through a construction zone, remember to:
  • Pay attention! There’s lots to take note of.
  • Follow the directions given on warning signs and by flaggers
  • Be prepared to react to other drivers near you. Don’t get too close.
  • Be prepared to slow down or stop.
  • Make sure you merge into the correct lane before you reach the work zone. You’ll get plenty of warning as you approach the construction zone.
  • Keep a safe distance between you and the car ahead of you.

Driving in High Winds

If you live in Nevada, you already know high winds. They’re a hazard to driving, especially to larger vehicles, trucks, campers, and vehicles with trailers. Here are some tips for driving in high winds:

  • Reduce your speed. Slowing down gives you better control over the vehicle and will give you more time to react in the event your vehicle gets hit by a strong gust of wind.
  • Maintain a firm hand position on the steering wheel. Strong wind gusts are unpredictable, and if you’re not holding the wheel properly, gusts can be strong enough to cause the steering wheel to be jerked out of your hands.
  • Be alert. Look well ahead and watch for any debris on the road. High winds can cause debris to litter the highway or can even throw debris directly into your path. By looking ahead you give yourself more time to react to road hazards.
  • Do not use cruise control. This will allow you to maintain maximum control of the gas pedal for when unpredictable gusts of wind occur.
  • Be proactive. Wait for the storm to blow over. It may be safer to pull over and take a break.
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