Firstly, what is gravel? Gravel is small, broken-down rock material. It can be obtained from a number of sources, such as rubble from a quarry, or from naturally occurring deposits in the earth. Particle size is generally no bigger than 2 cm (0.79 in.) in diameter. It is used as a base material in the construction of roads, buildings and other structures. On less trafficked roads, it is not viable to seal the road with bitumen, concrete or other materials, so the road is left with a gravel surface.
The major difference between driving on gravel and driving on a sealed surface is that there is much less grip on gravel. This is because gravel is a loose surface. To understand this, imagine walking on marbles. The wheels have a greater tendency to slip, which affects the driver's control of the vehicle. In real terms, this means that safe cornering speeds are reduced, and braking distances are increased.
The first technique you should learn for driving on gravel is simple: SLOW DOWN. This will make cornering safer, and braking distances will, of course, be reduced. Most accidents can be prevented simply by slowing down.
Secondly, drive smoothly. Due to the lower amount of grip on gravel, a car can respond unexpectedly to sudden inputs from the driver. If you turn the steering wheel sharply one way, the tyres don't have enough grip to change the direction of the car, so the wheels may point one way, but the car ploughs ahead. If the driver doesn't straighten up the wheel, but slows down, the front wheels will eventually grip, and the car will spear itself off the road quite nicely.
When braking, avoid brake lock-up. Brake lock-up is when the wheels on the car are stopped dead by the brakes, but the car does not stop. On bitumen, this produces that loud screeching sound that you always hear in movies and on television. Brake lock-up is caused by the wheels not having enough grip with the road to bring the car to a stop. The brakes stop the wheels, but the car continues to slide onwards, like a sled with rubber skids. Because gravel does not provide as much grip with the tyres as sealed surfaces, brake lock-up is achieved at much lesser braking pressure. When the brakes lock, the car does not steer properly, and may slide sideways, or, if worst comes to worst, you'll come off the road. You will know your brakes have locked up when you lose feeling from the steering, and you hear a crunching noise and gravel stones flicking up into your wheel arches. The trick is to give the brake pedal enough pressure so that your wheels are almost about to lock up. Remember, you will stop faster and in less distance when your wheels are NOT locked-up. This is why ABS (anti-lock braking system) brakes were invented. ABS monitors the speed of each wheel relative to the speed of the car. If there is a big difference, the ABS will intervene and ease off the brakes. If you have a car with ABS, forget about what I said about brake lock-up, just plant your foot on the brake pedal as hard as you can, and it will take care of all the skillful work for you. You may feel the pedal moving unusually under your foot, this is the ABS system in action. This is not a sign to ease off the brakes, as many people think it is.
One other aspect of braking on gravel, and this applies to cars with or without ABS, is the affect it has on the handling of the car. When your car slows down, the weight distribution of the car changes. This means that more of the car's weight is placed on the front axle, and less on the back. Conversely, when you accelerate, the weight shifts to the back. If you brake reasonably strongly, though not necessarily enough to induce brake lock-up, and turn at the same time, the car has a greater tendency to slide sideways. This is because the rear axle has less grip due to the change in weight distribution.
Try not to make the mistake that causes a lot of accidents on gravel roads: over-correction. When driving a car reasonably quickly (i.e. over 80km/h or about 50mph) on a gravel road, the car may seem "nervous" on the road. It may feel like it is squirming as you drive. If this bothers you, just SLOW DOWN. If you don't want to slow down, however, try to hold the steering wheel reasonably straight ahead. The car will probably settle itself. Accidents occur when the driver tries to correct the movement of the car, and steers the car, for example, to the left. The car then moves sharper to the left than the driver expects, so he or she panics, and steers the car hard to the right. They lose control, and eventually crash, often NOT on the side of the road they were originally headed towards.
Another factor in gravel driving is the vehicle that is being used. The single most important variable (vehicle-wise) is the tyres. The most effective tyres on gravel are those with a chunky tread pattern, similar to what is found on an off-road vehicle (a real off-road vehicle, not one of those pissweak SUVs). The large tread helps to clear away the looser gravel particles and grip on the harder, more stable parts of the road. Wider tyres are more effective than thinner tyres, since grip increases with the area of the contact patch of the tyres.
A four-wheel drive system is also desirable. This lessens the effects of oversteer and understeer that become apparent in rear-wheel or front-wheel drive. On the subject of which end of the car does the driving, front-wheel drive cars behave more predictably on gravel. This is because front-wheel drive pulls the car along the road, whereas rear-wheel drive pushes the car. To understand the effects of this, try getting a pen and pushing it along your desk (or any other smooth flat surface) with the tip of your finger, by its back end. To keep it going straight, you frequently have to change the direction that you are pushing it in. These direction changes could be considered turning the steering wheel. Also, try pulling the pen along the desk with a piece of string. This is comparable to front wheel drive. When pulling the pen, you'll find it much easier to keep the pen straight, as it tends to keep itself pointing forward.
It is also important to remember that not all gravel roads are the same. You can safely go much faster on a gravel road that has much of the loose particles swept off it and is wide, flat, hard and smooth, than you can on a bumpy track with plenty of marble-like rocks comprising its surface. Try to look further into the distance - if the road starts to look a bit sandier, or you can see lots of loose gravel on top, SLOW DOWN.
Due to the riskier nature of gravel roads, here are some general considerations for gravel driving:
- Keep left (or whatever is the correct side of the road for your country) as you approach and go around corners. If someone comes the other way at high speed, you mightn't have the time to safely move to your side of the road.
- Slow down as you approach crests. You don't know if the road goes straight or veers one way after the crest. If it does turn, you mightn't be able to safely get your car around the corner. Also, a car may be coming the other way and you won't be able to see it as you approach the crest.
- Gravel (especially Australian gravel) can be very dusty, especially in summer time. When you pass a car coming the other way, there might be a short period where you are enveloped in a cloud of thick red dust lifted up by the other car and can't see anything. This can be particularly nasty if you are, or are about to, go around a corner.
- Rain plus gravel equals mud. Mud is like gravel, except one helluva lot more slippery. Well, it varies. The mud you have to be most cautious of is the fine layer of mud that can form on the surface of the road during rain. This mud works as a lubricant between the tyres of your car and the hard surface of the road underneath. If the rain is really heavy, it may just turn the road into a quagmire. Then you'll get bogged, which really sucks.
The next issue is what to do if you lose control of your car. "Lose control" is a very loose term, and usually refers to a combination of brake-lock, sideways movement of the vehicle and panicked screams of "I'm going to die". If you find yourself in this situation, stop screaming and try not to panic. People in a panic do stupid things, and if you are in a car heading towards a rather large tree at high speed, you've already done enough stupid things for the moment. So get a hold of yourself. We've dealt with brake lock already - try to hold the pedal at that point where you are very close to brake lock. This is where you get the most effective braking. Also, due to the caster of the front wheels on a car, when you turn you raise the front outside wheel and inside back wheel very slightly. This means that you are more likely to encounter brake lock-up when you are turning.
To straighten up your vehicle when it is sliding sideways, steer out of the slide. This means steer in the direction of the forward side of the vehicle. Be very careful not to over-correct. A bit of sideways action scares the pants off many drivers, and they over-correct. Don't be afraid to use the entire width of the road (unless there's other cars around of course), it's easier to slide gently over to the wrong side of the road than it is to try and wrestle the car into line on the correct half of the road. While you try to steer out of the slide, ease off on the accelerator (the gas pedal, for you americans), and maybe gently apply the brakes. Remember, you don't want to do anything too suddenly.
Of course, there comes a point where you cannot rescue yourself from an out of control situation. Not even the best drivers can avoid the laws of physics. It is at this point that you should try to crash in a way that will minimize damage to yourself and other passengers. You don't want to be brought to a sudden stop. This will hurt you the most. If given the choice between a single, large tree and a fence, 20 metres of loose scrub, then a ditch, choose the latter. It will stop you slower. If you must hit something, hit it with the front of the car. The front of a car is designed to cop the brunt of a serious collision. Once you have stopped, it would be wise to get out of the car if you can, as with all that fuel and heat around, there is a chance of a deadly fire. Be careful if you have to pull other people out of the car, you may be worsening possible spine injuries. You may have to make a decision for that person - death, or a wheelchair for the rest of their life.
After all that, I must say that I recommend option number one: slow down. Getting to work five minutes late is better than not getting there at all.