On the Road

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Q: I travel in rural areas a lot and I have heard of drivers loosing control of their cars when they accidentally drive onto the gravel shoulder, which causes the car to roll over. What causes this and if it happens, what can I do to avoid an accident?

A: When we see accidents where a vehicle has rolled or flipped over, we can assume two laws were likely broken.

The first is usually a law pertaining to the Highway Traffic Act, speeding for instance, and the second is definitely a law of physics.

Except when driven off an embankment or into a ditch, there are only two circumstances when a vehicle will flip or roll over while being driven.

The first involves uneven braking on different surfaces, which I will discuss later on, and the second occurs when the bead of one of the tires separates from the rim causing a sudden and rapid loss of air pressure, or in other words the tire is instantly flattened similar to a blowout.

Vehicles are held in place on the roadway by two forces, gravity and friction.

Gravity is self explanatory but the force of friction, known to police accident reconstructionists as the co-efficient of friction or the 'drag' factor, is in simple terms the amount of 'stickiness' a tire has to hold it in place on the roadway.

An example of this occurs during legal car racing such as the Molson Indy, where race drivers try to 'warm up' the tires on a pace or caution flag lap by swerving rapidly back and forth at slow speeds. Essentially this swerving action heats the rubber tires using friction generated from the asphalt thereby increasing the amount of 'drag,' which allows drivers to take corners at higher speeds without sliding sideways or brake heavily without causing an uncontrollable skid.

The same principle applies to our domestic cars, SUVs, vans and trucks.

The friction or drag built up by the tires keeps the car in the exact spot on the roadway where the driver steers, particularly when turning, changing lanes and braking.

Go around a corner too fast though and the vehicle suddenly slides sideways as the driver has exceeded that critical point of friction and the tires begin to break loose from the pavement. The same principle applies when a skid occurs during heavy braking. In both cases the vehicle is considered to be out of control as the driver can no longer input any steering until the proper friction point returns, usually when a slower speed is resumed. Sound complicated? Simply it's just the basic laws of physics.

When a vehicle slips off the paved roadway where one or more of the tires suddenly find the gravel shoulder, the typical driver reaction is to hit the brakes hard, certainly harder than under normal braking conditions. Hard or heavy braking intensifies the problem as the vehicle is now trying to stop on two very different physical surfaces, pavement and gravel, which will often cause the vehicle to go into a skid or a spin.

The laws of physics tell us the friction points or drag factors of both surfaces are significantly different. As such, rubber tires 'grab' far better on solid pavement than they do on loose gravel and the result is often a sudden and unexpected out of control centrifugal spin as one side of the vehicle begins to rotate faster than the other.

If during the spin all four tires suddenly re-enter the pavement as they often do, the sudden shift in drag and momentum can cause that tire bead to break similar to tire blowout conditions. At this point, if there is still enough ground speed, higher roof line vehicles such as SUVs, minivans and trucks typically flip over, perhaps several times, often with catastrophic results.

To avoid this often fatal situation, drivers who suddenly find a portion of their vehicle travelling on a soft or gravel shoulder particularly at higher speeds should avoid applying the brakes unless absolutely necessary.

Vehicles are engineered and designed to do two functions well: steer and brake, but unless the vehicle is equipped with anti-lock brakes, it won't do both at the same time when in an uncontrollable skid or spin. Hard application of the brakes usually results in a significant if not total loss in steering. Release the brake and steering suddenly resumes. The best method to recover from a soft or gravel shoulder situation is to immediately take your foot off either the gas or brake pedal (whichever is being applied) and concentrate on steering the vehicle slowly and gradually back onto the paved roadway.

Avoid any sudden movements to re-enter the roadway and try to ease back onto the paved surface at a slower, safer speed. Other drivers usually yield if they see a vehicle travelling on the shoulder trying to get back onto the pavement.

Inattentiveness and fatigue are two of the main reasons why drivers tend to veer off the roadway onto a soft shoulder, into a guardrail or into ditch.

At the first sign of fatigue, pull off the road and stop at a safe location. A short walk in the fresh air or a soft drink may give some temporary relief to the signs of fatigue.

The best remedy for fatigue is to stop driving and either get proper rest before resuming the trip or allow a more alert person to take over the driving duties.