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This article covers the last of the five phases of the System – Acceleration. In a sense, acceleration is actually the wrong word. It gives the slightly misleading impression that you have to accelerate, i.e. to increase the speed of your vehicle, after you have safely negotiated a hazard. We discuss this further below.
Chapter 2 of Roadcraft has diagrams illustrating various examples of using the System in practice. The self-explanatory captions include:
Right-hand turn – Choose the appropriate point to accelerate safely and smoothly away from the hazard with due regard to the amount of acceleration, the nature of the road and road surface, traffic conditions ahead, and the position and movement of other road users.
Left-hand turn – Accelerate safely and smoothly away once you have made the turn.
Roundabout – Choose an appropriate gap in the traffic to accelerate safely and smoothly onto the roundabout without disrupting traffic already using it.
When the road is clear you can usually accelerate up to the prevailing speed limit, or to the speed of the traffic in front of you if that is lower. Roadcraft discusses some further implications and risks of acceleration in Chapter 4, of which the following is a summary.
Developing your skill at using the accelerator
Jerky acceleration is uncomfortable for the passengers, puts unnecessary strains on the vehicle and adversely affects tyre grip. Use the accelerator deftly, making precise and smooth movements to depress or release it. Thin-soled shoes help you to do this.
Acceleration capability varies widely between vehicles and depends on the size of the engine, its efficiency and the power-to-weight ratio. Take time to become familiar with the acceleration capability of any vehicle you drive: the safety of many manoeuvres, particularly overtaking, depends on your good judgement of it.
Always consider the safety implications of accelerating. Sudden sharp movements of the accelerator reduce tyre grip and jeopardise steering control. The faster you go the further you will travel before you can react to a hazard. It will take you longer to stop and, if you crash, your impact speed will be higher.
Acceleration sense
Roadcraft recommends that you develop the skill of changing your vehicle’s speed by accurate use of the accelerator – called acceleration sense (or, by a traffic policeman of my acquaintance, ease and squeeze). It means simply increasing the speed of the vehicle by pressing the accelerator (provided you are in the right gear) and getting it to slow down by releasing the accelerator and letting engine braking take over. It is used in every driving situation.
Good acceleration sense requires careful observation, full anticipation, sound judgement of speed and distance, driving experience and an awareness of the capabilities of your particular vehicle. A lack of acceleration sense causes many common mistakes: for example, accelerating hard away from a junction and then having to brake sharply to slow to the speed of the vehicles in front; or accelerating to move up behind a slower-moving vehicle and then having to brake before overtaking. If you have good acceleration sense you are able to avoid unnecessary braking.
Accelerating on bends
A moving vehicle is at its most stable when its weight is evenly distributed, its engine is just pulling without increasing road speed, and it is travelling in a straight line. Accelerating to increase the road speed around a bend upsets these conditions.
If you accelerate hard and alter direction at the same time you run the risk of demanding too much from the available tyre grip. If the tyres lose grip you lose steering control. To get maximum steering control, you should avoid altering your road speed at the same time.
As soon as a vehicle turns into a bend it starts to slow down and lose stability, due to cornering forces. If you maintain the same accelerator setting as you go into and round a bend you will lose road speed.
To maintain constant speed round the bend and retain stability you need to increase power by depressing the accelerator. How much to depress the accelerator is a matter of judgement but your purpose is to maintain constant speed, not to increase it. Increasing road speed on bends reduces vehicle stability. When you need to steer and increase speed together, use the accelerator gently. Use extra care in slippery conditions or you will get wheel spin, loss of steering control and a skid.