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The tachograph is an instrument fitted to a lorry which produces a chart such as the one above to record the vehicle's speed throughout the day, as well as recording the total distance travelled and the hours worked by the driver. The circular chart takes 24 hours to make a complete revolution, and as it goes round the vehicle's speed is recorded as a graph. If you look at the sample chart above, you'll see the speed trace indicated. The further from the centre the trace is, the faster the vehicle was travelling at the time.

Look at the blow-up on the left, taken from a tacho chart of mine on a regular night run. Note how the speed varies continually. This tacho shows the normal speed trace from a vehicle without a limiter. (The speed is indicated in km/h, not miles! 100 km/h is about 60 mph, the speed limit for lorries on British motorways.) This tacho shows a vehicle under the driver's control. The speed varies, which helps to keep the driver alert, and the driver has to keep adjusting the accelerator pedal to maintain the speed as the vehicle goes up and down gradients. He also has to negotiate other vehicles which may be travelling slower than he is, which gives him the opportunity to change lanes and vary the vehicle's speed even more.

Now look at this blow-up, taken from a different vehicle I drove on the same run on another night. (This is actually a section taken from the chart at the top of the page.) This vehicle has a 56mph (90km/h) limiter fitted, and you'll see that the speed trace is now almost flat, because the limiter has taken over and is controlling the vehicle's speed. The driver (me, in this case) now has nothing to do except hold the pedal down. The vehicle drives itself along at exactly the same speed for hour after hour, and the driver has nothing to do. When the human brain has nothing to do, it does nothing. This quickly leads to loss of concentration and awareness of what's happening on the road around the vehicle, and can result in the driver failing to notice something which he should react to, and in some cases falling asleep.

If I'd been asked to devise something which would send lorry drivers to sleep I'd have invented the speed limiter. They want to fit one to your car, set to 62mph, which will make every long journey you make into a nightmare of boredom and fatigue. You're already at risk from speed-limited lorry drivers. Now they want to limit you as well.