We have no sense of speed, at all.
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What you're actually able to detect with your body is called acceleration or deceleration; the speeding up or slowing down in any given direction. This also includes any high-frequency direction changes over very short distances like vibrations, as well as any irregular oscillations which are usually caused by things like bumps.
However, if all is smooth and straight as you move along, once you're up to a constant velocity you don't feel the speed at all.
I'll give you a practical example that many of us have experienced. When everything goes as planned, a passenger jet feels fastest when it's actually at the slowest stages of a flight. Those stages are the take-off and the landing.
The simple fact is, once your momentum is stable you cannot feel the actual cruising speed with any semblance of accuracy, whether it's in a commercial flight, or a comfy closed car, or the space station lapping the earth every 90 minutes for that matter. That's because when there's no acceleration of any kind to feel - everything is smooth and calm and level - there's not enough sensory input on our body for it to feel fast.
Somewhat helpfully, on earth we also get clues from our other senses. For instance, we can feel the movement of air or water on our skin. We could be perfectly stationary and still feel this though. If it's wind and you are moving too, the intensity you feel is the result of whether you're moving into or with the wind, so it's even less helpful as an indicator.
Whether it's an object or ourselves, we can somewhat observe the relative speed (velocity) compared to the surroundings. But if this observation of speed were enough, in-car shots of motor racing would look equally as fast on TV as it does when you're physically in the vehicle. The other thing making mere observation from inside the vehicle terribly inaccurate is it's affected by how close or distant the other objects are (with closer making it look much faster).
Vibrations don't give a usable sense of speed either. An unbalanced washing machine vibrates and it doesn't go anywhere. Nor does a game console controller, or a mobile phone, or an electric toothbrush.
Our ears also try to help. But at the exact same road speed a V6 hatch for example, could be revving its valve springs off in a low gear, while its otherwise identical diesel counterpart could be in a cruise gear with the engine ticking over just frequently enough for the turbo to be helpful, and yet the speeds will seem to be different to both the vehicle's occupants and to any observers.
So to sum up, if we actually had a sense of speed a commercial jet cruising at 490knots (approximately 900km/h) would feel that fast. We'd also be able to actually feel the earth rotating at 1670km/h at the equator (and we'd notice the difference at each latitude), or lapping the sun at 30km/s (108,000km/h) or the Solar System going around our galaxy at 220km/s (792,000km/h). Yet we can't sense any of that. If we could, nobody would have ever believed that the earth is flat for a start. We'd have sensed it spinning.
Despite this, an alarming number of people rely heavily on a "sense of speed" - which, as we've just established, they simply don't have - to choose the velocity at which they drive relative to the ground they're on.
They're the ones who accelerate lazily up to the speed limit (especially on freeway on-ramps) and then, inexplicably, keep speeding up beyond that posted number, or they don't bother slowing down enough before the speed limit sign when entering towns and roadworks because it "feels" so slow, and then slow down too much in the centre of towns because the scenery has now closed in and altered their perception again.
We have a compulsory device on every registered vehicle that measures and tells us our road speed, yet it just doesn't seem to get looked at very often.
If we humans were capable of realising the earth is round without arguing about it with ancient Greek philosophers for at least three centuries (and some still argue it), then perhaps we wouldn't have this problem.
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.
