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If I am trying to park my car, a power steering system really helps.

However, you can't really feel the difference once you get up to speed. Power steering barely offers any assistance at motorway speeds. This implies that it's easier to steer a car at speed.

Why is this the case? Of course, the angles that you would turn are smaller on a typical motorway. But the whole effort of steering feels so much more difficult at lower speeds, even with power steering assistance. For example the kind of angles you might take such as to adjust a parking manoeuvre which seem similar to, say, a lane change on a motorway, it's much more difficult at lower speeds.

My guess would be something to do with static and kinetic friction. Because the wheel is turning the contact area is smaller, and it is constantly moving to a new part of the tyre, so there's less inertia(?) to overcome there.

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    $\begingroup$ Just try a turning radius of 20 feet at highway speeds! <-- the point is that, on teh highway, turns are incredibly gentle. $\endgroup$ – Carl Witthoft Dec 23 '15 at 14:57
  • $\begingroup$ Your guess is correct. $\endgroup$ – Lewis Miller Dec 23 '15 at 14:57
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When a tire is not rolling (or going slowly) the rubber needs to scrub a lot in order to change the angle of the tire. When rolling then the scrub is much less because the point at which you are turning is much further away.

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