The inability to stay in the center of one's assigned lane while in motion; the tendency to veer, literally or otherwise. I first heard this helpful phrase while learning to drive on the highway with my father. Highway driving, of course, generally involves staying in one lane for a long period of time, shifting lanes only to pass a slower driver on the left like a good law-abiding citizen. So the ability to maintain constant control over one's position in the lane, and on the highway in general, is important. My father found it necessary to point out every driver who edged from one side of their lane to the other, whether suddenly or gradually, jerkily or smoothly, and say "see, that person has poor lane control," with an expression of disgust.
Similarly, poor lane control can be a problem in swimming, especially in a crowded pool in which you must share lanes. It is obviously less of a problem while swimming than driving, largely because you are less likely to die a violent fiery death from colliding with a swimmer in the next lane than from colliding with a motor vehicle. However, you can get water up your nose, mortally offend someone by kicking them, or break your toe on the wall quite easily. And it is certainly more difficult to keep to your tiny half lane while doing wide frog kicks than to keep to your full lane while driving.
While poor lane control is generally then a concept of physical placement while in motion, it also can encompass mental or abstract placement. You might apply it to the thought process in general: if your mind tends to wander, you could be said to have poor mental lane control. This is not always a bad thing: it certainly makes for more interesting conversations than small talk about the weather. But if you need to discuss Kant's theory of the beautiful on an exam you are taking, I doubt the professor will be impressed at a series of vague associations.
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