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Reasons:

  • Save money by making the rest of the runway narrower
  • Larger margin of error left and right
  • Easier to turn around, if necessary

And if the reason that it isn't done is merely for regulation/legal reasons: Is this something that would be practical if it were allowed?

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    $\begingroup$ It sounds like the underlying assumption is that it is easier to stay centered after touchdown. I think that might not be true, crosswind and e.g. uneven braking power can be problematic even after the wheels are already on the ground. $\endgroup$ – jpa Jan 14 at 10:26
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    $\begingroup$ Consider the same question phrased a different way could we make some parts of the runway narrower than the rest?, and you'll probably come up with quite a few counter-arguments. $\endgroup$ – Toby Speight Jan 14 at 13:19
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    $\begingroup$ How would it give you a larger margin of error? It's no use having a wide touchdown zone if you're then going to go off the runway as it narrows. $\endgroup$ – Dan Hulme Jan 14 at 17:22
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    $\begingroup$ @dtgq What problem are you trying to solve? Planes running off runways is pretty rare. $\endgroup$ – zeta-band Jan 15 at 0:15
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    $\begingroup$ "if you touch down off the side of the runway" then you've screwed up badly and should have gone around. $\endgroup$ – Roger Lipscombe Jan 15 at 11:49
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Because most runways already are as narrow as safely feasible.

Let us look at your question piece by piece:

Save money by making the rest of the runway narrower

You seem to be assuming we are currently deliberately wasting asphalt on making runways wider than needed, and could build them narrower. As they say on Wikipedia: [citation needed].

Larger margin of error left and right

The touchdown zone is the whole runway, as long as the aircraft in question can stop afterwards, so if you start down this alley you'll find it cheaper to complete the runway in the same width.

Easier to turn around, if necessary

This is already done by including turn pads, on top of the nominal width. For an example, see: OGZ

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Because planes that landed towards the edge of the touch-down zone would run off the side of the runway when it narrowed.

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    $\begingroup$ That's (one of the reasons) why we have rudders and tillers - so that planes touching down off-center can be steered back to the centerline. $\endgroup$ – Sean Jan 15 at 4:04
  • $\begingroup$ @Sean and those aren't too effective until speed drops significantly for ground handling. $\endgroup$ – jwenting Jan 15 at 6:28
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    $\begingroup$ @Sean To provide enough time to slow so you can steer back safely, you'd probably need half the runway to be full-width and the second half could be tapered off. But that second half is the first half of the opposite-direction runway, so it has to be full-width too. $\endgroup$ – Dan Hulme Jan 15 at 9:44
  • $\begingroup$ @jwenting: The tiller isn't too effective until speed drops significantly. The rudder is very effective indeed at high speed, and its effectiveness decreases as the aircraft slows down. $\endgroup$ – Sean Nov 8 at 2:44
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At least a few runways are divided by length. For example, once at Boeing Field (BFI) four general aviation light aircraft were cleared to land approximately at once on two runways. My VFR landing clearance was to land in the first half of runway 14R while another aircraft above and slightly ahead of me had to stay at 500 AGL(? I forget the exact number) until the threshold and then land on the second half. The left runway had something similar occurring at about the same time.

Having irregular width runways doesn't help this much. Also it is difficult to imagine that the non-ends would be graded any differently. In the overall project of constructing a runway, it is highly likely that the land preparation, utilities, and grading are like 80% of the cost of the surface. A forum exchange breakdown of a Canadian 7000 x 100 foot runway is:

$   250,000   geotechnical analysis
$ 4,000,000   gravel (60 cm of 7.5 cm minus, 25 cm 1 cm minus)
$ 2,500,000   labor and equipment rental
$   100,000   quality testing (compaction, etc.)
$ 2,000,000   asphalt (15 cm)
$   200,000   lighting
$    10,000   painted markings
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$ 9,060,000   total

Would making the shape narrower increase any of those costs? Probably. Certainly less materials should decrease the overall cost, and maybe labor could be economized.

For strong crosswind landings, I usually use all of whatever width there is!

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    $\begingroup$ Interesting mixture of units. Runway area in feet and depth in centimetres :-) $\endgroup$ – uɐɪ Jan 15 at 11:33
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    $\begingroup$ @uɐɪ So volume is in square feet centimetres! $\endgroup$ – Oscar Bravo Jan 15 at 14:12
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    $\begingroup$ Similar thing happens at Oshkosh every year, right? $\endgroup$ – yshavit Jan 15 at 16:06

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