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MyZmanim says:

According to most Poskim, it takes 18 minutes for an average person to walk a distance of one mil, and therefore 72 minutes to walk 4 milin.… Others say it takes 22.5 minutes to walk a mil, and therefore 90 minutes to walk 4 milin.

(Emphasis in the original.)

If it takes me 18 minutes to walk a distance, it's going to take me more than 72 minutes to walk four times that distance. People walk far distances more slowly than short distances as far as I can tell.

More extremely, I see the same proportion applied to four amos, which is even more unrealistic. Surely if it takes me 18 minutes to walk a mil (2000 amos), it doesn't take me 18 ÷ 500 minutes to walk four amos! I don't walk a mil at anywhere near the speed I walk four amos at!

Have any pos'kim dealt with the idea that those numbers don't match up?

Edit: Double AA pointed out in a comment that I'm asking this backward: the original source says it takes twelve hours to walk forty mil, and the other distance-time conversions are derived from that one. Either way, the question remains.

msh210
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    My guess is that we have a fixed "standard walking speed", even though actual walking speeds vary. The same way we have a "standard olive" even though not all olives are the same size and a "standard amah" even though everyone's arms are different. – Heshy Sep 09 '19 at 19:25
  • @Heshy that's definitely how Rabbenu Tam et al take it, but I don't see why it has to be that way if you reject (with good reason) his whole zmanim worldview. For RT the Gemara in psachim is discussing Halakhic shkiya/tzeis, like the Gemara in Shabbat, and must be using an objective unit. If you reject that then the latter discusses Halakhic times and the former discusses traveling expectations. – Double AA Sep 09 '19 at 19:45
  • @DoubleAA it's still reasonable to have a standard approximate speed to use in calculations even though it's not exactly right. – Heshy Sep 09 '19 at 20:22
  • @Heshy only if there are calculations to do. Outside Rabbenu Tam world is there ever a need to calculate hillukh X mil from Y mil? (Even if you find some instance of Parsa let's say, there's no reason the times need to be proportional 1:4 like the distances are.) – Double AA Sep 09 '19 at 20:23
  • @doubleaa Rabbi Yehuda in Yoma 6:8 – Heshy Sep 10 '19 at 10:23
  • @Heshy good source! But was the distance there really exactly three mil anyway? Was the need for timing the reading that important anyway? I'm not sure you can build a Binyan Av from that. And remember they weren't calculating X mil from Y mil. They both walked 2 mil, and then one walked a third mil and one waited a mil so any error wasn't being compounded, maybe they just knew to wait a slightly longer mil because they were tired hungry and in the desert. – Double AA Sep 10 '19 at 11:55
  • @DoubleAA the implication is that the third mil takes the same amount of time as the first two, so we can approximate distance as a function of time as linear at least up to that point. I agree you probably don't have to be that precise in the timing. – Heshy Sep 10 '19 at 12:14
  • @Heshy no, they all walk the first two mil. If walking multiple mil is slower then they've accounted already for much of the error. It could be if they hadn't walked 2, R Yehuda would say wait 3+. In fact why does he insist on walking 2/3? Maybe to account for this. (That's always been my theory since they didn't have stopwatches.) – Double AA Sep 10 '19 at 12:17
  • @Heshy I think the important point there is that wasn't a Halakhic Shiur. If I want to estimate a trip for a particular person all "average person" rules don't matter at all. My question was really where do we ever need to calculate a Halakhic Shiur of time-distance by reference to a different such Shiur? – Double AA Sep 10 '19 at 13:30

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