When you're in a path and you encounter another group going the other way, it is said that whoever is going down should stand aside to let those going up pass through. Why?
There are three good reasons for this:
- The harder work an uphill hiker has to do
- The smaller field of vision of an uphill hiker
- They are in that "hiking rhythm" zone which shouldn't be interrupted (Inertia)
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And most important and most ignored, everything else being equal, give the right of way to the hiker going uphill!
Why the latter? Think about how most people hike - usually focused downward, a few feet ahead on the trail, watching their footing to try to avoid a face plant. If you're headed downhill, you'll generally see some distance ahead of you on the trail at the same time as you're looking down to where you will set your feet the next step or two. You'll see any hiker coming up hill well in advance of meeting. On the other hand, if you're hiking uphill, and looking down at the trail, your range of vision is really just a few feet in front of you. Clearly the hiker going downhill has the visual advantage, and time, to adjust his position so the uphill hiker can pass. And, it's a lot easier for the downhill hiker to stop and resume than it is for the uphill hiker. Think about the interstate trucker or you on a bike - it takes more energy and time to restore speed if starting from a dead stop. So it is with the uphill hiker, especially if carrying a backpack - let her come on through so she can benefit from the "body in motion" principle in physics. This applies to hikers running downhill too. And just because you're running downhill (nobody is impressed anyway, and secretly hoping you face plant) that doesn't give you the right of way.
On a narrow pass, hikers going uphill have the right of way. Simply put, hikers moving uphill are generally working harder than those on the downhill slope. It is courteous to move to the side and let the uphill trekkers pass through.
It seems that many hikers—even experienced ones—may not know or always remember this, but hikers going uphill have the right of way. This is because in general hikers heading up an incline have a smaller field of vision and may also be in that “hiking rhythm” zone and not in the mood to break their pace. Often an uphill hiker may let others come downhill while they take a breather, but remember that’s the uphill hiker’s call.
In addition to the fact that it is easier to go down than up, I think the biggest issue is safety. If you were to fall while going down, there is risk of injury to those below (coming up). It is much easier to lose your footing and fall while moving downwards, and knocking people off a trail can lead to serious injury. If you were to fall while going up, you would injure only yourself, and typically it is easier to catch yourself.
I suspect it's just inherited from the "rules of the road" extant before most road traffic was motorised and before most roads were widened to 2 lanes.
On single-track roads e.g. in Scotland, it's still the rule of the road, and any motorist who remembers it is much appreciated by the cyclists working hard uphill.
The underlying reason is that going uphill - and especially starting uphill - is much harder (especially if you have to pick up toestraps!) and though this applies to some extent to hikers too, I suspect it would simply be too confusing to have one rule for the road and a different rule on footpaths.
See point 155 in the current edition of the Highway Code regarding single track roads :
"Give way to vehicles coming uphill whenever you can. "
I think hiking involves an element of psychological momentum.
You time your rests to be either very short or very long to control momentum loss. You rest with your pack on to avoid momentum loss.
In this case, I think it's about the mental momentum because of physical momentum. It is way easier to overcome inertia to continue down the hill when gravity gives you a boost than it is to overcome inertia restarting an uphill hike against gravity.
That is to say, I think it is less about the ease of going than it is about the difficulty of resuming.
There's a simpler answer that ignores any kind of "field-of-view" discussion. In the vast majority of cases involving right-of-way on a shared road, the party going faster yields to the party going slower. So cars yield to bikes which yield to pedestrians.
For example, imagine a flat trail which is shared by bikes and pedestrians. You would expect the bikes to yield to oncoming pedestrians. Since they're going faster, they have a greater responsibility to avoid collisions. In the same way, hikers travelling uphill are assumed to be going slower than downhill hikers. Thus they have the right-of-way.
Under the "going-down person stops" convention, the up-going hiker will automatically and immediately have some warning that something is wrong if the down-going hiker fails to stop because he is unable to do so, allowing him to stop and/or take evasive action with minimal communication between the parties.
If a nearby cliff, steep trails, slippery conditions, and foreign languages are involved then this convention makes a lot of sense.
I think it's similar to the rule that sailboats are given right of way over motorboats, and sailboats going upwind have right of way over sailboats going with the wind. With the boats, it's that motorboats have more options than sailboats, and sailboats with the wind have more options than those going against it. It's similar with hikers: those going uphill have to work harder, and possibly have fewer options.
Also as someone said above, when going down the vision is better than for people goin down.
In addition, if a bigger group has to pass another bigger group, the group going down can much more easy anticipate on having to pass the other group and find a suitable spot to stop. This is much harder to see for a group going up.
And while the group going down will stop at an appropriate spot or area, they will wait for the group going up.