How do ball bearings and difficult terrain stack?

Question

There is already this Q/A on whether difficult terrains stack (they do not). And this Q/A on whether ball bearings and caltrops stack (they do not). But when looking at the description for ball bearings I realized it states:

A creature moving through the area at half speed doesn’t need to make the saving throw...

And when looking for other references to "half speed" all that existed were caltrops (with a similar description) and this bit from the "Difficult Terrain" section:

You move at half speed in difficult terrain--moving 1 foot in difficult terrain costs 2 feet of speed--so you can cover only half the normal distance in a minute, an hour, or a day...

I had always assumed you could choose to move at half speed through ball bearings to avoid the saving throw but perhaps that is not the case...

Regardless, it is unclear to me what happens if ball bearings are thrown onto difficult terrain. You would (outside of features such as the Ranger's Land's Stride) already be moving at half speed due to the difficult terrain but do ball bearings require it to be halved again, or is putting ball bearings on difficult terrain useless unless a creature that ignores difficult terrain?

Answer 1

Difficult Terrain generally negates the need to make a save for Ball Bearings

The rule for moving through difficult terrain states is actually found in 2 places (both Combat and Adventuring). I've quoted the second instance here because it includes an additional and important clause (in bold):

You move at half speed in difficult terrain--moving 1 foot in difficult terrain costs 2 feet of speed

Ball bearings state that:

A creature moving across the covered area must succeed on a DC 10 Dexterity saving throw or fall prone. A creature moving through the area at half speed doesn't need to make the save.

Two things are important to keep in mind here:

1. The rule for ball bearings does not distinguish between a creature that chooses to move at half speed versus one that is forced to move at half speed.

2. Two instances of the difficult terrain effect do not compound with each other.

Ball bearings do not explicitly use the term "difficult terrain. This is because difficult terrain forces a creature to move slower while ball bearings give the creature the option to do so. Because it's an option, they can't be described as causing difficult terrain.

But that is what they are effectively doing. One effect forces the character to move half speed, the other effect gives the character the option. But since the character doesn't actually have an option due to the first effect, the penalty for choosing to move at full speed through ball bearings is moot.

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One important caveat: Certain classes have abilities that allow them to ignore the movement penalty for difficult terrain (also the Mobile feat and spell Freedom of Movement). In these instance, ball bearings would still require the character to choose to move at half speed or make the Dex save because ball bearings, while similar in effect, are not "difficult terrain" (because of their optional nature).

Answer 2

Ballbearings generally have no effect

If the difficult terrain causes you to move at half speed, therefore you avoid the effect of the ballbearings. If, like the ranger you describe, difficult terrain doesn’t affect you, you have the normal choice to make with respect to the ballbearings.

Think of it like this, moving slowly allows you to avoid the ballbearings: it doesn’t matter if you are moving slowly by choice or by necessity.