Special speed is 1/2 walk speed + all speed increases by 10 = ???

[Greywander]

In the specific case I'm dealing with, you have a burrow speed that is half of your walk speed (aside: would it actually be a problem just to make it equal your walk speed?). I also want to give a flat +10 feet to all move speeds. How would this interact with your half-speed burrow? Would it...?

(a) Assuming a 30 foot base walk speed, new burrow speed is 15 + 10 = 25 feet, while new walk speed is 40 feet.

(b) Walk speed increases from 30 to 40, which increases base burrow speed from 15 to 20. Burrow speed then also gets +10, increasing to 30.

(c) Burrow speed is tied to walk speed and can't be increased independently. Walk speed increases from 30 to 40, causing burrow speed to increase from 15 to 20, full stop.

I feel like (b) is definitely wrong. E.g. a MotM lizardfolk has a swim speed equal to their walk speed, so a flat +10 to all speeds would not double up and apply twice to swim speed. By applying the +10 bonus to your walk speed, your swimming speed (which is equal to walk speed) will already increase by +10, and thus the +10 has already been applied. I feel like because these special speeds are tied to your walk speed, then a bonus to just your walk speed would still be applied to these special speeds, in which case I feel like (a) probably isn't right, either. I guess it depends if the +10 is a modification to your base move speed, or if it's a bonus on top of your base speed. If you frame walk speed as 30 +10 instead of 40, then it makes sense that burrow speed could be framed as 15 + 10. If you frame walk speed as 40, though, then burrow speed would need to be 20.

Is there any other cases where something like this happens? Up until recently, move speeds have been independent of one another; it's a recent development to give a race with a special speed equal to their walk speed. Currently I think all such cases always set the special speed equal to walk speed, rather than to half or double walk speed, but I might have overlooked something. As long as the special speed is exactly equal to walk speed, then both (a) and (c) are functionally equivalent, so it doesn't matter which is correct.

[Dualight]

Off the top of my head, climbing/swimming without a climb/swim speed is another example of speed=1/2 walking speed.
But for the main question, if the burrowing speed is defined as equal to 1/2 walking speed, then the +10 speed effect would translate to +5 burrow, as there is no independent burrowing speed, just the option to burrow at half waling speed.
At least, this is how I understand it.

[Keravath]

Usually, the other movement speeds either have a specific value or are derived from the base speed/walking speed.

Fly, climb, swim for example ... if you don't have a climb or swim speed then these are 1/2 of your speed (i.e. equivalent to the effect of difficult terrain).

Sometimes these will be set equal to your base speed/walking speed. (Winged boots gives you a fly speed equal to your walking speed while the fly spell gives you a fly speed of 60'). A thief rogue has a climb speed equal to their walking speed. A burrowing creature could have a specific burrow speed or a burrow speed equal to some fraction of their walking speed.

After that, you just need to check what effect is increasing your speed. If you have the mobile feat then your speed increases by +10' (this refers to your base speed)

"Every character and monster has a speed, which is the distance in feet that the character or monster can walk in 1 round."

If your burrow speed is 1/2 your walking speed then it increases by +5' if you have the mobile feat. On the other hand, if your burrow speed is listed as 20' then taking the mobile feat won't change the burrow speed because it has a separate value independent of your base speed/walking speed.

[Greywander]

On the other hand, if your burrow speed is listed as 20' then taking the mobile feat won't change the burrow speed because it has a separate value independent of your base speed/walking speed.
Mobile would increase you burrow speed by 10 feet in this case. Mobile doesn't only apply to walk speed, it applies to all speed.

Seems like this is one of those things where there isn't a clear answer. Option (c) is probably the easiest to use, but different DMs might run it differently. I suppose I'll just leave it up to the DM to decide instead of trying to clarify it myself.

[bid]

In the specific case I'm dealing with, you have a burrow speed that is half of your walk speed
That's the problem.
RAW uses "use 2 feet of speed per feet of movement" to work around that issue.
Either you have a fixed 15' burrow speed, or burrowing is difficult terrain for you.

[OldTrees1]

If this were an incremental clicker game, then B would be the obvious answer.

Burrow Speed = Base Burrow Speed x 1 + 10ft = (Walk Speed / 2) + 10ft = (Base Walk Speed x 1 + 10ft)/2 + 10ft = Base Walk Speed / 2 + 15ft.


However I think it is more intuitive to use model A in RPGs. Model A follows the principle that the bonus only applies once, but applies it at the most beneficial time.

[Keravath]

Mobile would increase you burrow speed by 10 feet in this case. Mobile doesn't only apply to walk speed, it applies to all speed.

Seems like this is one of those things where there isn't a clear answer. Option (c) is probably the easiest to use, but different DMs might run it differently. I suppose I'll just leave it up to the DM to decide instead of trying to clarify it myself.

It is a DM call in any case. However, the rules define "speed" as walking speed.

"Every character and monster has a speed, which is the distance in feet that the character or monster can walk in 1 round."

Mobile increases a character's "speed".

"Your speed increases by 10 feet."

Given that speed is explicitly defined as walking speed in the rules - I tend to lean toward mobile only increasing your walking speed and not any other speed a creature may have unless that speed is also based on the walking speed/"speed".

Burrowing speed is not speed, swimming speed is not speed, climbing speed is not speed - only walking speed is defined as speed RAW.

However, if a DM wants to interpret the increase in speed provided by Mobile to apply to any speed the creature has then that is perfectly ok too.

[Skrum]

(c) is the obvious answer to me. I really don't see the logic supporting A or B. Burrow speed equal to half your walking speed, walking speed goes up by X, burrow goes up by 1/2X since the burrow speed is still derivative to your walking speed. Why would a speed increase cause the burrow speed to deviate from the given formula?

[Greywander]

It is a DM call in any case. However, the rules define "speed" as walking speed.
[...]
Burrowing speed is not speed, swimming speed is not speed, climbing speed is not speed - only walking speed is defined as speed RAW.
I'm not so sure about this. There are abilities and features that specifically give you a bonus to your walk speed, so I've always interpreted an increase to speed, sans specification, to apply to all movement speeds you have. "Your speed increases by 10 feet" means that if you have a fly speed, that also increases by 10 feet, in addition to your walk speed. And so on.

That said, MotM seems to be moving toward a paradigm where all speeds are derivative of walk speed. In that case, I actually would expect to see some creatures with special speeds that are half or double their walk speed, and thus clarification on how speed bonuses are applied. I suspect they'll go with (c) since it maintains the multipliers between walk speed and special speeds.

Not sure how this will shake out for monsters with walk speeds of 0, though; most likely they'll just keep using set speeds like normal. Though they could be given a non-zero walk speed on the assumption that they'll rarely need to use it. Alternatively, something other than walk speed could be defined as their "base speed", and thus we might instead see special move speeds being based off of your "base speed", which may be walk speed or it may be fly or swim speed, depending on what type of creature you are. A merfolk might use swim speed as their base speed, for example, with a walk speed that is half their swim speed.