How does Spirit Guardians impact available movement for affected creatures?

For the spell Spirit Guardians, which has a 15' radius,

An affected creature’s speed is halved in the area.

If a creature with normal speed 30 entered the area after moving 15 feet, what is its remaining available movement?

If a creature with normal speed 30/halved speed 15 starts in the area and moves 15 feet to exit the area, what is its remaining movement?

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.

which defines "move at half speed" as "costs 2 feet of speed". How does this connect with the answers below?

PH Page 190 has a section on different speeds that I think would apply:

If you have more than one speed…you can switch back and forth between your speeds during your move. Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 ar less, you can't use the new speed during the current move.

If a creature with normal speed 30 entered the area after moving 15 feet, what is its remaining available movement?

You have a speed of 30 and move 15 feet. You then enter the area and you now have a move of 15 feet. Your already-traveled movement is subtracted from your move speed, which is 0 or less, so you can't use the "new" speed.

If a creature with normal speed 30/halved speed 15 starts in the area and moves 15 feet to exit the area, what is its remaining movement?

This is the opposite, you have a speed of 15, and move 15 feet and are out of the area. Your speed is now 30, and you've only used 15 feet, so have 15 more feet available.

Difficult Terrain

The section you are referring to is under the Speed heading, which states that "[t]he following rules determine how far a character or monster can move in a minute, an hour, or a day." Therefore, those rules wouldn't apply to in-combat movement since that has its own rules.

The rules for in-combat difficult terrain are found PH page 190:

Every foot of movement in difficult terrain costs 1 extra foot. This rule is true even if multiple things in a space count as difficult terrain.

Nothing about halved movement. Hence, I conclude that halved movement, at least in combat (since the spell in question has a duration of 10min, I don't see it being that useful outside of combat), means $\frac{speed}{2}$.

• How does the wording about different terrain which I added to my question above impact your answer? Jul 18, 2016 at 15:22
• @Robbie edited to add section on difficult terrain you cited. Jul 18, 2016 at 15:38
• Wouldn't it take you all the time imparted to you to move 15ft inside the slow area of effect, and thus once outside, you are out of time to do anything more? In other words, your speed is 15 inside the area, you reach the edge, you now have 0 left since you already took all the time you had to reach said edge (opposed to you offering an extra 15ft to your characters)? Jul 20, 2016 at 3:19
• Moving is incremental, not decremental: "On your turn, you can move a distance up to your speed," (PH190) . You start at 0' moved, then you move 15'. You are now out of the area and have spent 15' of your 30' of movement. Jul 20, 2016 at 3:24

In the first case the creature has zero remaining movement, in the second 15 feet of remaining movement.

Since Spirit Guardians involves slowed speed, not creating difficult terrain, the difficult terrain rules aren't applicable. (Mis)applying the difficult terrain rules would give a completely different answer, because they alter how you count distance moved instead of changing your speed — difficult terrain and reducing speed just work completely differently. To understand how this spell works, you just apply the normal movement rules, but with your changed/changing speed.

Normal movement has two moving parts: your speed and how far you've moved this turn. Your speed is your current limit, and how far you've moved is independently counted up. Player's Handbook page 190, “Movement and Position”:

On your turn, you can move a distance up to your speed.

As soon as how far you've moved equals your speed, you're done moving. (This is simplified by ignoring things like the Dash action which provide extra movement, since we want to look at the basic mechanics relevant to this question.)

As a result, a creature that normally has a speed of 30, moves 15 feet into an area of effect that cuts their speed in half to 15, they can no longer move because 15 feet moved now equals the distance limit from speed 15.

Meanwhile, a creature that starts with a halved speed of 15 who moves 15 feet — and manages to get clear of the effect halving their movement with those 15 feet — will have a speed of 30 and be able to continue moving. However, that only works if the 15 feet actually takes the creature out of the effect — if it only takes the creature to the edge, they're out of movement but still within the area, and have to end their turn an agonising hair's breadth away from freer movement.

Having found more information since first posting my question, I think it beneficial for me to consolidate my findings and those contributed by my fellow answerers.

The original question sought to understand the impact of halving (or multiplying) a character's speed in an area affected by a spell, which in this case has a 15' radius of effect.

I will break my answer into two parts, the first comparing "Movement" and "Speed", the second addressing the particulars of my original question and using movement within a round.

Part 1: Movement and Speed

The word "speed" is used interchangeably with "movement" in the PHB. There is little doubt that "speed" and "movement" are two words for the same thing: the former connotes the ability to travel notable distances, whereas the latter connotes short-distance movement in combat.

The PHB addresses time and movement in its chapter Adventuring, and it there makes clear that combat is a subset of adventuring (which is intuitively obvious) when it states on page 181 under Time:

In combat and other fast-paced situations, the game relies on rounds, a 6-second span of time described in chapter 9.

Under its table Travel Pace, on page 182, the PHB divides the Fast, Normal, and Slow spaces into time distances of Minute, Hour, and Day, which are directly proportional to a standard character's single Round movement of 30 feet per six seconds. There is thus no difference between traveling ability and combat movement ability other than time scale.

During adventuring movement, there is a speed-altering category called difficult terrain which impacts travel and combat alike and has clearly defined effects. A character moving through difficult terrain is said to move at half speed. Under the heading Difficult Terrain on page 190, the PHB defines half speed as follows:

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.

Whether or not a spell's AoE counts as "difficult terrain", "half speed" is clearly defined as "moving 1 foot ... costs 2 feet of speed".

Within the Combat chapter of the PHB, on page 190, when talking about difficult terrain, it says:

Every foot of movement in difficult terrain costs 1 extra foot.

So we see that difficult terrain is defined exactly the same way inside and outside combat, written in two slightly different ways to make the point clear.

Regarding having different speeds available to you, and easily extrapolating movement in and out of difficult terrain and spell effects, we read:

If you have more than one speed ... you can switch back and forth between your speeds during your move. Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can't use the new speed during the current move.

The mechanisms for difficult terrain and spell areas of effect (and all movement everywhere in the PHB) are clearly interconnected and, in fact, the same.

Part 2: The Particulars of a Round

For the spell Spirit Guardians, which has a 15' radius,

An affected creature’s speed is halved in the area.

By "speed is halved", as above, we clearly understand "moving 1 foot ... costs 2 feet of speed" and "Every foot of movement ... costs 1 extra foot."

A character moving at 30 feet per round of speed would move at the equivalent rate of 15 feet per round inside the area of effect because each foot of movement would cost 2 feet of speed. @Alexis Wilke insightfully stated:

When you are slowed down, you just count each hexagon or square as 10 feet instead of the usual 5.

Here are a few scenarios:

• Our character moves 15 feet before crossing the border of the affected area. Having 15 feet of movement remaining, the character can move 7.5 feet within the area, rounded down to 7.
Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater. (PHB, 7)
• Our character starts at the center of the AoE and must move 15 feet to escape, at the cost of 30 feet to his movement for that round. Upon reading the border, he has has 0 feet of movement remaining. His total physical movement this turn is 15 feet.
• Our character starts 5 feet off of the center of the AoE and must move 10 feet to escape, at the cost of 20 feet. Upon reaching the border, he has 10 feet remaining of his 30 feet per round movement and, using it, moves a total of 20 feet this turn.
• @Alexis-Wilke also said:
As a side note, when going in diagonal (squares), you are authorized to see each square as a simple "1.5" the distance (which is close to square root of 2). So if you have 30 ft to walk in normal terrain you can go 4 squares in diagonal. (i.e. one square diagonal is 1.5 x 5 ft = 7.5 ft) You could apply similar math to slowed down straight line, but then it becomes complicated because you do not end up in one specific spot. Fights are clear if you are in a specific spot. Not in between two. So I would not allow it. You have to forfeit that last half movement.
If our character moving from the center crosses along the diagonal, the 15-foot radius of the spell matches two square diagonals of 7.5 feet each, and so he would escape the AoE by landing on the third square diagonally with 0 remaining movement.

Furthermore, we must remember that a round is

a 6-second span of time described in chapter 9.

Movement is given by the maximum distance a character can physically travel in a given span of time, in combat 6 seconds. If a character were to move as suggested in other comments, moving 15 feet to escape the half-speed area and then regaining a speed of 30 feet to move an additional 15, then the character, who has a base speed of 30 ft/6 sec = 5 ft/sec and a halved speed of 15 ft/6 sec = 2.5 ft/sec would move 15 feet at 2.5 ft/sec for 6 seconds followed by 15 feet at 5 ft/sec for 3 seconds for a total of not only 30 feet but also 9 seconds--in a 6-second round.

This is impossible without a time-dilation device or an effective speed of 45 feet per 6-second round. Effectively you are arguing that a character is not only moving faster than humanly (choose your race) possible--even faster than at a fast traveling pace--but that he is doing so while magically slowed down!

• I think that what SevenSidedDie and Christoper are trying to say is that the distance moved does not change, even if it takes you twice the amount of time to travel it. So if you have to move one square and seemingly use up 10 feet, you would still have 25 ft of your 30 ft left if the next squares are not affected by the slowdown effect. Which is why SevenSidedDie has "an edge case," too. Jul 21, 2016 at 18:30
• But if that were the case, then moving into the AoE after moving 15 feet, you would still have another 15 feet of "slow" movement, and you would still reach your target "slowly" within the same 6-second round. Jul 21, 2016 at 19:35
• @Robbie Nope, you'd step into the effect and then have to stop, because now your speed = 15 and you've already gone ≥15 feet and what your speed is at any instant dictates how many feet you may move that turn. But that's all in my answer already. Jul 21, 2016 at 23:00
• This answer is plain incorrect. Speed and Movement have different meanings and are not synonymous. Also nowhere in the description of difficult terrain does it generally define 1/2 speed as spending extra movement. It clearly is saying the only for difficult terrain. -1 Dec 26, 2018 at 14:36

I think that the simplest is to look at it graphically. When you are slowed down, you just count each hexagon or square as 10 feet instead of the usual 5.

 Normal      Slowed down
+---+---+---|---+---+---+
5   5   5 | 10  10  10
|
Going in:   |
--->--->--->| (15 ft)
|--->  (10 ft, cannot use the last 5ft)
|
Going out:  |
|<---<---<--- (30 ft)
|
Going out when closer to the edge:
|<---<--- (20 ft)
<---<---| (10 ft)


So, you have 15 ft left when hitting the slowed down area (30ft - 15ft walking toward that area) and can move one hexagon while in the slowed down area.

When exiting, you used up your 30 feet to reach the edge of the slowed area.

If you were closer to the edge, we can see that you used up 2/3rd of your movement within the slowed down area, and then use the last 1/3rd once outside. Your speed reverted to normal so distance wise it looks like you moved twice as much on the last 1/3rd of your movement!

You could use the same mechanism if the character was also slowed down by a spell, on top of the slowed down area. Just multiply all the numbers by two:

+---+---+---|---+---+---+
10  10  10| 20  20  20
|
Going in:   |
--->--->--->| (30 ft)
|
Going out:  |
|<--- (20 ft)
<---| (10 ft)
|
Walking within:
|    <--- (20 ft, cannot use the last 10 ft)


Now you burn the 30 ft just trying to get to it and you can only move one hexagon within it, although if you reach the edge, you can move another hexagon once outside (otherwise you lose the 0.5... speeds, like most everything in 5e, are rounded down.)

As a side note, when going in diagonal (squares), you are authorized to see each square as a simple "1.5" the distance (which is close to square root of 2). So if you have 30 ft to walk in normal terrain you can go 4 squares in diagonal. (i.e. one square diagonal is 1.5 x 5 ft = 7.5 ft) You could apply similar math to slowed down straight line, but then it becomes complicated because you do not end up in one specific spot. Fights are clear if you are in a specific spot. Not in between two. So I would not allow it. You have to forfeit that last half movement.

• This is not how movement works or is counted in D&D 5e (you don't count slowed movement as double). It's not even mathematically equivalent, as it gives different results than the actual rules do. Jul 17, 2016 at 18:17
• Well... that's my interpretation and I think it makes the most sense. If it is twice harder to move, you get exhausted faster and thus use up your speed 2x faster for the same distance. There is nothing illogical in that to my point of view. Jul 17, 2016 at 18:33
• This changes the order of operations of a mix of multiplication/division and addition/subtraction, which we all know changes the result of an equation. This would be improved by citing rules that support this order of operations. Jul 17, 2016 at 18:39
• The is an interesting house rule but nothing more. None of this has any rules support. If it does please add it. Dec 26, 2018 at 14:41