In OD&D time is measured in turns (10 minutes) and rounds (1 minute), with 10 rounds per turn. You check for wandering monsters at the end of every turn, with an encounter being indicated on the role of a 6 on 1d6. Hence, the probability of a random encounter is:
| 1 turn | 16% |
| 1 half hour (3 turns) | 42% |
| 1 hour (6 turns) | 66% |
| 2 hours (12 turns) | 88% |
Hanging around in the dungeon obviously isn’t conducive to a long or healthy life. (Not that this should come as any sort of surprise.)
However, it should be noted that — if the PCs are in a position where the wandering monster would not necessarily be aware of them — surprise is achieved 33% of the time (1 or 2 on 1d6). There’s really no way to calculate this into the numbers above (because too much depends on circumstance), but certainly in practice I found that this gave the PCs a not infrequent ability to avoid the wandering monsters. (Particularly once they realized what was going on and made certain preparations — like shutting the doors to a room while searching it — which would make it possible.)
What the 16% chance of an encounter every 10 minutes really boils down to, however, is a very active dungeon complex: The monsters are not just sitting in their rooms waiting for the PCs to kick down the door. (This is a topic I’ll probably be re-visiting in later essay.)
Working out the probabilities for wandering monster mechanics can tell you a lot about the nature of the setting. (And, conversely, when you’re designing a setting you should work out the probabilities to make sure you’re doing what you think you’re doing.)
For example, I’ve been homebrewing a structure for 3rd Edition wilderness exploratory adventures. The strucutre is based around a 4 hour watch (with 6 watches per day). The length of the watch was chosen because it’s convenient for the hex scales I’m using for my wilderness map.
Since I’ll be setting up random encounters for the various wilderness regions, I whipped up this quick cheat sheet for the probabilities involved:
(I may eventually end up standardizing these checks to X in 1d20 — 2 in 1d20 is identical to 1 in 1d10 and 3 in 1d20 is fairly equivalent ot 1 in 1d6 — but I wanted to start off with a more traditional approach.)
Obviously, the higher the probability the more likely the PCs’ journey will be interrupted. If I set the probability very high (2 in 1d6), then I’m virtually guaranteeing that their progress will be slowed to a crawl. If I set the probability very low (1 in 1d20), then I’m allowing them to potentially move through an entire region without ever meaningfully interacting with its contents.
ARCHIVED HALOSCAN COMMENTS
Justin Alexander
Dungeons, though, aren’t usually as densely populated as an office building, a mall, or apartment building.
A housing subdivision might be getting closer to the mark. But if I go outside my house at midday and just stand there, it’s quite easy to find long spans of time during the day in which I won’t see anybody walking past.
More generally, however, I was talking about them specifically in the context of the typical dungeon complex. Both in comparison to the rather static affairs found in virtually all modern modules, and — more generally — in terms of how it actually plays at the table: If you play strictly by the book in OD&D, random encounters happen a lot.
Consider that a character with a move of 6″ will only move 120 feet in a turn. (And that’s assuming they just move. If they slow down to precise mapping or to search, they’ll make even less progress.)
IME, characters were facing more random encounters than keyed encounters. I think roughly 3-to-1 is the ratio I was experiencing.
This ratio would hypothetically shrink if you chose to eliminate encounters after they had been met (creating the sensation of a complex hunkering down), but nonetheless the sense left with both me and my players was one of high activity: The dungeon was coming to the PCs rather than the other way around.
Thursday, March 26, 2009, 3:15:41 AM
Johan Larson
“What the 16% chance of an encounter every 10 minutes really boils down to, however, is a very active dungeon complex…”
I don’t think that’s quite right. Imagine walking around a human structure or community (an office building, a mall, a housing subdivision) and expecting to see someone only every half an hour or so. If that’s in a mall, the mall is closed, and your encounter is with a janitor or a security guard. If it’s a housing area, you’re either in cottage country in low-season or you’re walking around in the middle of the night.
If the dungeon is a place where people (well, things) live and work, encounters should be much more frequent.
Saturday, March 21, 2009, 5:47:37 PM
Justin Alexander
The decimal system has corrupted me utterly, apparently.
Thanks, I’ll get that fixed.
Saturday, March 21, 2009, 12:28:41 PM
glaurung_quena
Correction: 10 minutes per turn gives 6 turns per hour, not ten.
Friday, March 20, 2009, 8:40:25 PM
One of the things I have been thinking about using for hexcrawls and random encounters is adding some ‘flavor’ to the wilderness with a large portion of random encounters being of a natural outdoor event type. Living in the middle of nowhere (though it’s gotten a lot less so over the past decade) for the past decade or so I’ve noticed that there are a lot of things that happen that I have no real explanation for. Noises in the woods, sudden flights of flocks of birds, rather louder noises in the woods, and that kind of thing.
A lot of that kind of thing happens and you hardly have any idea of exactly why it happened. Sure, I know what it sounds like when a squirrel or small bird is moving through the woods (and it’s a lot louder than a city-dweller would think). A deer is really remarkably loud. Particularly in the fall when the leaves are all over the ground. But maybe a sudden flight of birds out of a stand of trees is actually due to a party of orcs or some other critter passing through the area? I think this kind of thing can add a lot of the flavor of what being outdoors is really like to overland travel. It might be particularly useful to add a bit of a feeling of unknown dangers lurking just out of sight to wilderness travel.