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Hex Crawls

One of humanity’s defining impulses is exploration and discovery, and hex crawls primarily cater to that. Hex crawls are about going to new places and finding new things until you have explored and documented a previously undiscovered wilderness like you were Lewis and Clark but with manticores, and then broken into the homes of and subsequently massacred any native populations you find to be inconvenient like you were Andrew Jackson but with goblins. The difference between a hex crawl and a wilderness campaign is that a wilderness campaign places the emphasis firmly on lurking evils threatening to overwhelm civilization. If the status of other regions is unknown at all, this is mainly an issue of reconnaissance, with potential dangers lurking underneath a fog of war, dangers that can be assumed to eventually come to your house and eat your lunch.
A hex crawl places the emphasis on exploration. The wilderness may well be full of dangerous things, but those things aren’t particularly imperialistic and probably won’t become a problem for your homeland in the foreseeable future. You don’t explore to reveal threats, you explore because you like exploring. There might be some other quest involved – maybe you’re trying to recover artifacts stolen from your kingdom and scattered through the wilderness, or maybe you’re mapping the area for future settlement, or maybe you’re exploring a newly discovered wilderness to try and make allies amongst the locals and convince them to join your fantasy federation. Ultimately, though, players are exploring, and whatever the main plot is, it shouldn’t make them feel bad about sticking their nose into every hex on the map just to see what’s in there. In other words, that thing open world games do sometimes, where they set up the main plot as being a countdown to Armageddon and then set you loose in the map and you chase collectibles and unrelated side quests for five hours before you remember that whole “demonic invasion” thing? Don’t do that.

The Scope of a Hex Crawl

The process of traveling from one hex to another is a process. It can take 5-10 minutes, and it’s a lot of die rolling and assigning of roles and taking notes and may include a random encounter that will take up another fifteen or twenty minutes on top of that. You can cut down a lot on this by drastically oversimplifying the process, but you shouldn’t, partly because each step involved adds important options to travel that at some point your PCs will want to take advantage of (i.e. you can limit all hex travel to fixed speeds, but at some point PCs will want to get on a horse and ride through the night and there is no particularly good reason they shouldn’t be able to do that), but mostly because this process makes for some good winding down time between hex encounters/adventures.
The important corollary to this is that most hexes the party enters should have enough stuff in them to justify a 5-10 minute simmering down period afterwards. At least 70% of all hexes should contain, at minimum, a significant encounter that will hopefully take at least half an hour to resolve. The remainder should have at least some kind of feature or landmark to them just so that each hex has something to discover, even if it’s just an ancient wizard’s tower that is now completely, genuinely empty. And again: These kinds of things shouldn’t be more than 30% of the hex crawl maximum and preferably less than that. The party shouldn’t be spending more than a quarter of the session just walking places, even in a hex crawl where walking places is the selling point, so they should have a high probability that after going through the process of traveling from one hex to another, there will be something in that next hex.
This means that when you make a hexcrawl, you are committing yourself to creating quite a few encounters and adventures. A 6×6 hex crawl, about the minimum size before it starts to feel like exploring a prison cell, has 36 hexes. That means you need at least 26 encounters or adventures to shove into them, and you’ll probably want at least one urbancrawl town somewhere to use as home base. That urbancrawl is going to probably be at least two or three adventures (or adventures’ worth of material, depending on how you distribute it) of material. The average published adventure has an average of about three main encounters in it, which means that a 6×6 hexcrawl has approximately (counting three urbancrawl adventures) a dozen adventures’ worth of material. That’s a full campaign, and it’s also the minimum size of a hex crawl, so definitely think carefully before making your hex crawl any bigger. If you use 30 mile hexes (which can definitely seem like an odd size at first glance, but we recommend it for reasons we’ll get into later), a 6×6 hex crawl is about the size of the entire island of Ireland, the modern nation of Austria, the state of South Carolina, or the Japanese island of Hokkaido. In other words, it’s the size of a respectable medieval kingdom and thus perfectly suited for a complete D&D campaign from low to mid levels (high levels usually go to some lower plane to stab Satan in the face, or you go underwater to fight the sahuagin queen, or something).
You can get larger than 6×6, much larger if you’re particularly ambitious. A 6×6 hex crawl is large enough for the average published campaign, but the average published campaign lasts less than a year in play (5e releases them about as fast as the average group can complete them, and their release schedule is twice annually). An 8×8 hex crawl is about the size of Greece, the nations of England and Wales combined, the state of Illinois, or the Liaoning province of China. This will require at least 15 adventures’ worth of content in the hexes, and if you still have only one major city, close to 20 adventures’ worth total, which is close to a year’s worth of play. Not-England is the setting for quite a few medieval fantasy stories, so this is also a pretty defensible size, though not recommended for new players.
If you want to add in Scotland for the entire island of Great Britain, you will need a 10×10, or 21 adventures in the hexes, and you’re also probably going to want at least two cities at this point, probably three, which means you’re looking at over 30 adventures, which is definitely going to take at least a year to play, and we haven’t even added in Ireland yet. We’re far behind the size of Germany or Japan, both in the neighborhood of 13×13, or 119 encounters plus multiple urbancrawls for a total of 50+ adventures, which can take the better part of two years. It’s even further behind France, a 16×16, 54 adventures and you’ll probably want a minimum of four major cities for 70+ adventures’ worth of material, putting us in the neighborhood of three years of content for a weekly group that moves fairly quick. So, we’re starting to see the limits of a hex crawl here.
Taking this to a crazy extreme just for fun, covering all of China would be a 64×64, and while modern China is much larger than the ground covered by Romance of the Three Kingdoms, if you wanted to add in the ground covered by Journey to the West (and the other two of the Four Classics while you’re at it, but those both take place in the same area as RotTK) that would actually be slightly larger than what the PRC covers and would take about 30 years to clear if you played 45 times per year (i.e. a weekly game with occasional canceled games). Want Europe and the Middle-East to play fantasy Crusader Kings? You’re now at the unimaginably large 100×100 hex crawl, and clearing a hex crawl of that size properly packed with encounters would take actual centuries. Just creating such a crawl would require either tons of crowdsourcing or else would have to be a generational project, and that’s with us using larger than normal hexes. If you decide to convert to six-mile hexes, the number of encounters needed to fill in a hex crawl the size of [region] is even bigger (which is not why we recommend thirty-mile hexes, but it is a nice bonus).
Going larger than 10×10 is generally not advised and even a 6×6 is plenty big for most purposes. It is theoretically plausible to go as large as 16×16 but it will be monumental both in creating and running it, so it is recommended only for dedicated groups who want to play very long term exploration campaigns. Remember, there’s nothing stopping you from stringing different campaign types together if you’d like to keep playing the same characters, so even if you do want a very long term campaign, it still might be wiser to run an 8×8 hex crawl and then have the characters move to a different location where they have a wilderness campaign or an intrigue campaign or whatever. If you really want to explore a France’s worth of hexes, though, 16×16 is the upper limit of reasonable doability, but it is reasonably doable if you’re committed.

Hex Design

What kind of encounters or adventures do you put in a hex crawl? Hexes can obviously contain complete adventures and you should have at least a few of those lying around. Multi-level dungeons and mysteries are by far the most obvious and easiest to do, while wilderness journeys are generally speaking incongruous with the hex crawl, but not always. While a multi-day journey makes little sense in the confines of a single hex, the wilderness journey mechanics work just as well for a short journey through an area densely packed with danger, for example, if there’s a hidden temple in a forest thick with dark fey, you could use a wilderness journey adventure for getting through the forest to the temple. Mass combats are also a strange sort of adventure to have in a hex crawl because the party might discover the adventure, walk away from it for weeks or months, then come back, and it’s odd that the military situation hasn’t shifted at all. Some hexes might indeed be home to some kind of military stalemate or cold war that won’t change until the players arrive to tip the balance one way or the other, and others might simply be home to a large army of enemy monsters, such that clearing the hex requires players to gather up an army of their own and drive them off.
Complete adventures are big and take a lot of time both to design and play, though, and there’s no reason to make all 70% (or preferably more) of your significant hex encounters include them. All that a hex encounter absolutely needs is a hook, a bit of build up, and then an encounter for pay-off. As such, it’s acceptable for a hex encounter to be a simple two-scene mystery, in which you have a hook that immediately draws the players’ attention (the burned out remains of a caravan, for example), three clues that all point towards the same scene, and then in that scene there is a confrontation with whoever sacked the caravan, some goblins in a cave or whatever. One-page dungeons with just four or five encounters make for good hex fodder (Matt Colville has a few videos on how to make a one-page dungeon, and that general formula can be repeated a couple of times without getting old so long as you don’t put any two of them immediately adjacent to one another, so players don’t get them one right after another).
Once you get used to designing these relatively brief encounters, it becomes easy. Then the trick is to come up with enough new ideas that it doesn’t become stale. Any adventures you’ve had fun running or playing can probably be converted to work as a hex encounter. Adventure paths can often be chopped up and have their pieces planted into a hex crawl like a macabre garden. Generic dungeons like the Temple of Elemental Evil or the Keep on the Borderlands can be dropped in practically any setting. Material from entirely different genres, like the Warhammer 40k or Warhammer Fantasy RPGs, the Star Wars RPGs, or Call of Cthulhu can often be adapted.
Material from totally different media can provide the ideas needed to create encounters or adventures from scratch. Mine books, movies, and games for plothooks or setting ideas. Bioshock is about an underwater society gone mad due to the corrosive psychological effects of superpower granting drugs, which is easily fodder for a D&D dungeon crawl. The Terminator is about a nigh-invincible assassin sent to kill a hapless victim, one so powerful that he most be evaded and worn down over multiple encounters rather than simply confronted and killed. Alien is about being stalked by a monster that will 100% kill you if it catches you alone and unawares, but will be more vulnerable if confronted by a large, prepared group, making it a game of trying to find the monster without being separated and picked off. The sequel Aliens is about those same monsters forgoing the whole “hidden stalker” thing and just Zerg rushing the party through the air ducts.
And when you inevitably run out of ideas or energy for the project, just give it a few weeks to simmer and then come back to it. Hex crawls of any appreciable size will take a longtime to complete, so don’t expect to get it all done in a week between session zero and the start of the campaign. It’s surely possible to hammer out three shoddy encounters a day and jam them all into a 6×6 hex crawl, but it won’t be any fun.
The hex crawl is also going to require a random encounter table. This table should be a d100, but you’re going to roll it on each and every hex the party enters, so don’t be afraid to stock large parts of it with “nothing happens.” Each major region of the hexcrawl should have its own random encounter table, with each random encounter table covering no more than about 40 hexes. This means that it’s fine if your 6×6 hexcrawl has only a single encounter table, but an 8×8 should have at least two and a 10×10 should have at least three. “Regions” don’t necessarily have to be contiguous. You might have a plains table, a hills table, and a forests table for something resembling medieval Britain or France, and none of those regions would be particularly contiguous. Splitting things up by terrain often makes more sense than splitting them up by political boundaries, but sometimes it makes a big difference whether it’s the Knights of Solamnia or the Knights of Neraka who are patrolling the region. Pick whichever is more appropriate for the hex crawl you’re running.

Crawling Hexes

So, you’ve got a bunch of encounters and adventures out there in the big, unexplored wilderness, you’ve set your party down in their starting hex, and now they would like to go to an adjacent hex. How do?
Traveling from one hex to another is a ten step process, although many of the steps are just “check to see if X is happening,” where the answer will usually be no. The checklist is below, and it’s recommended that you print it out and use it as an actual checklist. If you have a DM’s screen and any part of that screen is not already occupied with information you actually use regularly, tape the hex crawl checklist over that part of the DM’s screen for as long as you’re running a hex crawl. Otherwise, just make sure it’s as handy as your monster stats or your adventure outlines. This checklist is optimized for speedy resolution, trying to remember all ten steps off the top of your head will slow things down and often doing them out of order will require redoing steps you did earlier than you were supposed to. The process of traveling from one hex to another is meant to be the first stage of building tension towards the climax of whatever encounter is found in the next hex, but doing it from memory can often be frustrating instead.
  1. 1.
    Assign roles
  2. 2.
    Choose bearing and pace.
  3. 3.
    Record speed loss from travel.
  4. 4.
    Roll Constitution saves.
  5. 5.
    The party guide makes a navigation check.
  6. 6.
    Mark off supplies and/or hunter rolls Survival.
  7. 7.
    Enter the new hex (if the party has enough speed to make it today).
  8. 8.
    The guide rolls Stealth (if sneaking).
  9. 9.
    Roll for random encounter.
  10. 10.
    The spotter rolls Perception to find the hex encounter.

Step 1: Assign Roles

There are three roles in a hex crawl. Attempting to do two or more of the roles grants disadvantage on all checks related to the roles, so it’s worth splitting them up. The guide is in charge of making navigation checks, which can be either Survival, Investigation, or History. History is rolled at disadvantage, however, because knowing the terrain in theory is very different from knowing it in practice, so preferably the guide will have either high Survival or high Investigation. The spotter needs high Perception, and the hunter needs high Survival. Party members with no role assigned may assist party members with their role checks, granting advantage on the check, however party members with a roll assigned may not assist other party members with their roles.

Step 2: Choose Bearing and Pace

Once roles are assigned, the party chooses a bearing and pace. Bearing is pretty straightforward. Every hex has six faces in the same cardinal direction. If you have true columns, these directions are north, northeast, southeast, south, southwest, and northwest. The party picks one of these directions to head in.
The pace the party can move at determines their speed. Each hex is thirty miles across and requires 30 speed to cross by default, however some hexes require more than 30 speed to exit (see step 3 for details). A creature’s strategic speed is equal to their tactical speed in feet, i.e. most medium creatures move 30 miles per day, some move 35, etc. A party moves at the speed of its slowest member, but anyone riding a mount may use the mount’s speed instead of their own.
A party moving at a slow pace gets advantage on either their Stealth check to avoid detection in step 8 or to their Perception check to find the hex encounter in step 10, but not both, they get advantage on their Survival check to hunt in step 6 no matter what, and their movement speed is multiplied by 0.75. On the other hand, a party that’s moving at a fast speed gets disadvantage on their Survival check in step 6 and their Perception check in step 10, can’t roll Stealth in step 8 at all, and their movement speed is multiplied by 1.25.

Step 3: Record Speed Loss From Travel

Now that you have your pace set, you know what the party’s speed is. Every time the party exits a hex, their speed is reduced by an amount depending on what kind of hex they’re leaving. Tangled jungles require more speed to exit than open plains, roads make things much faster, and so on. If the party has some leftover speed and can’t quite exit a hex, then mark down how much speed they’ve spent on it today and they can add more on the following day. It may take the party several days to exit particularly treacherous terrain. If the party is moving with fly speed instead of land speed, they treat all hexes as being plains hexes, regardless of their actual terrain, however they do not benefit from roads (air currents and the like slow down long distance air travel just like bumps and gullies do on the ground, but cannot be paved over).
Speed cost to exit
Blue Water
+Water Crossing
*Blue water hexes can only be crossed by creatures with a swim or fly speed. Creatures who cannot breathe underwater may not end their movement for the day on a blue water hex (unless the hex has some kind of small speck of land to rest upon, although most hexes with islands in them are coastal rather than blue water). If forced to do so because they lack enough speed to exit the hex, they drown.
**Creatures with a swim speed or some other means of crossing rivers, straits, etc. etc. without looking for a ford may ignore the speed cost multiplier for a water crossing. Finding some way to cross a river with a grappling hook and feather fall in order to avoid the speed cost multiplier if perfectly acceptable. Note that a bridge still counts as a water crossing, unless the hex also has a road. Roads lead directly to bridges, but otherwise the party must follow the river until they find the bridge, which is not ultimately all that different from following the river until they find a ford.
Subtract the speed required to exit the hex from the party’s total speed for the day (or remaining speed, if they have exited a hex already on that day).

Aside: Why Thirty Miles?

30 miles is a very long distance for daily travel. It’s weird that characters can move that quickly by default. Why use 30 miles as the default hex size? There are several reasons. First of all, it means that all questions about how many hexes you can cover if you’re riding [creature] from the Monster Manual are immediately answered. Rather than plugging creature speeds into some complex equation, you just take their fastest relevant speed and that is the number of miles they can travel in a day. Divide by 30 and round up, and that is the number of hexes they can crawl in a day.
Secondly, it converts very easily into maps that use the DMG’s recommended size for kingdom or continent scale while still having each hex be big enough to justify the process of walking through it. The DMG recommends kingdom scale be 6 miles per hex, small enough that you’d expect travelers to move multiple hexes in a single day, which is useful for troop movements when being 25% faster means actually covering an extra hex, but bad when you want each individual hex to include a major feature. It means drastically increasing the work required to populate a hex map or else filling up the map with tons of empty hexes that are disappointing to enter. The continent scale is 60 miles per hex, so big that it would certainly take more than one day to move from one hex to another on foot, and requiring enormous speed boosts before you move noticeably faster. A 30 mile scale sits comfortably in between these two. 30 mile hexes are 5 hexes across in kingdom scale and half a hex across on continent scale. This means any hex map that uses either of these scales can easily be retrofitted to use hex crawl scale.
If you aren’t planning on using any official maps as the basis for a hex crawl, or if you don’t mind changing their scale significantly (they’re fictional locations, after all, and changing the distance between Waterdeep and Baldur’s Gate isn’t that different from changing how critical hits work, which is certainly within the GM’s purview), it’s justifiable to instead use thirty kilometer hexes, which is much closer to what you’d expect the average person to cover in a day while also preserving the translation of tactical to strategic speed. The system works identically, although it does mean that you will need more hexes to cover the same physical area, so a 6×6 hex crawl would only cover an area the size of Belgium, and a sizable 10×10 would only be the size of Portugal or Iceland.

Step 4: Roll Constitution Saves

This is an optional step. If the party is out of speed but would really very much like to leave the hex they’re currently in before the end of the day, they can roll a Constitution save for a forced march. For every hour of forced march, they add 0.1 to their speed multiplier for pace (so a slow pace goes from 0.75 to 0.85, a standard from 1 to 1.1, and a fast pace from 1.25 to 1.35), but everyone must make a DC 10 Constitution save, +1 for each hour past the first (so DC 11 for the second save, DC 12 for the third, and so on). A character who fails the save must either take a level of fatigue or else fall behind.
Characters may not take a long rest immediately after exiting a hex but before rolling for random encounters or to spot the hex encounter, so any fatigue taken while traveling will continue to affect them during a random encounter as well as during any hex encounter, although in the latter case the party can usually take a long rest before engaging with the hex encounter (although sometimes the hook for a hex encounter is “something tries to murder you”).
If the characters are marching for an extra nine hours, then there is no longer enough time in the day for them to take a long rest, unless they are elves, in which case they need only four hours of trance for a long rest and it takes thirteen hours of forced marching to prevent them from having one. A party who’ve forced marched this long receives only a short rest whenever they finally go to sleep, unless they sleep in to get a long rest, in which case their speed multiplier for the next day is reduced by 0.1 for each extra hour they must sleep to get a long rest. Fortunately, all of this is fairly rare, since succeeding on nine Constitution saves in a row is really quite difficult.

Step 5: The Party Guide Makes a Navigation Check

Ignore this step if the party is following a road, river, or some other landmark, or if they’re moving directly towards some clearly visible landmark like a mountain or tower on terrain flat enough to keep the landmark in constant or near-constant view. In any of these cases, the party stays on course automatically.
If the party has insufficient speed to actually exit their hex this turn, skip this step. If the party is exiting a hex, the guide must now roll Survival, Investigation, or (at disadvantage) History. The DC for this check varies based on terrain. Coastal, desert, and grasslands terrain make this check at DC10. Arctic, forest, hill, mountain, and blue water terrain make this check at DC 15. Swamp and Underdark terrain make this check at DC 20. If the check succeeds, the party leaves the hex and enters the hex they intend.
If the check fails, the party veers. Roll a d20. On a 1-8, the party has veered left. For example, if they were going north on a map with true columns, they veer left and go northwest instead. If they were going south, a left veer would take them southeast. On a 13-20, the party veers right in the same manner. On a 9-12, by sheer dumb luck the party goes in the direction they intended.

Step 6: Mark Off Supplies And/Or Hunter Rolls Survival

This step is completed only once per day, regardless of how many hexes the party travels through. If the party hunter elects not to hunt in a hex (presumably in order to save his daily hunting for a hex with a lower DC), he cannot reverse that decision after leaving the hex.
The party hunter may make a Survival check at DC 15 in coastal, forest, grassland, hill, and mountain terrain during spring, summer, or fall, DC 20 in any of those terrains during winter (note that some forests, grasslands, etc. etc. are located in a climate that does not have winter, and in these areas the DC will be 15 year-round) or in a swamp or Underdark terrain regardless of season, and DC 25 in desert or tundra terrain. If the hunter succeeds on his check, the party does not have to mark off supplies for the day, however there is not nearly enough time to properly store and preserve hunted or foraged supplies for long term transport, so no amount of success will allow the hunter to increase the party’s food supply.
If the party goes hungry, everyone in the party takes a level of fatigue and does not gain the benefits of a long rest at the end of the day.

Step 7: Enter the New Hex

Ignore this step if the party doesn’t have enough speed to actually enter the new hex today.
There’s nothing to actually do in this step, however all steps from here on in are made based on the hex the party has entered, not the one they were leaving.

Step 8: The Guide Rolls Stealth

If the party is attempting to avoid detection, the guide rolls Stealth and records the result for step 9. Otherwise, ignore this step.

Step 9: Random Encounter

Roll a random encounter on the random encounter table appropriate to the section of hex crawl that the party is now in (and remember that the party has left the hex they were entering and is now in a new hex). Compare the Stealth check (if any) made in step 8 to the passive Perception of the monster (if any) in the encounter. If the Stealth check succeeds, the party has snuck up on the encounter.

Step 10: The Spotter Rolls Perception To Locate Hex Encounter

Every encounter in a hex crawl has a Perception DC required to locate it, ranging from DC 5 for obvious things like castles on a grassland up to DC 30 for incredibly well-hidden features. A single hex can even contain multiple encounters or features in it, each with different Perception DCs (this is not recommended, but mainly just because hex crawls are major undertakings to begin with and it’s probably a bad idea to increase your workload). At this point, the party’s spotter rolls a Perception check, and locate any hex features or encounters that they meet or exceed the DC for. What the party does with that information is, of course, up to them.
If the party has veered off course and become lost, spotting a landmark will usually clue them in as to where they really are. Or maybe they’ll assume there are two exact copies of the same landmark in different locations. How long you let them puzzle over this depends mostly on how funny you think it is watching them try to figure out how and why this village is in two places at once.


A hex crawl can be split into three main features. Most of the map is covered in hexes that contain quick encounters or short adventures. A few places on the maps contain complex, multi-level, multi-faction dungeons. Then there are the cities where the party can go to rest and recuperate between excursions. Cities are hardly a stranger to adventure and danger, though. In fact, cities usually concentrate crime and corruption just like they concentrate everything else. Put a hundred thousand people in close proximity to one another and the absolute number of thieves, assassins, vampires, and changelings will get high enough for each of them to have their own guild even if the overall density doesn’t budge.
So you’d expect cities to contain some adventure just like the wilderness and the dungeons, and the structure for that adventure is the urbancrawl. Just like hex crawling and dungeon crawling allow a party to explore wilderness and fortresses according to their own curiosity and courage (or foolishness, as you prefer), an urbancrawl lets them explore a city.
Urbancrawls are divided into neighborhoods, each neighborhood containing at least one city adventure, often several. In general, travel between neighborhoods takes no significant amount of time and aren’t impeded by random encounters. Players say they want to go to the docks and they simply arrive. Some specific encounters might trigger “the next time the players try to leave [neighborhood]” or “the next time players leave any neighborhood,” but these are rare exceptions to the general rule. Similarly, the basic layout of the city is immediately obvious. Players know what all the neighborhoods are called and what they look like from the start. There is never a question of terrain like there is with hex terrain, where players generally speaking have no clue what even the most basic features of any hex they haven’t been to will be. Traversing an urbancrawl isn’t about literally traversing the distance, it’s about finding needles in haystacks.
Each neighborhood has a number of encounters and adventures associated with it, and each one of those has a discovery DC. Players can roll Investigation to go looking for trouble in a neighborhood, and the DC they’re looking to hit is the lowest DC of any discovery DC in the neighborhood. If they exceed the DC by at least five points, they discover another encounter, provided they also meet that encounter’s discovery DC. For example, say there are three encounters in a neighborhood with discovery DCs of 10, 10, and 15. The party rolls a 15 on their Investigation check. They hit the lowest DC, 10, and they exceeded it by 5 points, so they discover an extra encounter, getting both the DC 10 encounters. They do not get the DC 15, however, even though they rolled a 15, because they’ve already discovered two encounters and they only exceeded the lowest DC of the neighborhood by 5 points. As another example, say the party is in a neighborhood with encounters at discovery DC 15 and 25 and they roll a 20 for their Investigation check. They find the DC 15 encounter and they have five points left over to discover another encounter, but since they didn’t meet the discovery DC of the DC 25 encounter, they don’t find it.
Discovering an encounter almost never reveals anything particularly alarming, but instead just reveals the hook for the adventure plot. For example, hitting the discovery DC means finding out there’s been a murder in the local flophouse, but doesn’t reveal the identity of the person who committed the murders, or it might reveal that giant vermin from the sewer are making off with people’s stuff, but finding out how INT 3 creatures the size of a warhorse figured out how to catburgle requires completing the dungeon that is their sewer lair.
Multiple encounters and adventures in an urbancrawl might all be related to one another, for example, a thieves’ guild might have lots of encounters relating to their racketeering, their dark magic item, the corruption of the town guard, their assassination gigs, and so on, spread throughout multiple neighborhoods. Opposition in the same sub-plot as already cleared adventures might start reacting differently, for example, the thieves’ guild might increase security after a few of their operations are shut down and they realize someone’s after them. If the party isn’t particularly covering their tracks (or especially if they’re bragging about putting a stop to crime), the thieves’ guild might send assassins after them.
There might also be some adventures in a sub-plot with absurd, 35+ discovery DCs, but which can still be located through the other adventures in that sub-plot, for example, the thieves’ guild lair might be extremely well-hidden, but interrogating captured thieves might help the PCs discover it, or you can use the three clues method for a mystery adventure, only instead of placing the three clues that lead to the thieves’ guild lair all in the same adventure, you space them out between several, which means the players will probably have to complete multiple thieves’ guild adventures before they stumble across one of the clues leading to the hideout. Additionally, finding and defeating the leader of a sub-plot antagonist organization, like capturing or killing the head of the thieves’ guild, might automatically clear some or all of the remaining encounters or adventures in the sub-plot.