Ackbar Said It Best...

I have seen a lot of questions recently on this 5e D&D Facebook group about how to handle trapfinding- so I wanted to give my two coppers on the situation.  I'm basing this off of only the information in the PHB and DMG since I haven't read the whole section in XGtE yet.

WARNING: I laid out a lot of basic information about the rules as written just to make sure that we are all on the same page- if you want to read some potential house rules and homebrew options, then jump down a ways...

I was surprised to see that there is not an entry for passive perception in the index of the PHB, but instead there's a more general passive check listing (which refers to the listings for ability checks- nobody said D&D was easy).  Regardless, PHB 175 says that a passive check does not involve a die roll and can represent the average result for any task done repeatedly "such as searching for secret doors over and over again."  It also explains how these scores are calculated.

There's been some confusion over when you make a perception check or an investigation check that has been perpetuated by inconsistencies in published modules.  PHB 178 says that perception lets you spot, hear or "otherwise detect the presence of something" while investigation is about making deductions based on things you see and allowing you to "deduce the location of a hidden object."  Jeremy Crawford has reiterated the separate status of these two skills, but admits that there is some overlap...

Let's assume that perception is the key ability for finding most traps- and this is mostly true as we can see on DMG pages 122-123.  Passive perception has been calculated for all the members of a party, the DM is aware of each party member's score, and they are all walking down a 10' wide corridor towards a collapsing roof trap.  The DC to spot the trip wire is 10.  How should the DM determine if the trap is spotted before it is triggered?

Option One: Compare the passive perception of each party member to the DC of 10 and then let those PCs become aware of the danger.  

• Practically anyone trained in perception (with a non-negative wisdom modifier) will succeed automatically beginning at 1st level- and even clerics, monks, and rangers who are not trained will have a good chance at spotting it as well.  This trivializes the trip wire based traps and the basic pit traps in the DMG.  This does reward player characters that make this consideration in their builds though.

Option Two: Call for perception checks from the PCs to see if they meet or exceed the DC since (conceivably) they are looking for danger as they proceed down the corridor.

• This gives all the PCs a chance to spot the danger and is more exciting in the short term- because rolling dice is much more fun than not rolling dice.  It also increases the likelihood of metagaming when everyone tanks the roll and then stops and says "What did we miss?"  Most tripwire and pressure plate based traps will be spotted with lucky rolls regardless of skill modifiers- and it can result in rogues not seeing what barbarians do (which is potentially funny)

Option Three: Say nothing unless a PC announces that they are going to make a perception check. 

• This is the way it has been traditionally done in editions prior to the inception of "passive" checks.  Since it is the old school method it has become the old habit of most DMs that have played through previous editions.  It may be the most fun for the DM (it is probably the easiest way to catch players unaware of traps), but it can be taxing on those on both sides of the screen as it can bog the game down with the players constantly checking for traps every 5' of an empty corridor.

I was thinking about how easy it is to spot traps when I started to consider how I'm going to make it more interesting (within reason).  These are some of the rules that I would consider instituting at my table and I'd be interested to see if they would be helpful to any other DMs out there.

• You have to move slower to best search for traps than you would if you were just strolling down the corridors of a dungeon carelessly.  I think unless you are moving at a slow pace (no more than 20 feet/round) you should take disadvantage on your passive perception and any perception checks.  A -5 to the passive check should make it less likely for the simplest traps to be spotted by everyone in the party unless they are moving slow enough to be looking around.  This was almost some sort of rule in 5th edition- proven in the fact that its referenced in the description for the feat Dungeon Delver (now omitted as errata): 

• You have to be in the front most rank of a marching order to best search for traps.  This highlights the difficulties of getting a good look down a corridor or at the ground of the person in front of you.  This also encourages rogues to take the role of scout- out a good 11' feet or so (at least) in front of the party and looking for danger.  If there are party members between the rogue and the activator for a trap, then they would have disadvantage on their passive perception and any perception checks they make to see it.
• I know that part of the idea behind the whole advantage and disadvantage system in 5th edition was to simplify the game and remove the need for so many conditional and circumstantial modifiers to a roll of the die- but you could also draw upon the guidelines from previous systems to adjust the difficulty as well.  In circumstances where the means of activation for a trap could be obscured by rubble, a low mist, darkness, or any other environmental factor you could add +2, +5, or +10 to the difficulty.

From my cursory look over the section in Xanathar's Guide it looks like a lot of care was taken to spice up and flesh out the rules for traps, but I haven't really given it the due diligence as of yet.  If you have any other suggestions or anything to add to this examination of the "core" trap rules, then reach out to me at critthulhu@gmail.com or on Twitter @Critthulhu.