At what area and distance can a character detect a trap?

Question

At what area and distance can a character detect a trap when actively searching? And on which with passive.

Hypothetically. Suppose that in front of the character is a corridor 30 squares long and 3 squares wide. There are five traps in the corridor, each 3 square wide and 1 square long. Traps are hidden. The player assumes that there are traps in the corridor and begins to actively search for them. How many traps and how far is he able to spot?

And exactly the same question but if the character is not looking for traps relying only on passive perception.

Answer 1

It is not a matter of distance

Strict trap detection rules, including distance, visibility, etc. is a 3.5e/Pathfinder thing. This approach works poorly within the 5e paradigm.

There is a reason why skill checks gone completely in 5e. Previous editions had the "Search" skill with strict rules about distance (10 feet, iirc). A DM was supposed to use this rules every time a PC might detect a trap. This could look like this:

DM: You see a 50 ft hallway... blah-blah

Player: I use my Search skill to search for traps.

DM: Make a Search skill check.

Player: (rolls poorly)

DM: You didn't find any traps.

Player: I search once more. (rolls great)

DM: You didn't find any traps.

Player: I go 10 feet forward and use my Search skill again.

This example shows two typical problems. The first one is that the DM has to ask for check even if there are no traps at all, because the rules say so. The second one is that the player clearly understands that there still might be traps, he just didn't find it yet because of a bad roll result, so he wants to roll again. Even if the roll was successful, it works only on 10 ft distance, so the player needs to go 10 feet forward and repeat the attempt.

This approach leads to a lot of unneeded rolls, wasting DM's and players' time (and it's real time, not game time). Many modern RPG systems had to rethink priorities, so does 5e. It diverges from the "using skills" mentality, shifting focus to the storytelling. Now it's the consequences that matters:

DM: You see a 50 ft hallway... blah-blah

Player: Do I see any traps?

DM: No you don't, at least not from the place you're standing now.

Player: Okay, I start to moving forward, carefully examining walls, the floor and the ceiling...

DM: Make a Wisdom (Perception) check.

Player: (rolls poorly)

DM: Well, you did notice a trap. The bad news — it's a bit late.. After 20 feet you hear a distinctive "click" under your feet. Make a Dexterity saving throw.

When you have a hallway tightly filled with five similar traps, I suggest you to treat this as one single trap with higher DC. Since your players now make one check instead of five, this might save you time (real time, not game time), allowing your group to spend this time on more interesting and fun things in game. Traps are a tricky game element, which can be quite exciting or very boring. I highly recommend this Angry GM article for more ideas about using traps, making them more interesting.

Answer 2

The limit of character senses is the limit of trap detection

Characters in dangerous situations are on the lookout for danger. Traps can be physical, magical, conversational, or whatever fits the situation as if you get caught in the thing badness ensues.

Detecting a potential trap

Detecting something that is amiss and arouses a sense of something dangerous could be a defect in the floor, peculiarity of the ceiling, sudden change in air pressure, or anything that the character can perceive as out of place for the setting. Noticing the out of place thing, whatever it may be, is a wisdom based skill.

If during a search in the character’s feelings zone there are several hidden traps, should he see everything?

Since looking for danger is an always on thing in situation that might include dangers, applying passive checks is appropriate. A character should pick up everything out of place that has a difficulty class (DC) less than or equal to their passive perception score. Weather that detected thing is actually a trigger for a trap or not is part of the story and may be fun to find out. Set the difficulty for picking up on the danger based on its concealment.

Actively looking for a trap

In a dungeon, a character is likely always "looking for a trap" which falls under passive checks. However, if there is a particular situation or cue that would cause a character to stop all other activity and movement to focus solely on looking for a danger, then by all means roll an active check.

Narrating passive vs active checks

As a DM, narrating these cues can provide a sense of ominous danger and the senses through which it is perceived. Sighting a change in the floor material when it changes from rough stone to well laid tile, smelling a change in the air composition, the sudden absence of conversation could all be valid cues precipitating an active roll.

Similarly, pointing out what characters notice as being amiss in a passive sense and giving the players a chance to respond provides the prompts that drive the game. They don't all have to be true traps. Some brief false positives can illustrate that the DM recognizes the party is being cautious. E.g. "Horatio notices a gap where mortar should be between the bricks of the wall up ahead."

Traps Adding to a Story

Dealing with static traps that characters can't interact with other than avoid or disarm is rarely fun unto itself. Consider how to use such devices to add to a story. Some examples include:

• Old broken traps that are easy to detect and harmless indicating the age of the location.
• A dinner party where a character notices the subtle shift in topic indicating an orchestrated danger illustrating the character's knowledge of the social circle their involved with
• An out of place object that is clearly placed to attract the naive and greedy showing what sort of thieves and interlopers the designers expected.

Follow up questions

Some questions to consider when designing and using traps that may be beneficial to story tellers:

• What kind of traps are fun for player characters to encounter?
• What are traps designs that player characters can interact with?
• What does a trap add to a location or adventure?
• What are traps that do not add to the experience?