[from another forum, in response to a question about whether having a Rogue with passive Perception 17 in the party makes it pointless for the DM to have DC 15 traps in a dungeon]
Traps have a perception component and a deduction component.
Perception: "Huh. There aren't any footprints on that brick."
Deduction: "Therefore, a poison arrow probably comes out of that hole when you step there."
There's nothing wrong with automatically giving the rogue the Perception component, and leaving the deduction up to the player. If he or she has a sufficiently high Investigation skill you can give them the deduction component too. "You enter the hall and immediately notice signs of a blowgun trap in front of one of the doors in the hall."
Even then, traps still aren't pointless because they are still dungeon features, e.g. the Rogue can potentially turn traps to his advantage by taunting monsters into them. Also, there will be some traps that can't be easily bypassed even if you know they are there (e.g. Glyph of Warding), and in those cases there's still a decision to make about whether or not to risk the trap in exchange for potential reward.
P.S. Additionally, I rather think passive Perception/Investigation is a bad match for traps because they do tend to make things static and boring: there shouldn't be THAT much difference between Perception 14 and Perception 15. Therefore I would recommend also allowing active Perception and Investigation rolls for anyone who is actively searching for signs of traps in a particular place, and if they ask for the specific thing ("are there any bricks without footprints?") I would give that fact to the player automatically without a roll.
In other words, the rather high DC 15 is just to notice signs of a trap automatically, without effort, as something you're always doing in the back of your brain. This justifies using even higher DCs for traps that are actually well-hidden and hard to spot even if you're looking for them, like noticing contact poison on a doorknob by the way it glistens (could easily be DC 25 = unnoticeable to a normal person without special training who isn't looking specifically for it).
--
I could not love thee dear, so much,
Loved I not honor more.
Traps have a perception component and a deduction component.
Perception: "Huh. There aren't any footprints on that brick."
Deduction: "Therefore, a poison arrow probably comes out of that hole when you step there."
There's nothing wrong with automatically giving the rogue the Perception component, and leaving the deduction up to the player. If he or she has a sufficiently high Investigation skill you can give them the deduction component too. "You enter the hall and immediately notice signs of a blowgun trap in front of one of the doors in the hall."
Even then, traps still aren't pointless because they are still dungeon features, e.g. the Rogue can potentially turn traps to his advantage by taunting monsters into them. Also, there will be some traps that can't be easily bypassed even if you know they are there (e.g. Glyph of Warding), and in those cases there's still a decision to make about whether or not to risk the trap in exchange for potential reward.
P.S. Additionally, I rather think passive Perception/Investigation is a bad match for traps because they do tend to make things static and boring: there shouldn't be THAT much difference between Perception 14 and Perception 15. Therefore I would recommend also allowing active Perception and Investigation rolls for anyone who is actively searching for signs of traps in a particular place, and if they ask for the specific thing ("are there any bricks without footprints?") I would give that fact to the player automatically without a roll.
In other words, the rather high DC 15 is just to notice signs of a trap automatically, without effort, as something you're always doing in the back of your brain. This justifies using even higher DCs for traps that are actually well-hidden and hard to spot even if you're looking for them, like noticing contact poison on a doorknob by the way it glistens (could easily be DC 25 = unnoticeable to a normal person without special training who isn't looking specifically for it).
--
I could not love thee dear, so much,
Loved I not honor more.
