Which dice for mine field?

[Balor01]

So here is the deal. I have kobolds who had two weeks to trap a narrow valley leading to an old tomb they are protecting for BBEG. They have great positions and will pepper my PCs from great heights (they also have some dispel magic rods to ruin day of anyone casting flying) while PCs presumably advance toward tomb entry.

Now, valley is trapped with a number of low-CR traps, but I have no desire to "map" entire valley. So I decided to roll each turn PCs move (they have no trap detection) to figure out if they step on a "land mine".

Which D should I use? Nasty DM I am, I like 1d2 (joke), but I would like to ask for some ideas redarding Ds or percentages.

Tnx playgrounders

[Person_Man]

d4, the pointiest of die!

Seriously though, I would describe the valley in general.

There should be one obvious approach with deadly traps. There should be a few dead animals lying about, but otherwise the traps are well hidden and reset on a daily basis by the Kobolds.

And then there should be one non-obvious approach, which the Kobolds use to move supplies in and out on a daily basis, and when they or an ally wants to get into the lair during bad weather when it'd be too dangerous to traverse the obvious path, like a narrow mountain pass or a small underground tunnel that opens up on the other side of the valley. This entrance should be a hidden door, and there should only be 1 trap (meant to keep animals out) with an obvious way of disarming or avoiding it.

If the PC's take the obvious route, then everyone gets hit by a near deadly traps - (Level * d8 or something similar). If the PCs bother to look for tracks (Survival) or search for alternative routes in (Spot), then they find the non-obvious way.

[Yora]

A little bit of math here:

Rolling a d10 for ever 10 feet of movement means a 10% chance that they will trip one trap when moving over 10 feet of minefield. However, it is actually easier to reverse it and say "there is a 90% chance that they get through without stepping into anything.
When they have to cover 20 feet of ground, the chance is reduced to 90% of 90%, or 81%. At 30 feet you have a 73% chance and at 60 feet you get down to just about 50% of being safe.

If the minefield is relatively short and has a small number of points at which the PCs can step into a trap, you should use a relatively small die with a high chance for traps.
But if you have a long distance of ground they have to cover, you should not make the dice too small and the chance too high, or they will step into a lot of traps.

Since the chance for a trapped spot and the chance to make a successful Spot check to find a trap are completely randomly rolled and the players have basically no way of affecting the outcome, you should keep the amount of traps they step into relatively low. When they roll the dice it may seem quite exiting at first, but when they stepped into the fifth or sixth one and realize that the outcome does not have anything to do with what they do, it will soon become just annoying and random.

So my suggestion is to look for a number of trap-rolls and a dice for trap chance, that will most probably have every character encounter two or three traps, not more. Not completely sure about the math here, but you could make it so that there is a 50% chance that each character is finding 2 or more traps.

[Lapak]

Roll 1d6.

Assuming there's enough traps in the valley to be a real threat, every player moving in the valley has an x-in-6 chance of triggering a 'mine', where x is the number of 5' squares they move through in a given round (max chance 5-in-6 regardless of movement.) If they are clever, and go with the plan of 'everybody step where I step' as they move across the map, only the leading character has a change of triggering a trap, but everyone in the follow-the-leader group has their movement rate reduced by half.

And I agree with Person_Man: if the players are sensible enough to search for alternate routes you should give them a chance at bypassing the valley-o-death.

EDIT: For people running full tilt across the field, reset the chance after every six squares of movement and roll again, so a player moving 50' across the field has a 5-in-6 chance AND THEN a 4-in-6 chance, and might actually hit two 'mines' along the way.

[Vladislav]

If you don't want to think about this too hard, I'd say just roll a d20 on every move action, and if the result is equal or less than the number of squares moved, you step on a trap. Replace d20 with d12 or d10 if you're feeling more cruel than usual.