I love pit traps. LOVE 'EM! My players are well aware of my pit trap love and are always trying to keep an eye out for them, so I have some quick rules I use for PCs noticing them.
Just walking along: you won't know until too late
Using a 10' pole: 1 in 6
Moving cautiously: 1 in 6
Moving cautiously and with a 10' pole: 1-2 in 6
Someone trained in identifying traps with a 10' pole: 1-2 in 6
Someone trained in identifying traps and moving cautiously and with a 10' pole: 1-3 in 6
Everything else: you won't know until too late
Moving Cautiously is really slow - about half the normal dungeon exploration speed. This doesn't seem like a big deal until the number of Random Encounter checks double ... and I don't pull punches with wussy random encounters.
Of course, the pit traps don't always open automatically - grotty old pit traps generally open up and swallow an unfortunate soul on a roll of 1-3 in 6. Fresh ones open on a 1-5 in 6. So those folks up front with the poles and the skills who are looking might miss the traps and the mage so desperately secreted in the middle of the party might still end up in the hole.
Depending on the specifics, pit traps get a 1d4-1 initiative roll against the party members. If the trap "goes first" you end up in the hole. PCs who win initiative get a saving throw (Feat of Agility vs d20p+0) to avoid falling into the pit. Running or being otherwise distracted negates the avoidance chance.
Being Tied Off
One of my players is pretty clever and always has a rope handily tied around his waist to avoid the worst of a pit trap fall (his characters have been the victim many times over). This will negate the damage form the fall a bit, but Rope Use comes into play as does a Feat of Strength for the person (or persons) on the other end of the rope. Also, it doesn't mitigate all of the damage (a sudden fall and slam into the side of a pit still hurts) and with a rope all in the way combat can be a bit more difficult - it could affect initiative, causing folks to trip up, or stop some crucial movement because the PC forgot to untie. One must weight the risks against the benefits.