For you, if a party is traveling along, what are the differences in perception checks they get to find an ambush (or whatnot) if they're traveling full speed and looking around as they go vs. traveling half speed so they can take more time being observant?
in my game I will normally take a look and see if anyone's passive bonus beats the DC to see them... if they do I tell everyone what the area looks like then tell the PCs with the high passive only "But you see more" and explain... this will give them a chance to get a surprise round on those laying in ambush.
if no ones passive is high enough I will ask everyone for an active... anyone who makes the DC is not surprised those that did not make the DC are surprised roll initiative.
IF the PCs are setting the ambush I have them roll stealth (or other relevant skill/ability) DC equal to the creatures they are ambushing passive perception... I then skip the active check entirely unless there is a big reason to have it and give or don't give the suprise... it is a little unfair to monster/npc but that is what I do