Wilderness Encounter Avoidance

Last Saturday's session featured a reconfigured party with improved wilderness exploration skills, including an explorer (an ACKS class that, somewhat oddly, merges abilities from the halfling and ranger classes in B/X-era D&D). The party generally responded to random encounter checks by attempting to avoid contact entirely, with a great deal of success -- almost to the point of making the session feel slow and uneventful, which is never a good thing.

To ensure high tension, I'm going to make some official rulings on how to use encounter avoidance techniques. There are three basic ways to avoid an encounter:

Hobbits: Difficult to spot!

1. Hiding. This is possible only for certain classes. Thief-types have a hide in shadows ability that improves with level. Once hidden, thieves can either remain motionless, or try to move (away, or in for an ambush) using move silently. A few other classes (explorer and elven ranger) have the difficult to spot skill.This is a hiding ability that works very well in the wilderness, and moderately well in dungeons, but does not function at all in crowded urban areas. Any other class can only be hidden through the use of magic, like the massmorph spell.
2. Evasion. Evasion is, essentially, running from a combat before it even has time to start. It is an obvious activity and will result in the party being detected temporarily and at least briefly pursued. If successful, it means that the pursuit has been abandoned. It will, however, result in lost travel time and some risk of getting lost. In particular, it typically requires leaving any marked path or road, and at least one navigation check will be required to locate the road again.
3. Parlay. Every encounter requires some sort of reaction roll, to see if the monsters encountered are hostile. If they are intelligent and someone has a language that allows communication, then it may be possible to advance that person forward to function as a spokesperson, to add modifiers to the roll from things like charisma or diplomacy. (In narrow dungeon passages this is only possible if the speaker is already standing in front. In an outdoor environment, anyone can easily step forward out of the group)

These options are mutually exclusive. If you are hiding, you aren't running away or attempting negotiations, and so forth. Hiding and evasion become impossible if the party is surprised, which happens 2 in 6 times. (Exception: Explorers have a 10% chance to allow the party to evade even when it is surprised.)

A fourth option is to simply run away after the encounter starts. This is always possible, unless the party is slower than the monsters. However, it always has the same drawbacks as evasion, and becomes very difficult to execute once in melee contact.