("Fast Blast" = "I had an idea but haven't thought it all the way through yet"... and I've been listening to Coast to Coast AM again lately...)
120' movement per turn is ridiculously slow, right? Even if you do the double move described in some versions of the game (in which case why not just say movement rate 240'?). So how to explain it?
Keep in mind that underground, it's dark. Available light sources don't really illuminate very well. Watch the Descent movies, or go walk around a cave (or dark woods) with a lantern.
There is no casual movement in dark, unfamiliar, hostile environments. Period.
Exploration Movement (movement rate per 10 minute turn)
- Allows mapping with "good enough for government work" measurements given.
- Allows automatic spotting of obvious traps (open pits, etc).
- Nonobvious but exposed traps (tripwires) are only triggered if the party is surprised (roll as if it's an encounter) by them.
- Normal chances of surprise in an encounter
Walking Movement (movement rate per minute)
- No measurements given for mapping
- Obvious traps are not noticed if they surprise the party
- Nonobvious traps are automatically triggered
- Double chances of surprise in an encounter
Running Movement (movement rate per round)
- Automatically trigger all traps in the way
- Automatically surprised in an encounter
- No mapping, and the Ref doesn't even have to tell you about passages or doors you pass... in fact, if in a totally unfamiliar area, save vs Paralyzation (every round?) or run into a wall or trip or otherwise have your nose come into swift contact with a hard surface.