It's been remarked often that the movement rates in D&D are ridiculously slow in real world terms. I had never quite grokked just how slow, though, until a comment from Charles Taylor of Spells and Steel on my post about lightly armored adventurers spelled it out in clear, real world units of measure.
A movement rate of 120' per turn equals an unbelievably sluggish 0.136 miles per hour! (About 0.219 kph, for my metric readers.) The official rationale for this is that the party is moving cautiously, in poor lighting, while mapping and keeping an eye out for traps and hazards. Now, it's perfectly reasonable to expect that a party exploring dark and dangerous places might move a bit more slowly than the average human walking speed of 3 mph (4.8 kph), but a mere 1/20th of that? It seems clear to me that Gygax and Co. did not start out by asking themselves, "What's a realistic speed for people carefully exploring a cave?" Instead, they seemed to be interested first and foremost in a movement rate that did not allow a party to zip through a moderately sized labyrinth in under half an hour, and concocted the cautious movement rationale as a post hoc justification.
Realistically, a movement rate of 1 to 1.5 mph (1.6 to 2.4 kph) should amply account for caution and poor lighting. That translates to 880 to 1320 feet per turn. Charles Taylor's suggestion of 1200' per turn falls within this range, and has the additional benefit of modifying the original movement rate by a tidy factor of 10. That's about 1.36 mph, for those keeping track of all the numbers.
This has a pretty major implication as far as movement in the dungeon: You really don't need to track time spent moving. Yes, that's right: No more counting off squares on a map as the party wends its way down dark corridors to see how far they get in a turn. Advancing 60 feet down a hallway takes all of 30 seconds - not even worth the effort of bookkeeping. Instead, moves will end at natural stopping points (typically keyed locations or other points of interest on the map), regardless of distance. Time is tracked not for the movement between points of interest but for the actions taken at those points. The heightened caution aspect of timekeeping comes into play when the party chooses to stop and search an area for hazards or listen for signs of trouble farther on, not during normal movement. Carefully searching a room for traps and treasure takes time, as per the rules as written. Fighting, loading treasure into packs, and other activities are assumed to take about one turn. Tallying up all those items should serve perfectly well to make sure a dungeon expedition doesn't turn into a mad dash from one end of the dungeon to the next in five minutes flat, without the need for a ridiculously slow standard movement rate.
Encounter speed is also slow, though not nearly in such absurd proportions. A 40' per round encounter movement rate equates to a bit over 2.7 mph (4.3 kph), which is slightly slower than average walking speed. Why don't you and the goblins take a nice leisurely stroll toward one another before you try to beat the crap out of each other?
If we assume a 6-second combat round instead of 10 seconds, though, that bumps our speed up to a healthy 4.5 mph (7.2 kph) - a brisk trot. A running speed of triple that is 120' per round, or about 13.6 mph, which is a pretty good sprint for the average human, and really a pretty desperate and reckless thing to do in the dark twisting corridors of a dungeon.
Outdoor movement, according to the rules, is calculated in yards rather than feet. A movement rate per turn of 1200 yards works out to just over 4 mph. Not bad. (Not that there are a lot of situations that call for a per-turn movement rate outdoors, but hey, it's nice that the math works out.) An outdoor encounter speed of 40 yards per 6-second combat round equals about 13.6 mph. That's about how fast the average person sprints in a 100-yard dash. Good for running speed, but way too fast for a standard encounter rate. Half that would be a brisk jog, which sounds about right.
Large scale, overland movement doesn't need much adjusting. Just divide the base movement rate by 50 instead of 5 as in the normal rules.
So, to recap:
Dungeon exploration rate would be 1200' per turn, standard encounter speed 40' per round, and up to 120' per round sprinting.
Outdoor exploration rate would be 1200 yards per turn, standard encounter speed 20 yards per round, and up to 40 yards per round running.
Overland movement rates would be equivalent to the base movement divided by 50; e.g. 24 miles per day for a base movement of 1200 per turn.
A few thoughts on encumbrance
The standard rules have all movement rates scaled by encumbrance; e.g. if you're lightly encumbered, all of your movement rates are reduced by 25%. Exploration speed in the dungeon drops from 120' to 90' per turn, wilderness exploration speed drops from 120 yards to 90 yards per turn, etc.
When you think about it, this is more than a little absurd. If moving along at 1.3 mph instead of 3 mph is cautious, then it's cautious. You don't need to slow down even more just because you're wearing plate and carrying a sack of loot.
My tentative impression is that encumbrance should set a cap on a character's absolute maximum movement rate, not take proportional bites out of every situational movement rate. A character carrying a light load might be able to run at only 3/4 speed, or 90' per round, but should be capable of any lesser speed, including normal exploration and encounter speeds.
It also seems logical to apply some kind of fatigue effect, though, which would accrue more quickly the more weight a character carries, and require rest to recover. A character in plate and carrying five weeks of rations and 1,000 gold pieces should tire more quickly than one with just the clothes on his back. I'm not sure how that would best be accomplished mechanically without bogging down the flow of the game, but it's worth pondering...