For our Lost City game, the players originally were looking at some fairly heavily armored guys - mail, scale, etc. and hefty loadouts of gear.
Once we settled on jungle, they started to peel that back a bit.
Sending them the rules for how the jungle and heat will affect them caused people to roll things back even further, since they had a concrete understanding of what a little less armor and gear would mean. It went from "be light, it'll help" to "be light, because it means I suffer 2 FP less per fight and start 2 FP higher when one happens."
It's worth noting that the axe-and-shield knight, a self-described heavy fighter, will be wearing leather armor (albeit ornate and enchanted.) We may have one guy in mail, he hasn't decided yet, but it might just be a willingness to suck up the pain to ensure he's got extra DR despite the downsides. Or he might revise and get lighter armor.
But it's not their travel speed that they were concerned about. It was the fatigue penalties. The climbing penalties (there will be climbing). How long it takes to get there? That just decides how much food to bring. Twice as long means twice as much food, not a big deal in most cases.
Here is something I have learned as a GM:
You can't get players to trade armor and equipment away for movement if, when they arrive at a combat, it's a heavyweight championship.
Nobody sprints to the heavyweight title fight. Nobody brings a light tank to a heavy tank battle, no matter how much faster the light tank is.
If the fight will always be waiting for them, it behooves them to bring the heaviest and most effective combat loadout possible.
Adventures which depend heavily on movement rate, but which ultimately have a setpiece end (the lost temple, guarded by tireless guardians) or are just a place to explore at your leisure, don't incentivize faster movement. If you have a bunch of hexes to explore and a lost plateau to reach (X1!), does it matter how long it takes?
You can ensure the travel is a grind in some environs - food issues, disease, wandering monsters, etc. That way a shorter travel time and better speed is good. You can make sure being slow in the dungeon means you can't run away and burn through more torches.
But if armor and a heavy loadout are more of an advantage in the combat situations than a downside in a non-combat situations, players will load up. I've found most players will choose "better equipped to fight" over "better equipped to get away." And why not? Winning fights is how you get the loot from the monsters. It's how you clear out the evil temple or send the semi-materialized death god back to his plane. Running is how you leave the problem for later.
Here are some ways I think work to incentivize lighter loadouts and less armor:
- Bonuses for speed. If having extra speed is an advantage, people will seize it. "If we're fast, not only can we sack the Temple of Woe but also get there before the high priest puts the solid gold chalice back in its unopenable box!"
- time limited adventure locations. If you only have X days to loot the area before the army shows up, you will have a real incentive to skimp on heavy stuff. If the chance to adventure literally ends at a certain time (not just becomes harder), then people will take a hard look at lighter loadouts.
- level playing field. If the place you end up with is the middleweight championships or lightweight championships, people will be tempted to overmatch it with heavy armor but will also feel like lighter armor is an option.
- legitimate tradeoffs. If armor gives a lot of benefits and slow movement gives few real downsides (the adventure waits, non-combat problems aren't movement-sensitive, etc.) than people will armor up. GURPS nicely comes with two of these - FP loss from combat and marching is tied to encumbrance, and so is Dodge (which is a legitimate and extremely valuable defense.) D&D5's way of putting DX bonuses only for light armor is a good example of a legitimate tradeoff.
- remove the fear. This is a more general suggestion - don't make being without your armor the worst thing in the world. Make sure it's also a good thing part of the time. No one wants to bet their guy's life on light armor instead of heavy if there is no upside. But if they know that it's not going to be more lethal if they go light, they'll consider going light. If they suspect you're pushing them to remove their armor so you can really let them have it, they will keep it on.
Most of those work pretty well in combination - my Lost City setup is a combination of Level Playing Field (foes aren't hanging out in scale or plate, either), Time Limited Adventure Location (the players have 5 sessions, 6 max, to loot as much as they can, and slow movement means more encounters which means less session time in the city), and legitimate tradeoffs (heavy loaouts and metal armor = more fatigue, and this is potentially costly.)
Some players will still choose heavy armor and heavy loadouts. That's a legitimate choice. They may choose to lose out on some opportunities for loot or to avoid monsters or to flee and bet on the value of armor in combat and their extra equipment being useful. That's fine, too.
There are heavy-handed ways to ensure the characters strip down, like heat traps and magnet traps. Or nagging at the players until they take off their helmets and remove their plate armor. Or you can just tell people it's hard to sleep in armor. But I'm reminded of a book I'd read that said the conquistadors that took out the Incas didn't remove their armor to sleep. I believe it - if you feel like the fight takes X, and that fight can come at any time, you'll drag X to the fight no matter what it costs to get it there. Incentivizing things so going without the usual maximum combat gear is a better way to go. Don't make it a punishment for not doing it, make it - at the very least - a mix of punishment for not doing it and a reward for doing it.
I think I managed that.