4e, and to some extent 3e, have done away with random encounters. This has been brought up before and I don't intent to discuss the pros or cons of random encounters, merely what has replaced it: the ambush on the road.
These were incredibly common when I was playing Living Greyhawk where the module would have some seemingly random fight between point A and B that had no bearing on the story save to round out the XP budget. I played a double round mod that was a double rounder because it had four encounters unrelated to the main plot as you travelled overland. Despite this, I still regularly have ambushes in my encounters.
A What Now?
I'm using the term "ambushes" to describe any encounter on the road that serves no plot purpose: a small side-trek without any actual story. Most of these are just encounters but small tangential side-quests that can be introduced and end in less than a single game session, between the goals and story you hope to accomplish during said session.
Most of the time they happen during overland travel. They're little sub-stories added for pacing so travel isn't as uneventful and boring as an Indiana Jones red line. A wagon with a broken wheel, a predator attacking for food, a band of bandits attempting robbery, or chance meeting with some NPC with a unique personality.
I tend to think of Dragon Age when I discuss these, picturing my path along the map interrupted by crossing swords. I've even described an ambush as such to my players, who are avid Dragon Age fans (given 1/2 my 4e group has worked for BioWare at some point). Although, many of these actually had a plot element, introducing potential new companions or introducing other plot elements.
If ambushes are just tacked-on encounters, why even have them? Well, part of that harkens back to the days of random encounters. In many ways ambushes are just pre-planned random encounters, as 4e doesn't play as well with actual randomized encounters. Fights need more than just monsters, they need terrain and an environment. An ambush is a little like having a pre-rolled random encounter where you know what the terrain is like and have the monsters stats ready.
As mentioned earlier, this is a little bit of gaming convention. When travelling between cities or adventure locations it just feels natural to include a fight. Possibly because the PCs are moving from dungeon to dungeon and you just want to use some monsters not found underground. Possibly for a narrative break between locations. Possibly to just reinforce the idea of Points of Light and the wilds as a dangerous place. Or possibly because it's Dark Sun and that's what happens you leave the city.
Pros and Cons
The good things about ambushes include the narrative break between locations mentioned earlier, and the reinforcement of the world's danger or theme. They can also be a good way to inject combat and direct danger to an otherwise uneventful game. For example, in an adventure that only involves moving about the city talking to lords or interacting with urban NPCs a back-alley ambush might be a nice break and diversion.
An ambush can also be meaningful filler. I was reminded of this recently. I had a good idea for the start of my adventure and for the end, but the end had built-in choices that lead to the next adventure, and playing after that would mean writing three or more alternate adventures. It was easier to have a little mini-adventure in the middle, and use a monster I wouldn't normally be able to employ.
The problem with ambushes is the length of time they can take. Combat isn't fast in 4e, and an unnecessary fight might kill any tension being built in the story. A fight means you have to drop whatever else you were doing for an hour and focus on the tactical side of the game. And if not done well they can feel a little superfluous.
4e is also a fast game. Each simple encounter gets you 1/10th of the way to the next level, a significant step. If there are regular quests and fights of harder difficulty then it might be even more, perhaps 1/8th or 1/6ths of the distance between levels. A throw-away ambush might easily be one of the most significant fights of that short level, and hardly meaningful or memorable.
Adding a Little Randomization
This is an idea I've been refining for a while.
As mentioned earlier, 4e has pulled much of the randomness from encounters. So every encounter the DM presents is tailored for the PCs with a difficulty set to accommodate the players. This came up in my contribution to the sandbox v. rails blog carnival. If the PCs face a too hard fight they're either rolling terrible, using terrible tactics, or the DM made the fight unfair. The last option really trips up players, because they expect the DM to play nice and not set out to kill them. The random encounters of earlier editions gave the DM a little bit of a pass, as encounters could be too easy or too hard strictly from the dice. Now, complacency has set in, as the players think the DMs are unlikely to be unfair at the risk of alienating players.
So how to add a little randomization to 4e ambushes? This is tricky as I'm a proponent of fights not being defined by the monsters included. The kernel of the fight should be some interesting and descriptive hook. It's not the fight with the grell, darkmantles, and grick, it's the fight in the flooded room with the currents that drag you across the battlemap to the drain; it's not the fight with magical turrets and dwarf skeletons, it's the fight with the locked doors and music-related puzzle that needs to be solved to escape.
I start by stealing from the best, Sly Flourish. It's easy to have a static battlemap that you have prepared and re-use with different added terrain or features or from a different angle. One nice option is the reusable Gale Force Nine maps or Paizo's Flip-mats. I mention these because you can draw on them, allowing for repeated customization and tweaking. But paper battlemaps from adventures, or pre-made Dungeon Tile terrain kept on poster board with Blu-Tac would work as well.
Then the actual random element. Monsters are actually easy. It's simple enough to print-out some monsters (from the Monster Builder) that have been scaled to the PCs level (+/- 1 or 2). You can have a large stack of printed monsters (possibly printing 2 monsters per sheet then cutting the sheets in half) and simply deal a randomized encounter. Get a bunch of humanoids? Then they're bandits. A monster and humanoids? Bandits with a trained or leashed beast. Several beasts? Then it might be a main predator and several following scavengers.
Likewise, it's easy enough to have terrain cards printed out and shuffled. The bandits might have a barricade they're using for cover, or a hidden pit, or the ground could be wet and muddy from recent rain, or the fight is on a steep hill (uphill is difficult terrain and forced movement downhill gains an extra square), or a variety of other pre-planned effects. Just deal one or two and see how it integrates into the encounter.
This requires alot of prep work. But once done you can use it again in future. It's front-loaded prep work that can be repeated as often or as infrequently as desired.
That's it for today. You can follow me on Twitter if you want (I'll likely follow back). Next time I'll have a blog on ARs; the notorious paperwork from Living Bookkeeping.