This question comes from a discussion with a DM friend of mine, whose players asked him if they could take a short rest while traveling in a moving cart. His initial reaction was to think that wouldn't work, but upon considering the rules for short rests:
A short rest is a period of downtime, at least 1 hour long, during which a character does nothing more strenuous than eating, drinking, reading, and tending to wounds.
There doesn't seem to be anything explicitly prohibiting it there. One could argue that driving the cart could be considered a strenuous activity, but doing nothing more than sitting in the back seems to be a harder to justify as such.
Taking it a step further, we also considered the possibility of a long rest. Looking at the rules for long rests:
A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch. If the rest is interrupted by a period of strenuous activity – at least 1 hour of walking, fighting, casting spells, or similar adventuring activity – the characters must begin the rest again to gain any benefit from it.
We again considered that sitting in moving vehicle isn't explicitly called out, and intuitively doesn't seem to qualify as a "period of strenuous activity". Assuming one could rest on a sailing adventure (with the ship being a vehicle), and suddenly this seems less absurd.
And yet it's a concept that appears to go against the norm, with stopping and finding a place to specifically rest or set up camp, and I haven't seen the questions asked here or elsewhere. Is there something else written anywhere that prohibits this, or is our interpretation here wrong?
Best Answer
Short answer: rules seem to be fine with it for both short and long rests.
As a rule-based answer, I'd say there's nothing against it, as you noted. I've personally allowed characters to take short rests in a cart because it seems within rules. The only thing that would prevent the long rest is the bumpiness of the cart preventing the minimum 6 hours of sleep the long rest demands. But even then, the basic rules don't strictly go against it.
I'd simply suggest being consistent. If cart-rests would go against a grittier campaign or such, or you have encounters planned where you want them stopped and perhaps unable to run, then you could effectively houserule with your group that there's no cart-resting. But if it wouldn't break things for the campaign, be it mechanically or flavorfully, then the rules seem to support it, and it can also help speed things along for the plot.
Instead of saying they have to stop, then travel for hours with a generally monotonous journey that they may not get much description from anyway, they just rest through that while traveling (assuming someone is steering). And if they would encounter something along the way, it might be even more interesting now that they decided not to rest prior to traveling, instead getting interrupted during the travel and having to deal with the encounter not yet rested.