Planning your next trip?

Not quite. Just wondering whether gravel bikes are now compelling. My experience is that gravel roads nearby are perfectly tackled by folks on alloy road bikes—even with 23 or 25 mm tires—but some folks with carbon bikes grumble, perhaps out of fear that a rock will chip their cherished frame.

That said, I don't quite understand how "gravel road" is defined. If the road is covered with just 2-cm diameter gravel, is that still a gravel road suitable for a gravel bike. It seems that at some point an MTB is anyway necessary.

depends - some "gravel roads" are firmly packed and may even be graded. The adhesion between each rock is where the variance is.... crossing a loosely packed area is completely different, and momentum becomes a factor too.

@Sam 2cm pebbles are perfectly fine for gravel bikes unless it is very deep and loose

I think the situation is that a rigid, road style bike can really only vary so much

the limits of the design category are pretty easy to reach I suppose

Perhaps XC bikes are superfluous because of gravel bikes ¯\_(ツ)_/¯

@Sam If you are still wondering if they are compelling, you probably live in an area where there's a gap between roads and MTB trails and a gravel bike will be a bad compromise in most cases. I would say that here, 50% of the non-asphalted roads could be done with a roadbike, but carefully (cobblestones, dirt/mud - then you would have the double penalty of not having low ratios and riding slowly), we don't have so many "gravel roads", but gravel bikes are very sensible choices.

Typical example: komoot.fr/plan/… it's too smooth to benefit from the suspension of a MTB. When it's dry, the surface is too rough to be ridden comfortably with a road bike and 4+bars tires, and when it's wet, it's muddy and you benefit from deeper threads that typically require extra clearance.

@Sam The same road with the same gravel mix can vary drastically along a section, between different types of weather, seasonally, and with maintenance. A newly graded road can be quite dicey for riding on a road bike, with loose chunks of gravel and soft spots. After a couple of weeks of traffic, most loose gravel migrates to the verges and the soft spots tamp down. Perfectly fine on a road bike until repeated traffic starts to create ripples, at which point it is graded...

Because the surface is not fully stabilised, in wet conditions low areas where rain pools can collect a layer of poorly adhered mud that can break up into sandy spots when it dries. (This is less of a problem on well-maintained roads where they grade the camber back into the road, more of a problem on converted trails.)

Upgraded trails, as opposed to rural roads built for farm vehicles, tend to use smaller gravel (finings or screenings) which creates a smoother, more stable surface.

@DavidW There are also a lot of gravel paths without motor vehicle traffic, so they don't get "ironed" (but also no washboard ripples)