The largest designed curves in the game are the 13 meter radius sidewalk tiles used in places like residential cul-de-sacs and traffic circles. I went into the prefab editor to see what’s available and how useful they’d be. It turns out the only version of these shapes I see are the plates, which are suitable for sidewalk tiles but not for a banked turn. And more surprisingly, they’re not even available from the shape menu. They’re dev only blocks.
So it seems the largest radius you have specific blocks for is 7 meters. For a race track curve, this is probably the gentlest/most forgiving wall you can create.
Now I know you weren’t looking for a wall. You were looking for a banked curve, as in a sloped road surface. Options here are even more limited, and theFlu is probably right that the 5 meter radius “dome” shapes are the least sharp shapes you’re going to find. There are all sorts of ramps/wedges but their corner pieces tend to make the turn within one block.
As it turns out, a banked turn doesn’t work anyway. I drove a motorcycle around some steep terrain in Navezgane, and it confirmed my suspicion that the physics aren’t there. The bike will pitch up and down to match the slope, but it won’t roll. There is no centripetal force pushing you into the curve. So really all you can do to actually constrain the vehicle’s movement is to put up a wall like the above, whether out of terrain or blocks. Curves via terrain are somewhat easier now with the dev terrain modeling tool, but I would still find blocks to be cleaner to use.