Revised Rulebook p. 133 gives prices for 8-foot-wide and 10-foot-wide roads. But there is no indication of how wide a road needs to be in order to accommodate ordinary commercial traffic, whether one lane (for a low-traffic road where, if two wagons meet, one must pull over to let the other pass) or two lanes. (Pages 151–153 give wheel-track widths of up to 7 feet for military chariots. But they are silent regarding wheel-track widths or total widths of ordinary commercial carts and wagons.) How wide is a ruler supposed to build his roads?
Judge's Journal p. 218 states that an avenue within a settlement has width of 15 feet plus a 5-foot sidewalk on each side. Revised Rulebook p. 273 also mentions that a formation must be no wider than 15 feet (sufficient for five man-sized creatures, three horses, or one wagon and one horse) in order to fit on roads and trails. So does this 15-foot width apply to a road between settlements as well, making the 8-foot and 10-foot widths given on Revised Rulebook p. 133 total red herrings?
A typical Roman road was a one-lane 8’ wide road and you should consider that standard. I offered 10’ wide roads simply because a lot of people prefer everything be in 5-foot square boxes on the battlemap for simplicity even when it’s not historically accurate.
The rule that a column must be no wider than 15’, but no more than one wagon with a horse alongside it, actually assumes a standard 8’ road. Roman roads were built with shoulders on either side of tamped earth, sometimes lightly graveled. Infantry and horses often walked there, not on the central (paved) portion, which was primarily for wheeled vehicles. If you visit various Roman roads in Britain, Gaul, Italy, etc., you often see that the earth on either side of the road is a well-compacted trail, sometimes wider than the “real” road.
So the rules are assuming that the 8’ road has shoulders of well-compacted ground on either side, and that the infantry and cavalry are marching alongside the road as much as on it. Only the wheeled vehicle (if any) is “on” the road. That’s why the formation is limited to just one wagon with a horse (escort rider) alongside it. If the road is a two-lane 15’ road (at double cost) then you could fit two wagons, etc.
The rules are a simplification of reality because if we want to truly simulate every aspect of roads, we’d need to know the exact width of the shoulders, and what material they are, and separately apply weather effects to them; formation speeds would increase and decrease based on weather vs road and road-adjacent terrain; formation lengths would increase or decrease as roads narrowed or widened, with possible “traffic jams” that delay movement. We’ve essentially abstracted all of that away.
Most maps (historical, game sourcebook, or Judge-made) don’t specify the width of the roads, and most Judges don’t want to adjust the speed of formations on roads based on roads of varying thickness and thinness. (ACKS is relatively unique in even specifying things like weather effects by road type or column widths at all!) But if you do want to map to that level of detail, and you do want to track formation width and length, it could certainly add another layer of realism to troop movement and logistics.
TLDR: The column width rules are for the 8’ roads.