what about those bridges some cities have with a load of houses ON the bridge so you don't even notice you're on a bridge not just a normal street when you're walking over it?
My thought is that the tradeoff advantage of declaring a bridge "static" would be that you could build on it; the best Dwarven representation of our running example bridge Untertorbrücke in historical times would probably be 5-6 squares across, to allow for a square of fortifications down each side and room for a wagon to go down the middle; then when the fortifications are removed later it would be just wide enough to be two lanes. (The actual Untertorbrücke seems to be 8-9m across at the arch spans, and the support pillars extend out to about 12-13m total width. Without any fortifications in modern times this handles two narrow lanes of traffic and some sidewalks. It's not clear how far the earlier fortifications were built out from the edge; it would not be uncommon at all for them to overhang at least somewhat, both for more room and for better fields of fire.)
Part of the problem here is that DF wagons have been 3x3, which is unusually wide for the streets, tracks, and bridges in many historical eras; and additionally problematic when you consider the unimproved and frequently mountainous terrain involved in getting to an early fort. From the
Wikipedia article on Lane:
"The U.S. Interstate Highway System uses a 12-foot (3.7 m) standard for lane width. 11-foot (3.4 m) lanes are found to be acceptable by the Federal Highway Administration for automobile traffic, but as lane width decreases (9-foot (2.7 m) lanes are found in some areas) traffic capacity decreases. ... In the United Kingdom, many lanes are found in the countryside, and most of these lanes are wide enough for one car at a time and often have a lay by for cars to pass. In general, European laws and road width vary per country, with the minimum widths of lanes being anywhere between 2.5 m to 3.25 m"
From the
snopes.com entry on railroad gauges and the origins of
"Standard" gauge of 4 ft 8.5 in (1.435m):
"Historian James Crow, writing about Housesteads, the 3rd century Roman fort built along Hadrian's Wall, notes that:The wheel rut and gate stop in the north passage are well preserved, and a number of reused stone blocks formed part of the latest surface to survive. The gauge between the ruts is very similar to that adopted by George Stephenson for the Stockton to Darlington railway in 1837, and a 'Wall myth' developed that he took this gauge from the newly excavated east gate. There is a common link, but it is more prosaic, and the 'coincidence' is explained by the fact that the dimension common to both was that of a cart axle pulled by two horses in harness (about 1.4m or 4ft 8in). This determined both the Roman gauge and Stephenson's, which derived from the horsedrawn wagon ways of South Northumberland and County Durham coalfields.
What we're talking about in DF is usually more closely related to
Loading gauge and
Structure gauge, which has to do with the tunnel size (or other restricted space) that something can fit through. Modern European loading gauge width is 10 ft 10 in (3.29m), Great Britain an older 8 ft 6 in (2.6m), North America mostly 10 ft 8 in (3.25m);
An obvious parallel to the DF wagon is the
Conestoga wagon, specifically designed for heavy duty use in hills and low mountains, for fording streams and rivers, and for breaking new ground into new territories. "The average Conestoga wagon was 18 feet (~5.5m) long, 11 feet high (~3.4m,), and 4 feet (~1.2m) in width. It could carry up to 12,000 pounds of cargo (~5.5mt)." I'm a bit suspicious of the narrow width they list, but there seems to be some precedence for the actual Conestoga wagon to be that narrow so it could fit down certain trails; a more typical "prairie schooner" of later years and wider spaces would be wider, and a lot of the covered wagons you see are actually of this later type.
This photo shows something closer to the early Conestoga type, and is indeed quite narrow. Note that the narrow and curved bottom was supposedly at least partly to keep barrels from rolling about, certainly a concern for dwarves! (Also note that in this historical context, barrels were not only used for liquids and powders, but for many things that DF dwarves would store in bins, such as small finished goods. If you wanted a reasonably portable, standard sized, heavy-duty, waterproof container you used a barrel; boxes and the like were for light duty in-town use, and crates were typically larger items for use on ships and would require multiple people and/or a block and tackle to move.)
All of these really point to wagons being more sensible at no more than 2 squares wide. Transporting a 3x3 DF wagon would be an ultra-wide load taking up most of two modern highway lanes, special planning as it would be too large for many ramps and some bridges, and probably requiring police escort if not closing the roads entirely; it would never fit on a railroad. You could probably make a pretty good case that DF dwarven wagons should be 1x2, on the smaller end of a Conestoga wagon, designed for tight quarters and rough terrain; having human wagons at 2x3 as they are designed more for use in the lowlands would preserve the concept of "we need better roads and bridges to get the full human caravan" that was a useful DF gameplay mechanic in many versions.