How wide a tyre could a road wheel accommodate? I though 28mm is the limit?
It depends on the internal rim width and it'll depend on the
frame clearance too; my 700c road wheels on my town bike came with 37mm tyres but they wouldn't fit on the 700c wheels on my drop bar bikes, the rims will be too narrow and there wouldn't be frame clearance anyway. From
Sheldon:
Width Considerations
Although you can use practically any tire/rim combination that shares the same bead seat diameter, it is unwise to use widely disparate sizes.
If you use a very narrow tire on a wide rim, you risk pinch flats and rim damage from road hazards.
If you use a very wide tire on a narrow rim, you risk sidewall or rim failure. This combination causes very sloppy handling at low speeds. Unfortunately, current mountain-bike fashion pushes the edge of this. In the interest of weight saving, most current mountain bikes have excessively narrow rims. Such narrow rims work very poorly with wide tires, unless the tires are overinflated...but that defeats the purpose of wide tires, and puts undue stress on the rim sidewalls.
Georg Boeger has kindly provided a chart showing recommended width combinations:
Which tire fits safely on which rim?
[all dimensions in millimeters]
Note: This chart may err a bit on the side of caution. Many cyclists exceed the recommended widths with no problem.