Getting back to the original question - it would see logical to re-write it along the lines of-
"If an Order purported to impose different speed limits in different lanes* intended for different direction of travel on a single carriageway road, would such an Order be lawful" - which could then be considered as to whether it was intra or ultra vires in respect of the enabling legislation, and whether it fell foul of some other public law rule (such as being Wednesbury unreasonable), or was for some improper purpose.
Presumably, we are considering Orders made under the authority of s. 84 RTRA 1984. This provides for speed limits to be applied to a "road".
"Road" is defined in s. 142 (for England and Wales) as "any length of highway or of any other road to which the public has access, and includes bridges over which a road passes"
Absent any case law on the narrow point, it would seem to depend on whether a judge decided that the operative word was "any" or "length".
* Perhaps the first "obvious" question would be whether we were discussing different speed limits for the westbound and eastbound lanes (for example) of a single carriageway road, as opposed to different speed limits for the entire width of the carriageway depending on direction of travel.
Whilst "any length" of road might be interpreted to include the westbound lane but not the eastbound lane, unless vehicles travelling westbound could be considered a separate class of vehicle, I'm struggling to see any vires[/i for the latter.
