please empty your brain below

What's a binary road?
Penny's dropped now. Doh.
Perhaps reclassified, if the actual road is still in use, rather than no longer exists.
You should do more stuff about roads. Roads are intresting.
The B1010 from Althorne to Burnhan especially designed for motorcycles.
So what's the hidden agenda here?
What about binary C roads?
And do we get photos from A11, with X marks the spot?
Does the a1m count as binary?

dg writes: For this list, I decided the A1(M) was a subset of the A1.
Is there a convention that B roads always have 3 or 4 digits? No B10 or B1 for instance
If you include Northern Ireland B roads start at 1 - B1 Downpatrick to Ardglass.
Planning to walk all the Binary roads then?

dg writes: I've sort of done that.
There is such a convention - although I'm not sure why. It doesn't avoid duplication as A-roads run into three and four digits. In a rare example, the A1000 and B1000 actually meet each other in Welwyn.

Both the Isle of Man and Northern Ireland have single digit B-roads.

There was briefly a B38 in Birmingham - a downgraded section of the former A38, but it was quickly renumbered B384. (Still not following convention as road numbers in that area normally begin with a "4")

On numbering convnetion, it is no coincidence that all DG's "binary roads" are in a zone east of the A1 and north of (or crossing) the Thames.
Numbering convention:
You could go on. Ternary Road names would contain numbers 0-2, quaternary ones 0-3 and so on.
Alternatively you could say A roads must be at least base 11, B roads base 12 and motorways, I can't be bothered to count.
You could, but I wouldn't recommend doing so unless you want to clear a space around you.
The A1100 doesn't exist any more I'm afraid. It's now part of the B1113 since the Ipswich western by-pass was built.

dg writes: Updated, thanks.
Interesting idea from Still Anon about not using the phrase "doesn't exist", when something is renamed. You wouldn't say "Posh Spice doesn't exist", but you might say "the West Riding doesn't exist".
Perhaps we could have road numbers in Hexadecimal, the signs would be a bit smaller.
Well they're already in duodecimal, if you include the prefix.
...and ignore motorways.
What about C-roads?

There appear to be at least 2:
DG - you missed the M100 (the numbered pencilled in for a full London-Newcastle A1(M)) and M110/M111 (the numbers reserved for the rump section of M1 into Leeds, should a west of Leeds extension of the M1 to the A1 be built). But then again, I'm pretty sure that's deliberate.
Re: B roads and the number of digits

I gather the plan before 1922 was to not have any 4-digit A roads (and then the plan in 1922 was that they were short links, or Ax2xx roads in the County of London), so clashes were thought unlikely.

Certainly it is the case that 3-digit B roads were reserved for London (and Scotland if 7-, 8- or 9-zone). Conflicts were likely to exist in Central London, though AFAICS (in England), it was only the 303, 304, 306 and 401 pairs that had A and B meeting each other. And the 259, where there was a deliberate last-minute renumbering before it all became official to give the south coast route (which was B road initially between Newhaven and Eastbourne) between Brighton and Dover one number. Now it's just 401 and the 1000 example that timbo gave, though Toddington is to get A and B5120 meeting each other later this year when the Dunstable Northern bypass opens and the A5120 south of the M1 becomes B5120 (a duplicate number!)

In addition to Northern Ireland, the Isle of Man has single-digit B roads. And both, plus Jersey (where all numbers are unique with A roads having the lowest ones, B roads in the middle, then C roads), have double-digit B roads.
Don't the A306 and B306 still meet at the Red Rover?

you are quite right. Also I've found the 105s meet nw of Clissold Park and have done since '22.
The A1111 has had a rather fine song written about it.

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