.......distance numbering has its own problems.
Take M8 in Glasgow - J17/18/19 would be in the same kilometre, so you'd have (hypothetically) M8 J44A, 44B, 44C etc...
No, they would have been 43, 44 and 45 for such few instances. The Distance measurement doesn't have to be to a surveyor's precision, numbers can be adapted to suit.
I think that's exactly right, as the level of accuracy only needs to be appropriate for the purpose. When navigating on a motorway it isn't really necessary for distance information to be especially accurate.
There are several ways of doing this. The Americans, who have a lot of urban freeways with very closely spaced exits, seem to use letter suffixes (See: Los Angeles
. I think that is only really necessary where there are many very closely spaced exits, not just a few.
In South Africa there are a few places where there are a number of very closely spaced exits such as here in Cape Town
. Here though the letter suffixes are where a single exit (junction) has more than one physical ramp in a particular direction, such as a cloverleaf, where a driver has to take a different ramp depending on whether they want to turn to the left or right. In this case there probably aren't enough closely spaced exits in succession to suffix them.
All in all, I think the difficulties are less than with sequential numbering, but it's probably a bit late to change now. The other big advantage is on roads where there are numbered and unnumbered sections (I'm thinking of the A1/A1(M)) as the numbering can end and resume more easily.