Martin Underwood wrote:

Interesting that the junction was
constructed with single-lane slip roads leading from a (presumably)
3-lane M1 to a (presumably) 2-lane A1, rather than making the slip
roads 2-lane as you'd get at any motorway junction nowdays.

But it wasn't built "nowadays". It was built at a time when the amount of
traffic on the road, and especially the amount of long distance traffic, was
so much less than now that a time traveller going back would think all
drivers were having a non-driving protest day of some sort.