Where is it?
Chevening Interchange, junction 5 of the M25. The point where London's orbital meets the A21, a major route to the south coast, and the M26, a shortcut between the M25 in the south and the M20 towards the Channel Ports.
What's wrong with it?
It appears to have been designed blindfolded. The priority route is M25 to M26 — while a fair amount of traffic does go that way, surely the M25 should have priority over all else, what with it being a ring road to 6 milion people and the busiest road in the country.
Traffic from Sevenoaks is also badly served: to head east from there and join the M26/M20 route, traffic must use the narrow and winding single-carriageway A25 parallel to the M26 since there's no access to the motorway network eastbound from the A21 here. Kent County Council continue to lobby for a change to this situation, all to no avail.
Why is it wrong?
The M25 wasn't meant to be the through route here. The interchange is built around a section of the once-upon-a-time A21 Sevenoaks bypass, which used to be the north-south route here, explaining why that is a mainline carriageway. When the M25 was built, it stopped here, the A21 continuing north and the M26 continuing east. When finally the ends of the M25 were joined up, it was too late, and the junction had to be adapted. The M25 is now carried on little two-lane sliproads.
What would be better?
First of all, let's have access between the A21 and M26 — a left turn M26-A21 is easy enough to add; then A21 to M26 could be achieved by either shifting the north-western sliproad out a bit to make way for a loop, or tunnelling a new sliproad underneath the whole thing (this has been done elsewhere before). As for the M25, how about giving it more lanes and more priority at merges and diverges?
Right to reply
Well, I think we need a map drawing to decide that one. And a very large amount of money.
You say the slip for westbound M25 appeared unexpectedly, but there is a sign warning of the split at least two miles back. How much warning would you like?
I am not quite sure it ought to have caused a panic. There is a junction plan 2 miles out, repeated at 1 ½ miles, 1 mile and ½ mile. The road markings go to hazard lines for the left hand carriage way a mile out with carriage way markings. There are two overhead gantries at around 600 and 300 indicated yards out but the slip road is extended way beyond that.