It has occurred to me that there is another aspect to this problem which may explain why it so annoying.
In the mid-20th century, the amount of private car usage increases dramatically.
For High class passengers: 1935 33% 1945 40% 1950 60%
For Medium class passengers: 1935 3% 1945 5% 1951 21% 1955 25%
Although the horse cart distributionweight also falls (38 points until 1922, 20 points until 1935, 8 points until May 1954), my very subjective impression is that there are more horse carts around in the early 1950s than earlier, because routes that were previously running fine are now being delayed.
When we were doing the private car testing earlier this year, I noticed that a relatively small number of horse carts were able to eventually gridlock a whole map as every other vehicle queued up behind them.
I do like the realism of having horse carts around in the early 1950s, but I'm not convinced that British roads had more horse carts in 1953 than in 1923. Perhaps the distributionweights should be further altered? Basically re-use existing graphics so that there is a lower distributionweight for horse carts in the late 1940s and especially early 1950s.