You really think the idea of anything like bumper to bumper traffic existed more than a hundred years ago? Everything before 2000s (though surely car traffic existed in the 1900s) seems like a dramatization.
The number of horses in London was causing problems at the end of the 19th century - they don't scale well when you need to provide stables, food and of course leave piles of dung everywhere.
One of the bigger problems was all the manure on the streets. I believe this is one of the reasons why there are stoops in New York, the steps up to brown stones.
Also there’s a dump site in New York called Dead Horse bay.
Now doubt there’s many people here who have been to New York, I’ve just seen YouTube videos about this.
Stoops in London? I’m guessing quite a few properties have steps up to the entrance. Going through YouTube videos of shooting locations of the Sweeney and it doesn’t seem that prevalent.
I would think in some places there was bumper to bumper for short distances. I would expect in parts of NYC it existed. I think it was in the 1920s when traffic lights started to appear.