Do you mean Sydenham Road? Otherwise known as the Great Car Park of SE26? I should think people use Dartmouth Road for exactly the same reason I do - to avoid Sydenham Road. I live on Mayow Road, about half way along, and regularly drive from Crystal Palace to get home. As the crow flies, going through Sydenham is much the quickest route. But rather than sit in traffic on Westwood Hill just waiting to even get onto Sydenham Road, I take Sydenham Hill, Wells Park Road, Kirkdale, Dartmouth Road, Perry Vale, Mayow Road. Even at the peak of school-run time I rarely meet standing traffic on that route.

If I was going from Crystal Palace to Catford, I certainly wouldn't go anywhere near Sydenham Road. Or Penge. I'd use Forest Hill.