1954 n=49

1962 n=33

1977 n=120

1987 n=532

1987 n=666

1987 n=2392

1994 n=7397

1998 n=13509

2001 n=15112

2004 n=24978

Applegate, Bixby, Chvátal, and Cook (1998) found the optimal tour of the 13,509 cities in the USA with populations greater than 500.

(Picture is a link to a higher resolution version.)