Transport and Land Use has published a study examining the relationships between urban density, block size, and vehicle miles traveled. The study found that in older, more established cities, density and mixed-used areas were more important factors than block size for reducing miles traveled. But the study said smaller metro areas with lower density more effectively reduced miles traveled by shrinking their existing block sizes.
For more information: http://bettercities.net/article/block-size-not-just-density-key-cutting-auto-travel-19311