http://www.subsim.com/radioroom/showthread.php?t=117226
This thread talks about convoy sizes and such and how realistic they are...as for the traffic...I believe Tater has a mod that cuts down on the amount but not sure what it does for composition. Here's the link to that thread...
http://www.subsim.com/radioroom/showthread.php?t=116283
Hope this helps.