I don't know if it's in the FARs or the AIM; but our SOP dictates some speeds and guidelines.
No faster than a person can walk in the line area.
No faster than a person can run on the taxiways.
Max taxi speed of 15 kts - for FOD considerations.

It is interesting to hear what the Part 121 carriers use. I saw some T-38s one time in KS that were taxing so fast that I thought they were on a takeoff roll, but then again I know they were late too Heck - those aircraft look sleek just sitting on the line.

USMCFLYR