A "Safety Alert for Operators" (SAFO 09004) from 2/11/09 says "Slow the aircraft to a fast walking speed on the centerline of the landing runway prior to attempting to exit the runway. Taxi at a fast walking speed until parked at the ramp or until aligned with the centerline of the runway for takeoff."
Which, of course, isn't a regulation, in this example is specifically talking about winter conditions.
Riverside County in Southern California has an ordinance (regulation) that states:
Section 12.08.100 Taxi speed.
No person shall taxi any aircraft on the airport unless there will be no danger of collision
with any person or object. All aircraft shall be taxied at a safe and reasonable speed
commensurate with safe operation in relation to existing conditions and with due regard for
other aircraft, persons and property. (Ord. 5661 § 1, 1988; prior code § 5.11 (part))
So whatever a "safe and reasonable speed" is. I've heard it called a "brisk walking pace."
I had an airline pilot as a ground school instructor once, and she said that ground control was giving them a hassle one day, so everyone decided to taxi at a "brisk walking pace" around LAX. Got ground whipped into shape real quick!
The FAA's website, under "Best Practices" says to "Maintain an appropriate taxi speed."
Since it's not clearly defined by the FAA, I assume that most airlines limit taxi speeds via their SOPs, also due to the variety of equipment, it might be unfeasible for the FAA to mandate a specific speed.
- Max 30 kts: straight line with no close obstacles (for example: back-tracking on the runway)
- Max 20 kts: straight line with obstacles (for example: on the taxiway, or close to other aircraft/stands/ground vehicles)
- Max 10 kts: turns and entry onto ramp area