I copied this from another forum:
This is fairly accurate, but always check with each state:
AZ (only with 5W, with certain qualifications)
AR
CO (total max. length 70 feet)
D.C. (surprisingly no restrictions)
ID (total max. length 75 feet)
IL (only 5W, total max. length 60 feet on certain highways)
IN (total max. length 65 feet)
IA (total max. length 70 feet)
KS (total max. length 65 feet)
KY (total max. length 65 feet)
LA (total max. length 70 feet)
MI (total max. length 60 feet on certain highways)
MN (total max. length 60 feet on certain highways)
MS
MO (some exceptions or restrictions, total max. length 65 feet)
MT (total max. length 70 feet)
NE (total max. length 65 feet)
NV (total max. length 70 feet)
NM
ND (total max. length 75 feet)
OH (total max. length 65 feet)
SD (only with 5W, with certain qualifications, total max. length 75 feet)
TN (total max. length 65 feet)
TX (total max. length 65 feet)
UT (some exceptions or restrictions, total max. length 65 feet)
WI (total max. length 60 feet on certain highways)
WY (total max. length 85 feet)
States Where Towing Triples is NOT Legal, Period: AL, AK, CA, CT, DE, FL, GA, HI, ME, MD, MA, NH, NJ, NY, NC, OR, PA, RI, SC, VT, VA, WA, WV
Canada is covered too. Some provinces allow it, others don't.
When in doubt and not wanting to be involved in an accident where you are deemed to have been towing illegally, thus possibly contributory or the primary cause of the accident, contact the state's department of motor vehicles.
PS: The states of CT, GA, MA, NC, PA and RV have a 2-vehicle combined maximum length limit of 60 feet. MD is 55 feet and NJ is just 50 feet.
Source: Coast to Coast Magazine, July-Aug., 2006, "Rules of the Road and
Towing Laws," pp. 56-57.