http://www.fmcsa.dot.gov/registration-licensing/online-registration/onlineregdescription.htm
Visible from 50', to me that was always 1 1/2" talll, never had a problem since 1988 of doing that size. We do 100-150 trucks a year.
That is off the US dept of Transportaion site....USDOT...DUH...