Doubt full size trailer is very unsafe.

There is such a thing as triple length trailers as well. The American roadways were never designed for these trucks. Literally corporations are turning the American highway into a place for trucks that act as trains.

The only reason these trailer and tractor combinations exist is to increase profits to companies by one or two driver's salary. It is a hideous idea. The combination trailers add a great deal of sway to increase the  danger on the highway. Add to the sway some wind and it is a tractor-trailer combination that will kill.

June 12, 2015
By Geoff Pender
Jackson, Miss. — Mississippi's transportation commissioners (click here) are urging Sen. Thad Cochran to help quash a proposal that would force the state to allow larger double-trailer trucks on its roads.
"I don't like Congress mandating that states do this," said Central Transportation Commissioner Dick Hall, who signed a letter to Cochran along with the state's other two elected commissioners. "I'm a whole lot more interested in the safety of Mississippi's drivers than I am the financial welfare of a national corporation."
National highway safety advocates also are fighting the measure, along with another passed by the House that would extend a freeze on stricter regulations on when and how long truckers must rest between long hauls....

These trucks have been unsafe for decades.

This is an article in the New York Times dating back to 1988. These tractor-trailer combination rigs have not changed and the article is still valid.

May 3, 1988
By James Gleick

LARGE double-trailer trucks,  (click here) those highway behemoths with two trailers behind a single cab, are two to three times more likely to be involved in crashes as other heavy trucks, a study in Washington State has found. 

Large trucks in general crash far more often than other vehicles, accounting for 12 percent of the nation's deaths from traffic accidents, according to the study's authors, Howard S. Stein and Ian S. Jones of the Insurance Institute for Highway Safety in Washington. Most of those deaths are in cars hit by the trucks. Measuring Highway Crashes...

It is going to be hideous managing traffic in any city if these tractor-trailers are allowed off the highways. Count on ridiculous traffic jams as well as damaged infrastructure. They are too long for any city.