Editing Semi-trailer truck (section)

===Skirted trailers===
{{main|Trailer skirt}}
An innovation rapidly growing in popularity is the skirted trailer. The space between the road and the bottom of the trailer frame was traditionally left open until it was realized that the [[turbulence|turbulent]] air swirling under the trailer is a major source of [[aerodynamic drag]]. Three split skirt concepts were verified by the [[United States Environmental Protection Agency]] (EPA) to provide fuel savings greater than 5%, and four split skirt concepts had EPA-verified fuel savings between 4% and 5%.<ref>{{cite journal |url=http://papers.sae.org/2012-01-2043 |title=EPA Smartway Verification of Trailer Undercarriage Advanced Aerodynamic Drag Reduction Technology |year=2012 |publisher=EPA Smartway |doi=10.4271/2012-01-2043 |access-date=2 October 2012|last1=Wood |first1=Richard |journal=SAE International Journal of Commercial Vehicles |volume=5 |issue=2 |pages=607–615 }}</ref>

Skirted trailers are often combined with Underrun Protection Systems (''underride guards''), greatly improving safety for passenger vehicles sharing the road.