18-wheeler is a common term for a semitrailer (tractor trailer vehicle). WB-50 is one type, 50 refers to the wheelbase distance in feet from the front axle of the tractor to the rear axle of the trailer. WB-50 is the second smallest of the WB series, so I wouldn't use any less than that and would probably go larger, such as a WB-65. Templates are included in "A Policy on Geometric Design of Streets and Highways" from AASHTO and should be available from the Federal Highway Administration.