What windows should be used to soundproof from traffic noise?
There are a number of companies that provide windows with high STC ratings. The series 7000 windows from http://www.silent-guard.com appear to have STC of 40 and above.  There are many other manufacturers that sell sound attenuating windows. 

I believe you are correct that to get what you want you'll have to specify specific windows.  The way you describe the contract doesn't appear to be objective and measurable, and you really don't want to have to pay twice for windows.

You could also renegotiate the contract to specify STC rated windows of 40 or higher, however you might not get what you need then depending on how a given manufacturer rates their window.

Also, keep in mind that at higher STC levels the noise conduction through walls starts to become larger than the windows.  You may want to evaluate the siding/sheathing/insulation/wallboard plan to make sure that you don't put great windows in but still get significant low frequency road noise through the walls.