California Vehicle code:670. A "vehicle" is a device by which any person or property may be propelled , moved, or drawn upon a highway, excepting a device moved exclusively by human power or used exclusively upon stationary railsor tracks.26708. (a) (1) No person shall drive any motor vehicle with anyobject or material placed, displayed, installed, affixed, or appliedupon the windshield or side or rear windows.(2) No person shall drive any motor vehicle with any object ormaterial placed, displayed, installed, affixed, or applied in or uponthe vehicle which obstructs or reduces the driver's clear viewthrough the windshield or side windows.26708.5. (a) No person shall place, install, affix, or apply anytransparent material upon the windshield, or side or rear windows, ofany motor vehicle if the material alters the color or reduces thelight transmittance of the windshield or side or rear windows, exceptas provided in subdivision (b), (c), or (d) of Section 26708.(b) Tinted safety glass may be installed in a vehicle if (1) theglass complies with motor vehicle safety standards of the UnitedStates Department of Transportation for safety glazing materials, and(2) the glass is installed in a location permitted by thosestandards for the particular type of glass used.
Read through the codes above..... it looks to me like if the tinted windshield is DOT approved for use on the road then it would be legal per CVC 26708.5 (b) (2). The windshield on my spyder is smoked/tinted and is a factory windshield. You have to be careful with aftermarket accessories because often they will have a waiver saying "for off road use only" or something similiar because they haven't complied with the vehicle code.