FMCSA section 393.60
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[TD="width: 100%, colspan: 2"] Coloring or tinting of windshields and windows. Coloring or tinting of windshields and the windows to the immediate right and left of the driver is allowed, provided the parallel luminous transmittance through the colored or tinted glazing is not less than 70 percent of the light at normal incidence in those portions of the windshield or windows which are marked as having a parallel luminous transmittance of not less than 70 percent. The transmittance restriction does not apply to other windows on the commercial motor vehicle.[/TD]
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Link...http://www.fmcsa.dot.gov/rules-regu...sr/fmcsrruletext.aspx?reg=393.60&keyword=Tint
I just found a 10 year old copy of the regs and it says the same thing, so its always been legal.

I killed the lights as soon as I pulled off onto the ramp into the scale.