The luminance and color of a traffic sign depend on the angle, illuminance, and spectrum of the illuminating sunlight. The general method for measuring the effects of sun on traffic signs is to use Xe source or Xe source with filters as a sun-simulator to simulate the CIE defined average sunlight spectrum, D65. The disadvantages of using the sun-simulator are the high cost and measurement error that arises from the difference between the spectrum of the sunlight and that of sun-simulator. Using Xe source as the sun-simulator may produce an error of around 3.5% due to the spectrum difference. To solve these problems, a measurement method has been applied to eliminate the need for sun-simulator in measuring the sun effect on traffic signs. Measuring the spectral reflectance of the target and calculating a luminance and illuminance translation factor allow us to calculate the luminance reflected from the target, regardless of the light source illuminating on it. For instance, the effects of other sunlight spectra, such as D55 or D75, on the targets can also be determined by this method without the use of any light source to simulate D55 or D75. The sunlight spectrum in the presented simulation equation is obtained directly from the CIE definition, eliminating the error associated with the difference between the sun simulator and sunlight spectra. The experimental results demonstrate that the calculated luminance is about 1% different from the measurement results. Low cost, good accuracy and high flexibility are the advantages of this method.