A method for determining the dry, wet or frost state of the pavement of a roadway in a specific verification region ahead of a vehicle, comprising the steps of: - generating with an optical source composed by two optical frequency combs: - a first optical frequency comb with a central frequency f1 and a frequencies span of n*Δf1, being n the number of the comb lines and Δf1 the spacing between comb lines, wherein the spectral bandwidth of the comb covers a region of the spectrum where the imaginary part of the complex refraction index of water and ice cross each other; - a second optical frequency comb with a central frequency f2 and a frequencies span of m*Δf2, being m the number of the comb lines and Δf2 the spacing between comb lines, wherein the spectral bandwidth of the second comb overlaps the spectral bandwidth of the first comb, wherein frequencies f1 and f2 are in the optical region of the electromagnetic spectrum, and frequencies Δf1, Δf2, f= f2-f1 and Δf=Δf2-Δf1 are in the radiofrequency region of the electromagnetic spectrum; - combining the output signals of the two combs to obtain a dual comb beam and directing said beam towards the verification region; - receiving a back reflected signal at a photodetector (5.2) after striking the verification region with the said beam; - outputting an electrical signal S1, with a bandwidth BW, - obtaining the radiofrequency spectrum of the signal S1, - comparing the amplitude of the line of frequency f = f2-f1 with an amplitude of reference for the pavement in the dry condition; - if this amplitude is different from the amplitude of reference, then, - calculating the ratios of the amplitudes of the lines of frequencies around the central frequency f; - assessing the pavement state as "wet" if the ratio is above a threshold and as "frost" if it is below the threshold. - generating with an optical source composed by two optical frequency combs: - a first optical frequency comb with a central frequency f1 and a frequencies span of n*Δf1, being n the number of the comb lines and Δf1 the spacing between comb lines, wherein the spectral bandwidth of the comb covers a region of the spectrum where the imaginary part of the complex refraction index of water and ice cross each other; - a second optical frequency comb with a central frequency f2 and a frequencies span of m*Δf2, being m the number of the comb lines and Δf2 the spacing between comb lines, wherein the spectral bandwidth of the second comb overlaps the spectral bandwidth of the first comb, wherein frequencies f1 and f2 are in the optical region of the electromagnetic spectrum, and frequencies Δf1, Δf2, f= f2-f1 and Δf=Δf2-Δf1 are in the radiofrequency region of the electromagnetic spectrum; - combining the output signals of the two combs to obtain a dual comb beam and directing said beam towards the verification region; - receiving a back reflected signal at a photodetector (5.2) after striking the verification region with the said beam; - outputting an electrical signal S1, with a bandwidth BW, - obtaining the radiofrequency spectrum of the signal S1, - comparing the amplitude of the line of frequency f = f2-f1 with an amplitude of reference for the pavement in the dry condition; - if this amplitude is different from the amplitude of reference, then, - calculating the ratios of the amplitudes of the lines of frequencies around the central frequency f; - assessing the pavement state as "wet" if the ratio is above a threshold and as "frost" if it is below the threshold. - a first optical frequency comb with a central frequency f1 and a frequencies span of n*Δf1, being n the number of the comb lines and Δf1 the spacing between comb lines, wherein the spectral bandwidth of the comb covers a region of the spectrum where the imaginary part of the complex refraction index of water and ice cross each other; - a second optical frequency comb with a central frequency f2 and a frequencies span of m*Δf2, being m the number of the comb lines and Δf2 the spacing between comb lines, wherein the spectral bandwidth of the second comb overlaps the spectral bandwidth of the first comb, wherein frequencies f1 and f2 are in the optical region of the electromagnetic spectrum, and frequencies Δf1, Δf2, f= f2-f1 and Δf=Δf2-Δf1 are in the radiofrequency region of the electromagnetic spectrum; - combining the output signals of the two combs to obtain a dual comb beam and directing said beam towards the verification region; - receiving a back reflected signal at a photodetector (5.2) after striking the verification region with the said beam; - outputting an electrical signal S1, with a bandwidth BW, - obtaining the radiofrequency spectrum of the signal S1, - comparing the amplitude of the line of frequency f = f2-f1 with an amplitude of reference for the pavement in the dry condition; - if this amplitude is different from the amplitude of reference, then, - calculating the ratios of the amplitudes of the lines of frequencies around the central frequency f; - calculating the ratios of the amplitudes of the lines of frequencies around the central frequency f; - assessing the pavement state as "wet" if the ratio is above a threshold and as "frost" if it is below the threshold.