In your physics class, you have just learned about the relativistic frequency shift, and you decide to amaze your friends at a party. You tell them that once you drove through a stoplight and when you were pulled over, you did not get ticketed because you explained to the police officer that the relativistic Doppler shift made the red light of wavelength $650 \mathrm{nm}$ appear green to you, with a wavelength of $520 \mathrm{nm}$. If your story were true, how fast would you have been traveling?