Another thing people keep forgetting is the doppler effect. Since light can't go faster than c, the energy gained from momentum instead ****s the wavelength up towards the bluer end. So the light from your headlights is still at c, but is shifted way up into the gamma-ray end of the spectrum.
Conversely, light from your taillights lose energy, and therefore get downshifted, they'd fall down into the radio-wave end of the spectrum.
There are actually several differest effects experienced as you approch c. This page is pretty good at explaining them:
http://www.fourmilab.ch/cship/cship.html