To expand a little on the expectation that Chuck 'IT God & Lord of the Cyber Wave' Fickens mentions (that the light from the headlights would approach a stationary observer at 2c) you would certainly expect that to be the case. (Just as you would expect a rifle bullet fired at, say, 1,000mph, forwards from an aircraft travelling at 500mph to approach the observer at 1,500mph).
Simple Newtonian physics dictates that the combined velocity would be (u + v) (where u and v are the velocities of the two objects).
However, Einstein’s theories suggest that nothing (including light) may travel relative to something else at a speed greater than c. To accommodate this there is a formula for calculating the relative speeds:
(u+v) / (1+(uv/c2))
Substituting low values for u and v results in very little difference whichever formulae is used. In my case of the rifle bullet, the difference is just 0.0000000017mph.
However, two “Starships” (as in Star Trek) converging, with each travelling at 0.8c would converge not at 1.6c, but at a more reasonable 0.976c. Using the same formula and substituting 1.0c for the car and the light from its headlamps the result is 1.0c.
If you examine the formula you will find that as long as the individual values of u and v do not exceed c (and this is impossible under Einstein’s theories) then their relative velocities cannot exceed c.