What you say to them is, "A car can't go the speed of light." According to the theory of relativity, NOTHING that has mass can go as fast as light.
If you were in a car travelling at 99.999999% of the speed of light, and you turned on your headlights, the light from your headlights would shine out in front of you, and it would light up the landscape, and you would be able to see it.
From the point of view of a hitchhiker standing on the side of the road, the light from your headlights would be 'blue shifted' because of the doppler effect. Actually, 'blue' is a bit of a misnomer here, because at that speed, the 'light' from your headlights would appear to the hitchhiker to be a beam of extreme, high-energy X-Rays.
I'm not 100% sure, but I think that the light that you see, reflected off the hitchhiker's shirt and other objects in the road would be blue-shifted by exactly the same amount. (Wear your sun-glasses. :-)