Were you in auto focus or manual focus, the only thing that I can think at the moment as a possible issues causing you not to be able to take a photo is that the camera was trying to AF and because of the darkness not able to focus
I see from flickr that you've done some on a very high ISO, this is to be avoided at all costs, as all you will end up with is a blob of over exposure where the headlights should be, these would be improved by getting on location earlier
it would be a lot nicer to have some good colours in the in sky
If you can get there just after the sun has set you will get some great colours I generally find that you cannot get a decent light trail image until about 15 minutes after the sun has gone below the horizon as there is still just a little too much ambient light, but that 15-30 minutes after sunset on a long exposure you'll pick up some great colours in the sky
Try and stick to ISO100 with time and experience you will know when to push it to the max ISO that I would use and thats 200, I've never felt it wise to go above that headlights are just too bright :shrug:
I like your composition
and it looks like it could make a good addition to our light trails map so if you don't mind giving me the location details I'll ensure that it get's added onto the map
Matt