One of those weird ideas, I'm grateful they went with it.  

When you're out and about, you have to remember that all you are doing with your camera is capturing light on a little flat piece of silicon, if less light is around, less light goes in.  

In this case, it was after sunset and the light was fading fast, and there were cars whizzing by us on the street.  By dialing down my aperture and using the bulb setting I could handhold this shot and have my lovely assistant hit him with a burst of the flash at my command.  In this situation you are at the mercy of the cars going by, and how many go by at once, and in which lane, etc.  That's okay, because it gives you something different to work with in each shot.  

Be sure that your subject understands what you are doing and to try and be still, otherwise you will get light trails going through them as their superimposed image on the light trails will not be as bright as they are.  

Tons of fun!  =)