Well, I've been thinking on this for a while and with a new junction just having opened in town I went out this morning to put my theory to the test. First off, here's one of the resultant images:
The difficulty with urban traffic trails is trying to balance the exposure. To capture the vehicle lights you're looking at shooting around the ISO200 f/16 mark, but with street lights doing their job of making everything bright those settings can limit you to an exposure duration of maybe 10 seconds.
Once you add some ambient glow from twilight your exposure duration is reduced again down to maybe as little as a couple of seconds - and that's not long enough for your original plan of traffic trails. What to do?
These two images are single frames from the final stacked image up top. You can see there's no way I could have captured all the trails in a single frame without compromising the look of the image - the trails would have faded against a bright roadway or the sky would have blown out- or both.
The more I thought about it (which, if I'm honest, wasn't actually that long) the more this conundrum seemed akin to stacking star trails. So, having arrived on location I composed my shot and set the aperture to f/16 to capture the vehicle lights without blowing the highlights. An aesthetic possibility of shooting at narrow apertures is that any static lights in the frame take on the appearance of starbursts, according to the format of the lens' aperture blades - check out the rays of light coming off the traffic lights and streetlamps. f/16 and ISO200 gave me a base exposure of 2.5secs so I fired off several frames consecutively, making sure that for each lane of traffic I had traffic coverage where I wanted it. An advantage of this technique is to not worry too much about planning ahead for the right traffic density - if you see a truck or bus coming and want its high trails, keep the shutter on continuous whilst it passes through the frame; if there's periods with no traffic, you don't have to expose. Just go with what's there.
To layer up my images I used the startrails.exe (PC only) freeware application from http://www.startrails.de/ It's pretty easy to use but the help files are all in German so I'll cover the instructions in a future post. Mac users, I've yet to find freeware that does the same thing but it's pretty much the same as layer mode > lighten in PS.
I'll also be posting about my urban star trails series so check back soon for more LE tips.
