How to maximise the impact of car headlight trails in long exposure

I'm trying to do long exposure photos with car headlight trails and am finding the results a little disappointing due to the low 'impact' of the light trails. I believe there are a few things affecting this, namely

1. length of exposure (currently limited to about 90s at min ISO and aperture)


2. the amount of traffic


3. the level of light given off by street lamps

I believe 1 & 2 can be fixed by the addition of an ND filter and choosing a busier time to take the pictures, but am not sure how I can address 3. Is there some sort of filter I can use which will minimise the effect of the street lamps without cutting out too much of the light from car headlamps/tail lamps?

Here's an example of what I've done so far:

Answer

The solution to #3 is to find a location without those horrid Sodium Vapor Lamps, preferably one with no street lights at all. Then you can take much longer exposures and you don't need to worry about # 1 and #2. And you will have light at wavelengths other than a narrow band around 2700°K. On a night with a moon in its second quarter (which means it is already in the sky just after sunset when the most vehicular traffic at night is usually seen) you'd be surprised how much can be illuminated with the moonlight during long exposures.

This was a proof of concept shot I did a while back. Although there are no light trails in this one, the shadow of the parked car is thrown by a nearly full moon. The barely visible secondary shadow of me and my tripod (in addition to the darker one thrown by the moon) at the left edge is thrown by a bright street light about 1000'/300m away. At the time the trees near the water were dark silhouettes to my naked eyes. ISO 2500 for 30 seconds at f/2.8. Exposure reduced two stops in post.

Dropping the ISO to 100 and narrowing the aperture to f/11 would allow exposing for 1/2 hour at the same exposure! But that would make any short duration light trails pretty much invisible. So try using a wider aperture such as f/5.6 and you could expose for 7-8 minutes. If the light trails still aren't bright enough, raise the ISO a little and shorten the shutter speed by the same amount. The ambient light will be at the same exposure level, but the headlights and taillights will get brighter relative to the ambient. If the light trails are too bright then stop down a little and increase the shutter time by the same amount.