Another use for Smart Objects and Stack Modes on this image today. Before, it was for noise reduction, this time I've used the technique to make the traffic look a bit heavier in a single shot than it actually was.
The five images above, after editing one in Lightroom then syncing the adjustment across them all, were all opened in Photoshop as a Layer Stack. (Photo menu > Edit In... Open as layers in Photoshop).
After checking that all the layers were aligned, they were converted to a Smart Object (Layer > Smart Object > Convert to Smart Object). Back in the same menu, the Stack Mode is set to Maximum. This allows all of the lighter value light trails to show through each other. The Smart Object was then rasterized, converting it back to a single editable layer, and some further editing was applied to create the final image.