Nice image. I'm curious why you stacked instead of stopping down more for a longer exposure? Flexibility? It seemed to work well although the lines would be more contiguous with a single (or fewer) exposure(s).
Three main reasons:
1 - Traffic patterns. Traffic lights here changed every 2-3 minutes. Unless you could somehow get a 150-second exposure (at minimum) all the traffic would only be coming one way, with lots of stationary vehicles. I wanted traffic coming from all directions - the 16 exposures here weren't taken one after the other, but over the course of around 10 minutes.
2 - Erratic driving patterns. Traffic doesn't flow smoothly here. Cars and motorbikes stop in the middle of the intersection, vehicles make random U-turns and some just randomly weave between multiple lines of traffic. A single long exposure leads to lots of 'ghosts' of stationary/slow-moving vehicles where they stopped/moved slowly for prolonged periods of time, as well as a large number of random trails criss-crossing the general flow of traffic and ruining the composition.
3 - Exposure. Any longer and the scene would have blown out. Even changing the aperture f/16 would have only bought me 6-second exposures. And 15mm lenses don't tend to play well with ND filters - what makes the centre acceptably dark turns the corners into deep shadow.
