“Fifteen seconds MAX!” That’s really all the time you’ll need to create those welcomed dramatic exposures when rush hour traffic coincides with the light of dusk or dawn. ( short video clip of Boston traffic). Any longer exposures i.e. twenty, thirty seconds is over-kill as the taillights/headlights sit atop each other thus unable to really ‘add’ any additional energy to the overall composition.
Twenty minutes after sunset or thirty minutes before sunrise, you have at most, an 8-minute window where the city lights/car lights/sky are all within a 1/3 stop of sharing the same correct exposure, so all the more reason to keep your exposures limited to a maximum of 15 seconds...keep shooting, keep shooting, a low ISO i.e. 100 if not 50, is critical to get you to that 15-second combined with an aperture somewhere between F/11-F/16, and yes, if you have it call upon your FLW magenta filter!
Most of all be safe out there on those overpasses, stairwells, high-rise balconies or wherever you find yourself shooting the electricity of the city! Nikon D500, Nikkor 18-300mm at 30mm, F/13@15 seconds, ISO 50, NO FLW filter as I’d grabbed the wrong filter pouch earlier in the day. You keep shooting!