Last year, the city of Los Angeles finished the "largest LED street light replacement project" in the world, as Forbes reported then, swapping 141,089 streetlights in what's just the first phase of a full switchover from sodium lighting. (LEDs are more efficient and last much longer, so the city will save massively on both electricity and maintenance costs.) But LEDs, which cast a bluish light, look radically different from the old, yellowy high-pressure sodium lights, meaning a pretty straightforward municipal decision will translate into a completely new film aesthetic in one of the most-filmed cities in the world. (New York is planning on switching its lights out for LEDs too.) At No Film School, Dave Kendricken writes that "In a sense, every night exterior LA-shot film previous to this change is rendered a sort of anthropological artifact, an historical document of obsolete urban infrastructure."
Kendricken explains that non-tungsten artificial light sources, like LA's new streetlights, "produce a non-continuous or incomplete spectral output," so some colors just won't show up at all in anything filmed (or, more accurately these days, shot on digital video) under those lights (even with gels or post-production color correction).
High-pressure sodium, on the other hand, is pretty much monochromatic, producing light in only a very narrow part of the spectrum (the yellow part). That's much worse for lighting a film shoot, of course, but it does have a "starkly unique, artificially lit recognizability." (Kendricken has clips from Michael Mann's 2004 film Collateral to demonstrate this "before" look. Someone tell us, though, how PT Anderson got his beautiful blue nights in 2002's Punch-Drunk Love, which was shot in the Valley.)
So both types of streetlights have their aesthetic benefits, and certain filmmakers may prefer one or the other, but the difference is obvious (see the before/afters) and at this point pretty much a done deal; the way the world (and LA) will see Los Angeles at night has changed forever.