Modern Headlights

US law is that they have to be "as far apart as practicable". Automakers are given a lot of leeway for deciding how far apart is "practicable", but Ford got their nose swatted ("We're not going to make you recall them, but don't do it again!") by the Federal DOT for the turn signals on the '96 Mercury Sable, which were only about 18" apart. Same thing happened to Subaru for their '92 Legacy because they put the front sidemarker lights/reflectors (which have to be "as close to the front as practicable") behind the front wheels. Ditto Toyota for their original RAV4.

There aren't going to be any Federal light cops coming after '68 Valiants for turn signals too close together, but the main thing is whether the signals can do their job, that is tell other people close enough to matter, clearly and immediately, how and where the car's about to move. If you cram the front turn signals together so there's barely any separation between them, they can't do that.