External lights can absolutely have an effect on your plants, but it depend on multiple factors, like timing, frequency, intensity, etc. You can find studies and trials all over on the effects of street lights on plant growth for various plants, on flower reproduction, tree growth cycles, etc. But a lot of that too depends on the location of those things in relation to the light source.
Look at how grow lights are marketed, we quantify (put into measurable values) PAR light through PPFD, of which a map is included on virtually every grow light out there right now, including HPS/metal halides, and look how those intensities drop off quickly as you get further away from the source. So again, just because something is BRIGHT, doesn't mean that it's INTENSE (or providing adequate spectrum) that would promote a response from the plant based on proximity.
Now if you pull your pickup right up next to your flowering plants and turn on your brights, well. That's different. At my current farm I consult for; we run 24 hour lighting in veg in our greenhouse, which is very close to our flowering plants in the field, and nothing is hermaphroditing or having any issues flowering. The amount of actual light intensity coming from our grow lights, plus the transmission loss from the greenhouse material, makes it almost negligible to the plants in the field.
