When deciding on how to place lights on a layout it is important to consider how prototypical you want the layout to look. My experience is to look at real life lighting structures and then to scale them appropriately. As you will note, there are some rules of thumb for the spacing of outdoor lighting, however, when applied to any scale you are operating in, the rule spaces the lights too far apart.
The images below come from a presentation made by Michael Groves, Dwarvin Enterprises, LTD., at an 2020 NMRA clinic to give you perfect 2020 vision at night!!
Model Train Scale Heights
Here are some images for spacing that is used on Michael’s HO layout.
Water front globe lamps on
Swan Neck lamps
Industry standard spacing of lights
The following slide provides scaled spacing based on industry standard of spacing = 5 time the lamp height. As you will note, this sort of spacing seems to be out of proportion with the size of various layouts. For example, providing spacing of 20” for highway lamps on an HO layout would mean a highway to have even 4 to 5 lights on it would have to be 7 to 8 foot long.
This is a summary of the spacing recommend based on the above images and data taken from my own layout.
In summary, the spacing recommended above takes the industry recommended spacing and reduces it down to 2.5 times the actual lamp height used on a layout. Ultimately, this is only a guide as only you will know what is going to look good on your layout given depth of field and perspective etc.
I hope you find this helpful.