when you wish to define the visibility you must take into consideration variables like illumination (brightness), air humidity and weather conditions.
For instance, you mentioned that you wish to calculate the visibility during a rain. But you didn't mention variables like the daytime hour, climatological zone and intensity of that rain.
For example, a mid-day (12:00 hour) tropical rain (Rain forest) has lower visibility than a heavy rain at 19:00 (summer-time) at the Mediterranean region.
The roads will enter the grey hexes only on the "east" and "west" sides of the map. On north and south they only just go to the grey side hexes.
If this isn't what you mean exactly you'll need to attach the map so we can look at it but roads etc. will go right to the edge of the playable map area
Don
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"You are never to old to rock and roll if you are too young to die".--- What do you expect to be doing when you are 80?