Cities: Skylines: How the Mail / Postal Service actually works (rather than how you might expect it to work) …
So, as figuring out the optimum level for Postal Services isn’t exactly obvious from the Information Panels provided by Colossal Order, I decided to start from scratch and run some tests on <Fast-Forward> …
Next, I built a new Post Sorting Facility (PSF) adjacent to a Highway (so that Outside Trucks wouldn’t clog up the city’s road network); set the Budget to 100%; and pressed <Fast-Forward> again.
So at this point, there were no Post Offices in the city …
 One truck = 50 Import Units / 5,000 Mail Items
… therefore, 1 Import Unit = 100 Mail Items
 Imports are recorded as soon as they enter the map, not when they’re delivered
 Outside Trucks only deliver ‘Sorted Mail’
NOTE: The First-Person Camera will incorrectly show a truck as carrying 50,000 Mail Items
However, although Outside Trucks are always 100% full – they don’t deliver their entire load to the PSF – the amount delivered is instead a random percentage (in my tests, ranging from as little as 13% / 600 Mail Items up to 93% / 4,600 Items).
The non-delivered balance is freight-forwarded to the Truck’s city of origin, along with (in the case of my test) some of the 17 Units / 1,700 Items of Export Mail that had mysteriously appeared (evidenced by Outside Trucks being coloured blue on both the Import and the Export tab). But as that 17 Units of Export Mail weren’t coming from my city, I had to assume that figure represented ‘Intransit Mail’, being sent from City A to City C, with my city’s PSF acting as the City B go-between.
 Outside Trucks are always 100% loaded, however …
 Only a portion of that load is delivered to a PSF, the rest is taken to the city of origin, along with some Exports – they appear to represent Mail from other city’s Outside Trucks using the PSF as a drop-off / transit point
At this point, an Imports reading of 607 (60,700 Items) clearly didn’t represent the total Mail Items delivered to the PSF (12,700), so perhaps represented the total amount of mail being freighted around the map, of which only a small portion is delivered to the PSF.
 Taken at face value, the Import / Export figures are very misleading … Imports do not equate to Inbound Mail addressed to your city, and Exports do not equate to Outbound Mail sent from your city
After a brief cessation of activity, the Imports figure climbed again by another 89 Units, to 696 – represented by 5 more inward-bound Outside Trucks. It then dropped by 19, to 677. Meanwhile, Exports increased by 11, to 28.
Again, such peculiar figures only make sense if Imports / Exports perhaps represents the net amount of Mail on the map, partly derived from all the now partially-loaded Trucks leaving the PSF, plus any new fully-loaded Trucks arriving on the map. The problem with that notion is, there were Imports / Exports figures displayed even after all the trucks had vacated the map …
 The fact that Imports / Exports figures continue to display even when there’s no activity at all on the map suggests a bug
There’s another anomaly too, in that – from time-to-time – an Outside Truck doesn’t leave the PSF at all, it simply disappears inside, even though it has a residual load. It may then subsequently reappear some considerable time later, to continue on its way. And indeed, when one of the ‘disappeared’ trucks resurfaced to return to its city of origin, Exports jumped from 28 Units to 43 Units, equivalent to 1,500 Mail Items
Lastly, once the ‘Sorted Mail’ figure exceed 10% of the PSF’s maximum capacity of 500,000, Outside Trucks stop arriving, and both the Import and Export values then gradually decrease back to ‘1’, at the rate of 10%-of-the-previous-month’s-balance per month, which takes slightly over one ingame year. Goodness knows what that’s about …
 The Imports figure appears to be severely bugged, as it doesn’t even remotely approximate the amount of Mail delivered to the PSF, nor does it represent the total amount of Mail on the map
 Imports cease once ‘Sorted Mail’ exceeds 10% of the PSF’s maximum capacity (that’s with no Post Offices mind, it probably increases further when there are Post Offices in your city)
 In the absence of a PSF, Outside Trucks will deliver sorted mail directly to Post Offices, and also take away any unsorted mail
 The Post Office’s own Trucks will also take unsorted mail out of the city
NOTE: The First-Person Camera will incorrectly show a van as carrying 10,000 Mail Items
That means Post Office trucks are only really useful in a city that doesn’t have a PSF, in which case they will take unsorted mail to one of the cities next door.
Because the test City was a thinly populated collection of small towns spread out over nine tiles (and one Post Office was able to easily service about five tiles), it turned out that all I needed for an efficient Postal Service was two Post Offices on 100% Budget ($160) for a total of 20 vans, with one Post Office at each end of the map (101% is better though, as it provides an extra van and an extra truck at each Post Office for no additional cost).
Or alternatively, I could’ve maintained the same number of vans spread across four Post Offices on a 70% Budget ($224), along with double the electricity and water costs compared to two Post Offices.
So two at 101% looked like far the best way to go. And the main downstream benefit of that – apart from reduced overheads – was that the figure for the amount of Mail under ‘Imports’ looked a lot more sensible … less than 100, instead of 700+.
How the Mail / Postal Service actually works (rather than how you might expect it to work) … Detail
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