You can completely fill up the area shaded in green with anything, belts, pipes, other buildings, land(untested), etc. and still have trucks able to access it. Having the ports/inlets/outlets/inputs/outlets of the building being blocked or open has no impact, nor does the position of the blocked/open port/inlet/outlet/input/output.

Here you can see that you can have all tiles occupied 4 tiles away from a building, but as long as you either have 3 consecutive tiles open on the perimeter, or 1 corner tile open, trucks can still get to the building. Why is it 3 tiles? You will learn further in the guide. Why the corner? idk stop asking questions please your scaring the children

Expanding past the set area around the building, you also need at least 3 tiles of open space directly in front of the green 3 tile-line. For the corner, its a little more complex but simply put you need to have a 3x3 tile which at least contains the green open corner.

Before we continue on to the rest of testing, I need to show that the rules still apply regardless of the truck size (tested the pickup and the truck)

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With 4 tiles completely filled on all sides, neither truck can access the building.
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Now with 3 tiles all around, both trucks can access.
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Even with all 4 tiles filled all around, as long as there are 3 tiles open like shown it still counts.
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Even with 4 tiles filled all around, just having a single corner open still counts.
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Now that we have established that truck size doesn't matter, these show the extended testing.
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You need a 3-tile wide lane in front of the 3-tile opening
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You can also snake it around/build a maze, as long as its 3-tiles wide
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As you can see, if the opening to the lane is 2 tiles, it wont work.
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Or if you "rounded" the corner
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Just making sure that a 2-tile opening wont work with this.
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Extending the maze still works.
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PROCEDURE NOTES: All vehicles still count "ghost" objects (things placed on planning mode) as actual objects(early not-shown testing was with built objects, still showed same results). Between every step, I manually moved the trucks away so they would initiate a check on if they can actually access the building. Instead of moving cargo back and forth to do the "checks" I found out I can just assign a vehicle to it(early not-shown testing was with 2 unit storages, still showed same results)

thats about it, see ya