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Either way, based on your description, I feel your conclusion was in error.
"Huge" plastic containers, based on what I would define as "huge", as in "not just a little bigger than average plastic containers (ie: milk jugs) but MUCH larger, by at least two factors of magnitude" would be somewhere between the size of small cars and the size of standardized shipping containers. Such containers (were they still in condition adequate enough to be "containers" and not "broken pieces") would be too valuable to throw away so carelessly.
Second, landfills are isolated in an area owing to their being planned and often very large, so are removed from a highway. Being manmade for this purpose means they are deep and also not often put in a natural pit. A natural pit of sufficient size would be too valuable for geologic or astrological study to go filling with garbage. Finally, you did not describe anything other than the containers, and landfills (at any other time other than when they first begin, which mathematically you are very unlikely to be there for given their long lifespans of decades) have more than just a few items in there.
I can only conclude that they were put there as a form of temporary storage, and their owner intended to retrieve them at a later time. Perhaps they were for roadside construction.
If I am incorrect in my assessment, it is only because I have misinterpreted the scenario owing to a confusion based on your use of language. This style of communication of reality is terribly restrictive and subjective, after all.