What is your source for transportation costs in agriculture? According to the USDA, transportation accounts for less than 5% of what consumers pay for food in the united states [1]. The farm itself and labor account for 60% of the cost so even if transportation accounts for 25% of the cost of fresh produce (the USDA data is aggregated over restaurants and grocery stores), it still wouldn't make up for the cost of losing the economies of scale. Without a critical density of farmers producing the same crop in a region, many of the cost savings in automation and distribution are simply out of reach. Agriculture needs these savings more than most because of razor thin margins and natural volatility in yield.

I shop from local farmers in California all the time but they are rarely as inexpensive as SuperKing or Vons except when their produce is in season and they cut out all the grocery store middle men like the produce stands peppering the California freeways. I know that a lot of our best produce is exported because the farther they travel, the higher the profit even when accounting for transportation costs, so Id be surprised if transport is ever particularly significant. The cost of energy (electricity and fuel for equipment) is also higher than cost of transport so fuel costs will have an impact regardless of how far you move the end product.

[1] https://www.ers.usda.gov/data-products/ag-and-food-statistic...