Merging hashes, summing values that have the same key
One way might be to use #reduce (as you suggest) and Hash#merge with a block:

totals = codes_and_totals.reduce({}) do |sums, location|
  sums.merge(location) { |_, a, b| a + b }
end


The merge-block is only invoked if a key exists in both the hashes being merged, so we don't need to guard against nil or start with a zero value. If the block runs, it's because there are two number values that need to be merged (i.e. added, in this case).