So with the price of gas down, why have retail prices not followed? Most everything is shipped by truck and rail.
A couple general reasons, though when it comes to particular things there can be particular reasons why prices go up or down (or don't go up or down).
For one, there can to some degree be a ratchet effect - once people get used to paying more for something, there's sometimes less need to bring its price down in response to decreasing input costs than there originally was to raise its price in response to increasing input costs. For another, sometimes it takes a while (especially in light of that ratchet effect) for decreasing costs to have an effect on prices.
That said, in December we did see a slightly negative change in the Core CPI (though it was small enough that, rounded to one decimal place as it gets reported, the change was 0.0 rather than a negative number). That measure is for everything other than food and energy. Including food and energy, we of course had price deflation for December and even November - food prices were up some in aggregate but energy prices in aggregate were down considerably.
Going forward I don't know that I'd expect much in the way of price deflation, other than with regard to particular things (e.g. beef), as the deflationary effect of lower transportation costs might be offset by price inflation that occurs for other reasons. Prices on the whole might be flatter, in other words, rather than necessarily move down.