To help myself understand how they got to $14 I broke each area down into it's parts. Some is legitimate, but a huge majority...well judge for yourself. I think it's a joke.

Total Annual Oil Tax Subsidies:
Low estimate: $0.035/gallon High estimate: $0.06/gallon

The majority of this is actually tax breaks to oil producers not money paid to oil companies. They could just as well say I am being subsidized $30,000 a year because that is the amount I made that the government didn't take from me.

Total Annual Government Spending Subsidies:
Low estimate: $0.32/gallon High estimate: $0.95/gallon
Almost all of this (98%) is Road Construction and maintenance. We do pay for that already in gas tax and other taxes. What roads would we need with a Horse economy.

Total Annual Protection Costs:
Low estimate: $0.65/gallon High estimate: $1.05/gallon
$0.67 is Military and $0.29 is for Fire, paramedics, police.

So far not that much cost really considering we'd have to have some alternative (horses and wagons, sleighs in the north) that would have it's own expenses and controls.

It requires some creative assumptions from here on out to get to the big numbers.

Total Annual Environmental, Health, and Social Costs
Low estimate: $2.00/gallon High estimate: $8.13/gallon
$4 for Health costs, yet life expectancy has skyrocketed since the introduction of the automobile, another study could conclude that humans are living 30 years longer simply BECAUSE of automobiles, take your pick.
$0.30 for Visibility?
$2 for Sprawl? Would we not have sprawl with Horse and buggy, 90% agricultural society is the sprawl of the past.
$0.016 Oil spills, WOW not much.
$0.09 Noise pollution?
$0.20 Global Warming, that only twice as much as Noise pollution and 66% of Visibility costs?

It just get's silly from here.

Total Annual Cost of “Other” Economic Costs:
Low estimate: $1.59/gallon High estimate: $3.95/gallon
$1.30 cost of traffic delays. And Horse and buggy's would eliminate this how?
$0.57 cost Lost earning and productivity on the job due to accidents, none of which would happen with horses?
$1.49 Subsidized parking? No support, appears to be related to taxes not collected because parking lots aren't taxed? Enron played fast with potential profits in the same way. We could pay for global warming cessation, oil spills, and our military by eliminating this cost. No wait it's fake.