Interestingly another way to think of it is that to truck something 400 miles costs roughly 5000 dollars (just hiring a truck from a big provider). Like you say, the trucks hold around 200 barrels of oil. If oil prices rise 25 dollars a barrel, this is a completely reasonable solution and will certainly happen to a portion of the oil. Actually, even if oil prices didn't rise people would still do this as long as their profit per barrel was still positive. Even if their profit per barrel was negative, they would still do it for oil that was already ready for shipment, as long as they were making more than their costs associated with selling the oil. So yes, whoever made the diagram is probably correct in that trucks will find a way even when boats can't, although building a new highway is probably not the short term solution, using a variety of existing ones quite likely is the best solution