It would impact supplies, but not shut down travel within the country. There are many other refineries throughout the country as regional suppliers. But there would undoubtely be an impact to the Dow just as after 911. Depending upon the scope and success of an attack.
The effect would be more emotional than anything else. It would increase the jitters of people who don't realize we have alternatives, and the repercussions on the source would be far worse than anything they could do to us.
"It would impact supplies..."
That is an important statement that needs to be re-emphasized for new lurkers/posters to this thread. Any large scale attack would - at least temporarily - impact transportation. It is important that everyone stock up on extra canned goods, paper products, etc. Just think about what you wouldn't be able to live without for a couple weeks to 6 mos or so. Paper products, like t.p., will last forever and you will eventually use it, so why not buy enough to last several mos now. Imported goods, like coffee and sugar, may be hard to find if transportation is limited. Get in the habit of picking up a few extra cans of coffee or rolls of paper towels when you shop. You don't have to do it all in one shopping trip.
I think that some of us who live in "low target" areas of country may think that we would not be impacted if an attack happened. We would ALL be impacted if the stores were cleared out and no shipments were coming in to re-stock.
Being prepared helps me to sleep better at night too. :>)