Why does it have to be sudden? You either need to leave or you dont, it has nothing to do with how fast the danger develops. I am not really a SHTF person but I can play along for the purpose of fruitful discussion. All this talk about what type of vehicle is rather silly.. its just not that deep. We do not live on the tundra or salt flats.. most of us live in cities or other urban confines. Traffic jam or not, there isnt likely to be any of road travel from one city, state or region to another. A person needs a car which is reliable and preferably one that can carry multiple people and supplies. You will either get to where you are going via established roadways or you wont. The only other realistic alternative is to travel on foot or motor driven cycle.
What I am saying is that its probably not going to matter all that much if you are driving a honda civic or a ford350 dually. Its probably not worth worrying about. The question really boils down to whether you have a reliable vehicle or not. If you do, just check that box and move along to your next benchmark.
People tend to overintellectualize this issue far more than is practical. Its really rather simple and survival imperatives dont really change all that much within the same region. If were are talking about the difference between florida and maine... sure there are issues but the difference between florida, ga, ms, al or TN are not all that different. If you plan within your particular region, you will have a fairly practical system. It wont be perfect, so dont try to make it perfect.