In my case, it is a combination of issues which I fully accept are neurotic and life limiting. Heaven knows I have been told that often enough.
The first one is, as Withnail correctly surmises, the speed of your own and other vehicles, relative to your chances of surviving a mistake. As a passenger, I watch my husband react quickly and calmly to someone swerving/something falling off a roof rack immediately in front of us and know that I wouldn't have reacted like that. I'd have swung the wheel, hit some of the traffic which is EVERYWHERE and we would be dead.
Then there is the feeling of speed and proximity, all around me.
The number of times that a stretch of motorway will soar into the air, frequently bending at the same time, producing a feeling of total certainty that the car is going to go off the edge. There is a bit like this on the first bit of the M5, in Birmingham and worse, a huge long stretch outside Manchester. It was there, that, having attempted to drive across at a speed I could beat on the bike, I failed and had to be rescued by some nice police officers.
Then, if you avoid the certain death above, it is reasonably certain that you will be trapped on the bloody thing for six hours because someone else hasn't been so fortunate.
I have got into all sorts of scrapes avoiding motorways, particularly when having to attend meetings for work. Recently, when my sat-nav failed to recognise any of the local roads I stopped at a back-street car body shop outside which I could see some people. One chap was collecting his car and having studied my map, said that he was going quite a lot of the way in that direction and would put me on the right route.
Unfortunately, at some point he forgot I was there. I only realised this when he pulled in to the drive outside his house. I didn't want to embarrass him by reminding him of my presence so I carried on driving into the housing estate.
Off the motorways, I am quite a confident driver. Worse still, I have the nerve to get irritated with dawdlers, elderly pootlers and people who are clearly unable to drive at night.