Interesting...I always thought it was St Christopher as well, to go with the book he reads and that his 'journey' with Withnail was ending and he was being guided out safely - but I guess that's the crazy fans over analysing everything. If McGann said that's what it was, that's probably what it was, who'd know Marwood better than himself?
These aren't accidents! They're throwing themselves into the road, GLADLY! Throwing themselves into the road to escape all this hideousness! THROW YOURSELF INTO THE ROAD DARLING, YOU HAVEN'T GOT A CHANCE!