With the title, "How It Happened," Sir Arthur Conan Doyle sets up his readers to expect that they will read a straightforward narrative of a significant event, and the bulk of the story is just that: a combination of operator error and mechanical failure that results in a horrific car crash. Because the driver describes in such detail the sensations of the long downhill run and near misses on a series of three perilous curves, readers expect that the story will end with his miraculous survival, since he is telling the tale.
The story's ending really begins with the driver becoming "aware of my own existence once more," a clever and effective device that the first-time reader accepts as an account of the driver regaining consciousness after the impact. It seems plausible, since many people had gathered at the accident site, that a college acquaintance of the driver would turn up among the onlookers. And since Stanley was someone he regarded as "peculiarly sympathetic" and gentle, the reader accepts that Stanley would stand by his old college friend while others worked to free Perkins from the wreckage. That the driver was unable to move hints that he might be badly injured (but at least thrown clear of the wreckage). The fact that no one except Stanley is paying attention to him and do not seem to hear him suggest that the driver has died, but it is not until the driver remembers that Stanley had died in the Boer War does the reader fully understand that this is indeed a tale told by a dead man.