I`m sure that when the car came to a halt on top of the crash barrier both of them were still concious and had the look of of "Phew...that was close"...then, somehow the car rolled over onto it`s roof and they were both unconcious as if they`d been involved in a 90mph head on
also i would have thought getting from walford to tring would have involved busy roads brightly lit !
not the dark sleepy country roads they were on, unless like all cabbies he was driving via the yorkshire moors or something.