Not defending the driver, but there's no pavement/sidewalk on the piece of most of that road, although the visibility is good and it was 10am on a Saturday
He would most likely have been trying to cross the road to continue his run on the path on the other side. I've run this route along the river myself.
This area is part of the Fenland, so the land is very flat. As a result the road bridge over the river is raised up above the level of the surrounding area.
This means that as he was crossing the road he would have been invisible to any car approaching from the other side of the bridge until it crested the bridge. It's entirely possible that a car travelling fast - the speed limit on that road is 60mph but plenty of people exceed it - could have come over the bridge and the driver would have seen him too late to stop in time.
The placement of the footpath crossing, unsighted from one direction, is - as you can tell - far from ideal.
When I'm driving on that road I ease off approaching the bridge: in part this is because I know there's that footpath crossing the road on the other side. But it's also - and whilst I don't want to be too harsh to the driver involved here there's really no excuse for this having happened - because I'm not a complete moron and know that, regardless of how well I think I know the road, it's unwise to go over a blind crest at speed, and especially in a heavily agricultural area where large, slow moving farm machinery is a commonplace site on local roads.
Is it clear it happened out of town? I googled, and articles, including the one you linked, say "Newmarket Road" in Stretham. And within that town, there seem to be pavements.
Just curious, as you say doesn't make it any less bad for the driver. Though it would be that little bit more mysterious if the Autonomy codefendant was struck on the pavement.
https://www.google.com/maps/@52.3423362,0.2274592,3a,75y,335...
https://www.cambridge-news.co.uk/news/local-news/family-step...