I've recently found on Kindle a book on Bugatti by Boddy, published by Sports Car Press in 1960. Its description of Le Patron's declining years is very much at odds with WF Bradley's in his biography of Bugatti - as is the account of Jean's death. Boddy claims that the cyclist involved was a postman and was riding down the centre of the road, whereas every other version I've read says that the cyclist (possibly drunk) emerged from a side turning. I shall shortly be going back to look at contemporary French press sources - thank you Gallica!
The truth of this appears to be a combination of both stories.
The first thing to note is that the roads in the area have been altered, due to the building of Strasbourg Airport. Originally there would essentially have been one long straight from just west of the junction of the D392 and the D127 south of Altorf, all the way to Entzheim. On Google Maps this is named Route de Strasbourg, becoming Rue General de Gaulle on the outskirts of Duttlenheim and Route de Schirmeck outside Duppigheim. Bugatti used a 3 kilometre section of the road between Duppigheim and Entzheim for testing.
East of Duppigheim, just past a Total service station, the road now curves south, before swinging back towards Entzheim. There is a junction with an un-named unclassified road which continues the line of the straight and on this junction there is a pyramidal stele bearing a memorial plaque to Jean Bugatti. According to the plaque, this was the site of the crash.
The old road is the one which continues on the left of this photograph. If you switch Google Maps to satellite view you can clearly see the line of the old road continuing across the service station forecourt and then across the airport, although the road itself now degenerates into an unmade farm track when it meets and follows the airport perimeter, before returning to its old line. Intriguingly, a little further on to the east, on the south side of the road, there is what appears to be another memorial stone and plaque, with a motorcycle helmet on top.
Just before it enters Entzheim, the road becomes Rue Jean Bugatti, which terminates at the roundabout where the D392 rejoins the old route.
So - what of the discrepancies? In l'Intransigeant, August 13 1939, there is a telephoned report from 'Our Special Correspondent, Strasbourg'. Although not specifically stated, this makes clear that Bugatti was travelling towards Entzheim, since the cyclist is said to have been heading from Entzheim towards Duppigheim. L'Intransigeant speculates that Jean might have been dazzled by the cyclist's headlight, but that seems unlikely to be the primary cause. As is obvious on Street View, the road is barely wide enough for one car, while Jean was reportedly travelling at well over 200km/h. This website (in French) claims the cyclist was on the left side of the road, but it looks like his positioning on it would have been immaterial - although Le Matin does say Jean swerved to his left in his attempt to avoid him.
This is how Boddy describes it. My annotations in red:
The factory used a stretch of road near Molsheim for high-speed testing with the knowledge [but not necessarily the approval!] of the authorities. It was necessary only to clear the circuit [circuit?] and then to station a couple to warn anyone trying to enter it. Jean set off to test a car, and Roland, his young brother, was stationed at one of the entrances. A postman [at 10.45 at night? Time from Le Matin], mounted on a bicycle, came along, and airily ignored Roland’s fervent warnings. Roland was only a boy, quite incapable of preventing the man by force [he was actually just a few days short of his seventeenth birthday]. The postman cycled down the middle of the road, where Jean discovered him when he came roaring around a bend [????]. He could not avoid hitting the man and at the same time keep control of the automobile, except by the wildest of wild good-luck, and he must have known this, being a very skillful driver. He tried, however, and did succeed in the first part of the maneuver: he missed the mailman. But the car got away from him, rolled, and he was killed..
It seems certain that Roland was present, as L'Intransigeant says that Jean's brother and d'autres collaborateurs intimes were there. Le Matin names a mechanic called Homère, who had accompanied Jean on the first two runs out and back, but Jean then did the third alone. And Robert Aumaitre was certainly there:
The cyclist was named by both Le Matin and Le Petit Parisien as Joseph Metz, from Dornach, which is 100km south. According to Le Petit Parisien he was 18 years old and on holiday, staying with friends in Duppigheim.
Charles Faroux, writing in La Vie Automobile two weeks later, suggested that the cyclist had 'in all likelihood reached the road by some dirt road or path which had escaped the control of the two surveillance and warning posts.'