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Yes, there's a sign that indicates the start of motorway status. The road's already marked as D1 well before that junction, so just the status as D1 doesn't automatically ban bikes.
This junction looks very hazardous to me.
It seems to be a bit of a bodge job from when they constructed the motorway section. Drivers are prohibited from entering road 18 there (which is undoubtedly also why Björn continued to the service station there) by means of a sign and a solid white line. You bet that plenty of drivers will take chances there, though. This bit is also very good:
I daresay many people will disobey the banned turn and the 'no vehicles of any kind' sign there.
Oliver Schick-
The road's already marked as D1 well before that junction
Is it? I can see that the other side of the road (Westbound) is D1 motorway at that point, but Eastbound is still the E50 as far as I can tell.
(The various online maps all say different things, I'm going by the roadsigns visible on Streetview.)
Greenbank
Bjorn is fine. The E50 is still ok to ride on. Here's the junction that he would have taken to turn off the E50 on to the 18 after that service station:-
https://goo.gl/maps/ovDftMFTQ5N2
It becomes the D1 motorway (prohibited for bikes) after that point so he was ok, you can see the motorway signs a bit further along.