But those 50 kms are, according to the article, 'main commuter arteries' which to me makes it seem that they selected a small set of roads not to make the driving circumstances easier but to be able to more easily monitor the cars that drive there and to be able to reproduce similar situations more easily, to test improvements to certain algorithms or hardware.

So basically they're testing under the 'worst' circumstances, making it likely that it will perform well on 'easy' routes as well. This approach makes sense - what good is it to clock 1000's of kms of test drives on straight roads with no traffic and in broad daylight?