Autonomous cars have driven on the highway in real-traffic conditions [1], [2], although they had a driver at the ready in the event of any problem. I don't know about night or weather conditions for these instances.

[1] - http://en.wikipedia.org/wiki/VaMP (their citations pages aren't responding at the moment)

[2] - http://www.argo.ce.unipr.it/argo/english/index.html