Well, I have some sources for that information:

http://www.newyorker.com/reporting/2013/11/25/131125fa_fact_...

It's a long article, but search for "rain." Here's the most important quote for our purposes:

    Left to its own devices, Thrun says, it could go only
    about fifty thousand miles on freeways without a major
    mistake. Google calls this the dog-food stage: not quite 
    fit for human consumption. “The risk is too high,” Thrun 
    says. “You would never accept it.” The car has trouble in 
    the rain, for instance, when its lasers bounce off shiny 
    surfaces. (The first drops call forth a small icon of a 
    cloud onscreen and a voice warning that auto-drive will 
    soon disengage.) It can’t tell wet concrete from dry or 
    fresh asphalt from firm. It can’t hear a traffic cop’s 
    whistle or follow hand signals.