Do you have any sources for any of this information?

Because they seem entirely made up and the "issues when tires slip on ice" have been mitigated with computerized ABS systems for the past 20 years

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.

ABS allows you to take maximum advantage of the traction you do have, but it doesn't magically improve your traction. The problem the GP is referencing is that the driver (be it human or computer) operates the vehicle using some sort of model for the maximum available traction. (Some) Humans are able to notice when it is snowing, re-tune their model and drive at an appropriate speed. It doesn't seem impossible that some heuristic could be invented to do the same for a computer driver, but I don't think anyone has demonstrated that heuristic yet.

Funny thing about ABS systems is that the pump actually forces the brake pads open. The designer's thinking is that a turning wheel is able to steer, while a vehicle with sliding wheels is directed pretty much by momentum alone. This results in longer stopping distances.

So far as ice -- it's coefficient of friction in the real world is not a constant -- it varies based on local conditions. So you can't predict your ability to stop & steer on it - you can only react. Your best defense is to install ice-rated tires in early winter to increase your traction. Google will likely have to do some testing in Wisconsin to see how the software reacts to icy conditions, and is able to somehow automatically recognize ice/slush and adjust stopping/acceleration rates appropriately.

Well, in principle ABS shouldn't necessarily result in longer stopping distance: sliding friction is lower than static friction, so if you can control the braking precisely enough, not locking the wheels should result in shorter stopping distance along with not losing control. I'm not saying current systems are that good, though.

On dry surfaces, a professional driver is able to do Threshold Braking, which in non-ABS cars maximizes stopping power by taking braking just up to the limit before the tires start sliding. On cars with ABS, the intent is to minimize the rate at which the ABS pump activates in order to maximize stopping power (pulse ... pulse ... pulse vs. pulse.pulse.pulse)

ABS has an advantage over regular systems in that the computer is able to control braking effort on a per-wheel basis. Something a human with only one pedal to push can't do. IIRC, the best ABS systems are able to cycle at about 10Hz.

Something I vaguely recall reading about is that in a panic stop on ice, the heat from the sliding friction will melt a very small bit of ice under the contact patch, turning it into a hydroplaning situation. I need to see if I can find that reference again...

"ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces for many drivers; however, on loose surfaces like gravel or snow-covered pavement, ABS can significantly increase braking distance, although still improving vehicle control." (with citations) http://en.wikipedia.org/wiki/Anti-lock_braking_system

I've done a lot of work with perception systems and sensors. GP is right on the money with the problems. Rain is even difficult.

Snow is the worst though - it is both wet and super reflective. Super reflective dazzles your laser/obscures cameras. Wet screws up your radar.