I just have to say this AC & the 2 or 3 other AC replies to this must be some kind of anti-google car shill. This did intrigue me so I will answer your questions regardless:
1) The zone will have signs with transmitters...the guy holding the sign doesn't even have to exist anymore. Yay automation! /sarcasm
2) They are already programmed to deal with that. It is kind of a *must have* feature.
3) When people sit around thinking about how robots "see" they often imagine it's the same as watching a TV screen. This is not the case. The CCD sensors in these automatons see a MUCH broader spectrum of light. Things our eyes can't even discern like IR, UV, XRAYS, etc. It can see further, clearer, and faster than you can. It can see mosquitos crossing the street while it is doing 100mph. There are several cameras in existence that can see ice. Detection of ANYTHING is non issue with the available sensor packages of today.
4) Again, sensor packages make this a non issue. It'll see a swerving driver but what to do about it will depend on the rules set forth by the programmers. Since they are humans, which you imply are somehow superior, they can tell it to do exactly what you are talking about- stay 50m away.
That snarky bit about superior humans brings me to my point: EVERYONE thinks they are not the idiot driver in the next lane. Well if you aren't that guy and I'm not that guy then who is? Truth is folks, we ALL are the crazy drivers. As so many others have already pointed out we get distracted over stupid shit-- was that Natalie Portman driving a mint 1981 Delorean covered in hot grits??? No but that is the rear end of a semi trailer, thanks for playing. Computers don't get distracted. They simply malfunction which we are just as easily susceptible to (heart attacks, seizures, intoxication, etc). What they can do to negate this, which we don't do so well at, is redundant hotswappage. Even when we carry backup humans in the vehicle there is no easy method for a passenger to quickly and effectively take over. You have a heart attack and you kill your whole family too. The car can and WILL have several backups to backups. If it takes 15 cameras to make the risk acceptable then dammit they will have 15 cameras. They will have 5 THOUSAND if that's what it takes and the economy will make sure that they can produce enough cameras to make that happen. It always has. I am so certain that way back in the day when automobiles first started cruising the roads with horse drawn carriages that these exact same arguments were brought up. They held no weight back then and they hold no weight today. I mentioned earlier that I felt you thought humans were superior and in this respect we are and all you anti-whatever guys don't see it. If humans see something we want then barring an act of some deity we will have it! There is no possible way we will go backwards in technology unless an apocalyptic event should occur. What you anti-computer/tech & overly religious people need to understand and accept (not just tolerate) is that we have the power to make a whole bunch of awesome stuff happen. It is GOING to happen whether you like it or not. There will be a time that we will be chauffeured around the world in our automated vehicles hangin out with our clones, blown out of our gourds, banging an android prostitute on the way home from work which is being televised on Bawtporn.com because Verizon gave us $5 off our monthly bill (and the whole way the gov knew where you were, what you were doing, and deducted 1.7 millions credits from your bank account for lewd and lascivious behavior). Deal with it. We are humans. Not superior physically but mentally. Somewhere right now on this planet someone is doing something you find reprehensible. Apparently those Google cars were driving around with the general populace for quite some time and you didn't even know! We have the power to clone a human. Some lab somewhere secret is testing it right now. We already have fantastic drugs and make new ones everyday. And you can't possible tell me there isn't someone trying to build a sex droid at this second! We already got the tracking tech and the auto banking. What part of my not-so-hypothetical example above is not possible? This is gonna happen. Get used to it. Will we screw it up? Oh you bet ya. There is gonna be some horrific auto-car wrecks at first but there are still gonna be spectacular human auto wrecks too so it won't matter. We are MAN. We'll work those bugs out. Besides even if it did ultimately fail all we would do is shelve the project until technology stepped up a notch and the we'd revisit. The lab I work at is always retrying old ideas that fell through cause there wasn't a strong enough plastic or a powerful enough laser. Give 'em 20 years and auto-cars will be back on the road with iAI by Applesoft and sponsored by Allstate (complete with smooth talking & strikingly handsome Robot-American spokesdroid). That is the point of what is going on today. Google is testing to see if auto-cars are viable. As far as I can see with the results so far this is happening.
My clone and I for one, welcome our autodriving, sexbot laden, kept in good hands with Allstate overlords! See ya on Bawtporn.com!!!!