3 mph, 2 bays, anywhere the threat is likely to be coming from and only the area you cannot see in your mirror. Get a friend to stand alongside your bike, in the next "lane". Tell him to walk slowly backwards till you can see him in your mirror. The area in front of him is the lifesaver zone. The area behind him is mirror zone. Now pretend he's a car, and you'll understand why it's so important to look at that zone before a change of direction to the left OR the right. Also worth bearing in mind that I recall a bike cop once saying "I can sit in a bikers blindspot for 10 minutes without him knowing I'm there". Scary innit?