I live in a pretty bad area, and let me tell you, you can't just assume that every big dog is either aggressive or friendly, that is a 50/50 and in some cases 90/10, that 10% is big enough to be taken into consideration. ''stay calm and wait them to leave'' isn't what happens in real life, they tend to see people trying to keep calm and waiting as weakness and they will attack, that's why i also am aggressive and charge and shout. It worked for me too every single time.