Mirror, signal, shoulder check, move. These are the four steps for a successful lane change. Simple enough one would expect, until you watch what goes on around you in traffic these days. Many drivers will make lane changes where it is forbidden to do so. It is illegal to make a lane change over a solid line at any time. Solid lines are marked at places like crosswalk approaches or where merging traffic needs to gauge the surroundings before moving anywhere. They say to the driver "don't, it's not safe to change lanes here."
Others will jam themselves into any size gap that will fit their vehicle. The problem here is twofold. If we assume that the driver signals in advance as they are supposed to, courtesy is not shown by drivers in adjacent lanes in allowing the signalling driver some room to move in. Even though this driver cannot change lanes legally unless the movement is made safely and will not affect the travel of any other vehicle, they do it anyway, often accomplishing the whole process in a very short distance. Why plan ahead?
Drivers will also change lanes to move in front of the only other vehicle in sight, and do so leaving an unsafe following distance for the vehicle behind them. Never move until at least the point that you can see the entire front of the vehicle behind you in your rear view mirror. Remember that the two second rule also applies when you move in front of another vehicle.
"Tell the other drivers that it's illegal to change lanes in an intersection" is something that I hear frequently. This is not so in British Columbia. It is unsafe however, and a defensive driver will choose not to do this. It may also have negative consequences during a driving examination.
Remember to shoulder check before you move to protect against colliding with a vehicle in your blind spot, and that's the short course for changing lanes safely!