It is important to remain calm even when other drivers do something dangerous, stupid, or inconsiderate. One can expect to see such behavior often.
Here in New Mexico, the road fatality rate is about 30% higher than it is in the U.S. as a whole. And here in the U.S., the last time I checked, the road fatality rate was about 30% higher than it was in the UK, even though we have better roads than the UK has. That is the result of woefully inadequate driver training here; it's so poor that it's a joke. It is also the result of poor enforcement, at least here in New Mexico. When I tried to get statistical information on traffic citations and for what violations they were issued, I found that there are no records except for the total number of citations. Thus, I was unable to determine whether tickets are ever issued for failure to signal, etc.
A few years ago, when I took a driving course for geriatric drivers to get an insurance discount, I suggested that one way to improve driving was to follow the example of the police. Of course, everyone laughed because the police are among the worse drivers. It would be difficult for the police to give citations for the violations which they themselves habitually commit.
I'm convinced that with greatly improved driver education coupled with strict enforcement of the driving regulations, the fatality rate could easily be cut in half. However, doing that could be politically impossible. People expect to be able to get a license quickly, effortlessly, and cheaply. Forcing them to take a 160 hour course before even getting behind the wheel would meet with considerable resistance. So, with poor training and inadequate enforcement, people will continue to make lane changes and turns without signaling, not using lanes correctly when turning, following too closely, etc. etc.
Those of us who want to drive as well and responsibly as possible can purchase books which offer useful tips on defensive driving techniques and other information on how to reduce the risk of an accident. It is not enough only to follow the rules. It is important to anticipate the mistakes that other drivers are likely to make. Considering the expense, time loss, and inconvenience of an accident, even when one is not at fault, it makes sense to learn how to prevent accidents regardless of who would be at fault.