Europe's arbitrary post-colonial borders left Africans bunched into countries.
The medium population density countries are USA, Central America, South America, countries of eastern and southern Africa, north Europe, Saudi Arabia (Asia) etc. Remaining part of the world like Western Europe, rest of Asia etc. are the countries having the highest population density regions of the world.
The answer to this question is whenever you change speed or position. The natural tendency is we need to focus on what is going on ahead of where we are going, so we need to check our mirrors whenever we are the only car on the road. In order to prevent from accidents we need to be careful on driving in the road especially on express ways.
