Cars are large, powerful vehicles that people use to move themselves and their posessions around. Today, they are mostly powered either by internal combustion or by batteries. They have been around since approximately the late 1800s, although depending on your definition, they may have been invented earlier. Cars are generally driven on roads and highways.
Cars can go very fast! On a highway, a typical car might be traveling at around 70 miles per hour. Considering that cars often weigh two to four tons, an impact between a car at average speed and something else will likely cause considerable damage to both the car and the something else—to the point where laws require all cars to come with safety features designed to prevent the deaths of the people inside them when they come into contact with other things.
Further compounding the danger cars present is the fact that basically anyone can have or drive a car. The ability to operate a car is trivially easy to secure almost everywhere cars are available, even if the aspiring operator is an adolescent or a very old person. People in general are not known for being super responsible, and many regularly seem to forget that cars are big and potentially lethal, so people regularly operate cars when they should not (such as when they have been drinking) or without paying as much attention as they should (such as, perhaps, because they are simultaneously using a phone).
On the other hand, if no one is operating a car, it can't—and won't—do very much. Unoccupied cars are essentially inert.
The largeness and fastness of the car combined with their prevalence and the average amount of interest their drivers have in operating responsibly leads us to conclude that cars are pretty dangerous.
