There is one serious upside to those depressing billboards you sometimes see along the side of the road: They make people more cautious drivers. This can reduce accidents.
Researchers studied the way that people drove when they got to yellow lights. As the light switches, drivers have to decide if they are going to hit the brakes and stop for the red light or if they are going to accelerate and try to make it through before it turns red. While it is not illegal to go through a yellow light, it can be dangerous, especially when drivers misjudge the distance and end up running the red light.
In any case, the study found that people who had just seen a negative or depressing billboard tended to hit the brakes and stop. Those who had seen positive billboards or even neutral billboards tended to try to make it through the yellow light.
Why does it happen? The billboards subtly change the way that you think and the emotions you feel. When you experience negative emotions, you are also more likely to think about negative outcomes. So, as you decide if you should try to run through that yellow light, you are more likely to think about being involved in an accident, rather than making it through without incident. As a result, you tend to hit the brakes.
Of course, many things besides billboards play with drivers' emotions, and they can change the way people drive. If you get into an accident with a reckless driver who takes too many risks, you may be able to seek financial compensation for your injuries.