There are many different types of emergency vehicles around the world. The types of emergency lights they use can vary widely from country to country. In many areas, however, flashing red and blue lights are the standard emergency lights on many vehicles. There are a few theories about why red and blue lights become the most popular. For example, while the color red is associated with stop and warning, red emergency lights can get lost in heavy traffic because of the fact that most tail lights are also red. In these situations, blue lights really stand out and help to alert other drivers. Others point to studies indicating that red lights may be more visible during the day, while blue lights may be more visible at night. Using a combination of flashing red and blue lights thus helps to alert drivers ___ the time of day. Finally, some experts believe using both red and blue lights alleviates problems associated with drivers who are colorblind. People who have trouble seeing red often have no trouble seeing blue. ___, drivers who can't see blue can nonetheless see red. Using both red and blue lights thus helps to alert all drivers, even those with color blindness. [Question 37: alert drivers ___ the time of day]