Red is the most emotionally intense colour, it stimulates you, increasing your heart rate, your blood pressure and metabolism helping you feel more energised and alert. It is highly visible and carry's a sense or urgency, which is why it's commonly used for stop sign, warnings signs and traffic lights.
The colour of courage, it's prevalent in a lot or heraldry like the royal arms of England and Scotland and many flags of various countries around the world, also the queen's guard dressed in their red uniform.
As a stimulant colour it can help prompt people to make quick decisions, like buy now or claim discount or entice people in to a store with big 'clearance' banner beaming in the window. You can see around you that it is also associated with being erotic (like a poster with red stilettos appealing to woman to buy), dangerous (warning signs and alert signs) and high energy.