What do the big diamond-shaped signs with red, yellow and blue diamonds mean?

A typical sign that you're talking about might look like this:

You see these signs on any building that contains hazardous chemicals. The sign is called an NFPA panel. NFPA stands for National Fire Protection Association.

The idea behind these signs is to give firefighters some sort of advanced notice on what they are getting themselves into when they arrive at the scene of a fire. When there is a house fire, firefighters have a pretty good idea of what they are in for, but if there is a fire at a warehouse in an industrial park, it is very hard to know what chemicals might be stored there. The NFPA panel is a clear indication of what sort of dangers might lie inside.

The panel has four areas:

Red - Fire hazard
Blue - Health hazard
Yellow - Reactivity
White - Specific hazard

The numbers in the first three areas range from 0 to 4, with 0 signifying no hazard and 4 signifying a severe hazard. For example, in the Reactivity area:

0 = Stable
1 = Unstable if heated
2 = Violent chemical
3 = Shock or heat may detonate
4 = May detonate

In the Fire hazard area, the numbers indicate the flash point:

0 = Will not burn
1 = Above 200 degrees F (93 C)
2 = Below 200 degrees F
3 = Below 100 degrees F (38 C)
4 = Below 73 degrees F (23)

Concrete is an example of a class 0 fire hazard. Paper and wood are class 1.

In the Health hazard area:

0 = No hazard
1 = Slightly hazardous
2 = Hazardous
3 = Extremely hazardous
4 = Deadly

In the Specific hazard area, you will see things like:

OXY - Oxidizer
ACID
ALK - Alkali
COR - Corrosive

You may also see a "W" with a bar through it (meaning "use no water"), or the radiation hazard symbol for radioactive materials.

Now you can drive around your neighborhood and get a glimpse into what is stored in all of the buildings!

These links will help you learn more:

Hazard Sign FAQ

What is a hazard road sign?

Hazard markers are used to indicate a change in the direction of travel or an obstruction to any approaching traffic. Chevron alignment markers, bi-directional arrow markers and obstruction markers are common forms of hazard markers.

How do you read hazard signs?

The numbers found on a hazard sign are used to indicate the severity of the danger ahead. These numbers start from 1 (slight hazard) and go up to 4 (severe hazard).

What are the nine hazard symbols?

The nine symbols warn people of hazards associated with certain substances. These are poisons, health hazards (like cancer-causing agents or other illnesses that happen over time), immediate health hazards (skin or eye irritants on contact), gas under pressure, corrosives, oxidizers (chemicals that make flames burn hotter), explosives, flammables and environmental hazards.

What does a warning sign mean?

A warning sign on the side of the road indicates a potential hazard and requires your attention. Warning signs are placed before an obstacle on the road to indicate hazards that are not clearly visible or apparent to the driver.

What does an exclamation mark (!) on a hazard warning label mean?

The symbol of an exclamation mark warns you that the substance can cause immediate health effects, such as skin or eye irritation.