Railroad signals actually came first ... the motor vehicle traffic kind were a simplified version. Signals generally tell you one of two things: 1.) The status of a block ahead of you (Occupied, as in, there's a train there, or Clear, as in, there's no train there) and 2.) Speed control (as in, "Slow down to Medium Speed as you pass this signal, and when you get to the next signal, be going slow enough that you can stop if it says to.")
Imagine a traffic light that told you "You can pass this signal at 45 mph, but be prepared to hang a left at the next light, and be going no faster than 10 mph when you get there!" You'd have a lot more than "Green, Yellow, Red" to remember ... and it might vary from state to state, or city to city.
The other thing that can be confusing about railroad signals is that red doesn't ALWAYS mean stop. Sometimes it's just a placeholder; sometimes it means "Stop and Proceed," and sometimes it means "Stop.. and stay stopped!" So, a two story tall "Green over Red over Red" is sometimes the LEAST restrictive signal in a system (and means "Clear") while a tiny dwarf signal with a red lamp might require a train to stop and wait indefinately for a better signal to be displayed!
The broadway limited pages are really good primer if you want to understand the basics of how most American signalling (and some from other countries) works, and how it came to be. The whole thing starts here:http://broadway.pennsyrr.com/Rail/Signal/index.html