The Virtual Stop Light is the perfect tool to learn how traffic signals work (and its pretty fun, too!). Its a great app for babies, toddlers, and little kids (I made it for my 2 year old son :) ).
If you have a group, heres a fun game to try:
Open the app, prop it up where everyone can see the light, and just follow the prompts: red means stop, yellow means move arms but not legs, and green means walk.
All you need to do to get started is open the app (which begins with a red light), and wait until it turns green to GO!
A counter appears at the bottom of the screen which shows you how much time is left until the light changes, and once 5 seconds are left, little beeps lets you know to start revving your engines. You can also force a light change by just tapping the top, middle, or bottom of the screen.
If you want the Virtual Stop Light to take a break, just press the pause button at the top of the screen.
Have fun!