Automatic Warning System, or AWS for short, is the orange exclamation mark that appears on the right side of the Driver's GUI, which indicates the driver that he/she needs to slow down because there is a red/yellow signal ahead. You need to either press the orange exclamation mark or hit 'Q' on the keyboard within 6 seconds after the AWS starts to beep. Failing to confirm the AWS will result in an emergency brake, like the one you get after you SPAD. You can be ejected if you wait to long to release a break if you fail to leave the brakes.
As of now, the AWS does not warn you for speeding, but this may change in a future update.