You have a project to do. Great.
It might be appropriate for the Arduino. Great.
You have no requirements. Not so good.
Or, if you do, you failed to share them.
There must be something like 422 million projects on the internet involving traffic lights and the Arduino - none of them really worth a damn because they are unrealistic, but you can learn from other people's mistakes.
How are YOU intending to make a better traffic light system?