You are going to design traffic lights at an intersection of a city.  North and South (up and down) are going to be a main highway, East and West (right and left) will be a smaller cross street.  There will be no turn signals and no cross walks.  You only need to build/code one set of lights for each street (one set of green, yellow, and red for the highway, and another set for the cross street).  You do not need to build actual traffic lights or an intersection.  6 total leds (2 red, 2 yellow, 2 green); for the 2 traffic lights. 
1 photoresistor; for a car sensor on the cross street. 
1 button; to simulate 'loss of power' mode. 
Other necessary resistors, jumper wires

All traffic lights must cycle from green, to yellow, to red, to green… 
Lights must be red while crossing traffic has green and yellow. 
Yellow lights must be on for 500 milliseconds. 
Highway traffic is always green unless a car is waiting on the cross street. 
Highway traffic must always be green for at least 10 seconds. 
Cross street green light will be on for a minimum of 3 seconds. 
Cross street light will only remain green if it senses a car still on it. 
Cross street green light can only stay on for a maximum of 6 seconds. 
If 'loss of power' mode is activated (the button is held down), both red lights blink at the same time.