the red LED will become activated for few seconds after few seconds the red will become deactivated and the yellow LED will become activated for a less time period as compare to the red and the last LED green.  When the yellow LED will become off the green LED will become activated. When the green LED will OFF then again the yellow LED will activate and then the red, and this process will continuously repeat until the power is disconnected.

The circuit is using four 555 timer ICs, each IC is working to provide a certain time delay. And the time delay of all ICs are depends on their 100K resistors and the capacitor Cx, this means for increasing or decreasing the ON time of the LEDs can be adjusted with the 100K resistor and capacitor Cx. The can be operated with 5V to 12V DC. If you are operating the circuit with lower voltage for example 5V or 6V and the LEDs are not brighter on these voltages then change the value of all the 470 ohms resistors and 390 ohms resistor to 240 ohms.

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This article describes a project of a traffic light using 555 timer ICs. The circuit is using three colour LEDs which are red, yellow and green same colours as you have seen in the road signal traffic lights. The lighting pattern of the circuit is also same as the traffic signal lights. When the button S1 is pressed
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Traffic Light Project Using 555 Timer ICs