C :: Reading Button Input On Arduino Board

Okay so this is part of a small uni module around the arduino board. The task is to simulate traffic lights at a junction. At the moment my code below runs through the sequence with 2 seperate lights. When one set of lights turns RED , the other turns GREEN.In my main loop method I need to add a button for a crossing. I want to be able to check if the button is pressed at each of the delays but do not want to have to hold it down.I have thought about putting it at the top of the loop but then the program will only check if button is pressed in that point therefore the user will have to hold it down?

Code:

int green1 = 13 ;
int amber1 = 12 ;
int red1 = 11 ;

int green2 = 6 ;
int amber2 = 7 ;
int red2 = 8 ;
}

[code]....