Explain equation for Change Interval

y = t + v / 2a + 64.4G

Where:  y = length of yellow interval (sec)

    t = driver perception/reaction time (1.0 sec generally used)

    v = velocity of approaching vehicle (fps)

    a = deceleration rate (10 fps2 generally used) 

    G = grade of approach (percent/100)   

                         If G is positive percent or upgrade, use '+' symbol 

                         If G is negative percent or downhill, use '-' symbol