draw a curve between two lane marks

Hi, i have an aerial image for some road ramp,I want to draw ramp center-line which assumed to be located in the middle of the road lane,considering the widening factors that exists in some places, after determining the center-line, I want to divide it every 0.5 meter and extract the x,y coordinate at each point.. the line should be smooth.
please find the attached example of the aerial image..
if you could do it automatically or semiautomatic please contact me..i will send you the original image