Real time distance manipulation

My current requirement involves determining the distance of two lanes of a road on a real time basis. Extending the algorithm for "lane departure warning system", I think i can use the part of the code to specify the lanes on real time.

In general, the output can be as shown:

Now, to determine the current distance between the two yellow and magenta lanes, I need to consider the very first point of the yellow line and the parallel point (same y coordinate) of the magenta line. This will give me the current distance between the two lanes.

Other than this, I could use "bird eye's" image and process it in the manner such that I get two parallel lanes as follows:

In this case, I need to determine the parallel distance between the two straight lanes.

Then I can perform my further operations. Now, my doubt is, how can i determine the starting point of the yellow line and the parallel magenta line coordinate? Thanks in advance. Regards