How to measure distance of say a car from the camera

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Python

OpenCV

If my camera is my laptop webcam and I drive my car towards it. How can I tell how far it is?

I could do it by calculating the focal length and then estimating new distance. But a marker is needed for that (stuck on the car) . First, I haven't found a suitable marker that is reliable
Second, If there is a better way or any other way please do help

Once the car is close enough i want to freeze the frame and then detect faces.
That I can do.
I am using python2.7 and opencv 2.9. something

I have started opencv and python few days back. So please try and explain. Thanks

What i could do is draw a line from the centre of the car to the edge of the frame when viewed laterally. and when the line becomes too short capture the front view of the car on another camera.
But, even for this i do not know how to use two cameras with opencv.

Any help will be much appreciated