Approximate distance to detected object (human) using Tracking Pedestrians from a Moving Car tutorial

Hello,

I am using this tutorial to locate humans on video I recorded, however I would need to get an approximate or vague expression of distance from the tracked object to the camera for my project (it is used to send information to the vehicle's processing unit whether it should continue it's move, slow down or stop/move left or right based on distance to detected objects). Any ideas how I could implement such thing?

Cheers