How to determine the area covered by vehicles in a road

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If the image of the road is constant or the size of the road is constant then what about this image

When you detect the edges of road , then between those edges can you check how many vehicles (rectangle's) cover on the road like in the last image it clearly shows that two vehicles (rectangles) can cover approximately whole road or you can say 2.5 rectangles can cover the whole road , so what about using this technique like how many rectangles are in a row ? If its 3 rectangles then road is full , if 2 then 33% space is remaining , if only one row is in line then 66% road is empty , if no rectangle then 100% road is empty. This is just runtime logic , you may get some authentic answers soon as well.