First, we should note that the human vision is different from the artificial (computer) vision !

For more accurate understanding of the answer,  please consider the following three cases :


    A-  A proper height and angle ( in accordance with the technical requirements of  STA )  
             - A proper height ( about 18 meters )
             - A good angle of view ( about 20 )    
         
    B-  An improper height and angle ( different from the technical requirements of  STA )
             - An improper ( low ) height ( about 8 meters )  
             - An improper angle of view 

    C-  An improper height with an oblique angle ( completely different from the technical requirements of  STA )
             - An improper ( low ) height ( about 8 meters )  
             - An oblique angle
Why these conditions ( height and angle of view ) are chosen as the best mode?
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Case A:  A proper height and angle ( in accordance with the technical requirements of  STA )  

              As you can see in the picture below, in this case:

              - The vehicles are completely separable with good accuracy ( Which is useful in detecting and counting operations )

              - The camera preview encompasses all the road surfaces ( Which is useful in incident detection )

              -  All the details are identifiable  ( Which is useful for all the operations )

Note: In this case, STA is capable to detect up to  250  vehicles simultaneously ( a unique feature of  STA ).
Case B:  An improper height and angle ( different from the technical requirements of  STA )  

              As you can see in the picture below, in this case:

              - Some vehicles are hiding behind each other ( That impairs the detection operation )

              - The camera does not encompass enough the road and vehicles ( That impairs the incident detection )

              - Lots of detail and information is lost ( That impairs decision-making in different situations )



Case C:  An improper height with an oblique angle ( completely different from the technical requirements of  STA )

              As you can see in the picture below, in this case:

              - Some vehicles are hiding behind each other ( That impairs the detection operation )

              - The camera does not encompass enough the road and vehicles ( That impairs the incident detection )

              - Lots of detail and information is lost ( That impairs decision-making in different situations )

              - There is too much interference in the image of vehicles ( That impairs all the operations )