The imaginary line in a DWI arrest

As a Houston DWI Attorney I am constantly reviewing DWI video tapes. These video tapes show how a person performs the field sobriety tests. These are awkward tests that most people would never be doing in normal life, but they are suppose to be testing a person for clues to determine if the have lost their normal use of mental or physical faculties. One of the most annoying parts of the test for me is when a police officer asks a person to walk down an imaginary line. The officer does not indicate how wide this line is to the person when giving them the instructions, but counts it against them when the person steps off the imaginary line. I am always perplexed how the officer knows the width of the line if nothing was agreed to before the starting the test. For all we know that line may be two feet wide and the person never stepped off the line. Thus, an officer should give more detailed instructions in this phase of the tests before they say that this is an indication of intoxication.